* 
  * Copyright ( c ) Facebook , Inc. and its affiliates . 
  * 
  * This source code is licensed under the MIT license found in the 
  * LICENSE file in the root directory of this source tree . 
 
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *)

module Ast = Flow_ast

open Token
open Parser_env
open Flow_ast
module Error = Parse_error
module SSet = Set.Make(String)

open Parser_common

module type STATEMENT = sig
 val for_: env -> (Loc.t, Loc.t) Statement.t
 val if_: env -> (Loc.t, Loc.t) Statement.t
 val let_: env -> (Loc.t, Loc.t) Statement.t
 val try_: env -> (Loc.t, Loc.t) Statement.t
 val while_: env -> (Loc.t, Loc.t) Statement.t
 val with_: env -> (Loc.t, Loc.t) Statement.t
 val block: env -> (Loc.t, Loc.t) Statement.t
 val break: env -> (Loc.t, Loc.t) Statement.t
 val continue: env -> (Loc.t, Loc.t) Statement.t
 val debugger: env -> (Loc.t, Loc.t) Statement.t
 val declare: ?in_module:bool -> env -> (Loc.t, Loc.t) Statement.t
 val declare_export_declaration: ?allow_export_type:bool -> env -> (Loc.t, Loc.t) Statement.t
 val declare_opaque_type : env -> (Loc.t, Loc.t) Statement.t
 val do_while: env -> (Loc.t, Loc.t) Statement.t
 val empty: env -> (Loc.t, Loc.t) Statement.t
 val export_declaration: decorators:(Loc.t, Loc.t) Class.Decorator.t list -> env -> (Loc.t, Loc.t) Statement.t
 val expression: env -> (Loc.t, Loc.t) Statement.t
 val import_declaration: env -> (Loc.t, Loc.t) Statement.t
 val interface: env -> (Loc.t, Loc.t) Statement.t
 val maybe_labeled: env -> (Loc.t, Loc.t) Statement.t
 val opaque_type : env -> (Loc.t, Loc.t) Statement.t
 val return: env -> (Loc.t, Loc.t) Statement.t
 val switch: env -> (Loc.t, Loc.t) Statement.t
 val throw: env -> (Loc.t, Loc.t) Statement.t
 val type_alias: env -> (Loc.t, Loc.t) Statement.t
 val var: env -> (Loc.t, Loc.t) Statement.t
 val const: env -> (Loc.t, Loc.t) Statement.t
end

module Statement
  (Parse: PARSER)
  (Type: Type_parser.TYPE)
  (Declaration: Declaration_parser.DECLARATION)
  (Object: Object_parser.OBJECT)
  (Pattern_cover : Pattern_cover.COVER)
: STATEMENT = struct
  type for_lhs =
    | For_expression of pattern_cover
    | For_declaration of (Loc.t * (Loc.t, Loc.t) Ast.Statement.VariableDeclaration.t)

  FunctionDeclaration is not a valid Statement , but Annex B sometimes allows it . 
      However , AsyncFunctionDeclaration and  are never 
      allowed as statements . We still parse them as statements ( and raise an error ) to 
      recover gracefully .
     However, AsyncFunctionDeclaration and GeneratorFunctionDeclaration are never
     allowed as statements. We still parse them as statements (and raise an error) to
     recover gracefully. *)
  let function_as_statement env =
    let func = Declaration._function env in
    if in_strict_mode env then
      function_as_statement_error_at env (fst func)
    else begin
      let open Ast.Statement in
      match func with
      | loc, FunctionDeclaration { Ast.Function.async = true; _ } ->
        error_at env (loc, Parse_error.AsyncFunctionAsStatement)
      | loc, FunctionDeclaration { Ast.Function.generator = true; _ } ->
        error_at env (loc, Parse_error.GeneratorFunctionAsStatement)
      | _ -> ()
    end;
    func

  let rec empty env =
    let loc = Peek.loc env in
    Expect.token env T_SEMICOLON;
    loc, Statement.Empty

  and break env =
    let loc, label = with_loc (fun env ->
      Expect.token env T_BREAK;
      let label =
        if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
        then None
        else begin
          let (_, name) as label =
            Parse.identifier env in
          if not (SSet.mem name (labels env))
          then error env (Error.UnknownLabel name);
          Some label
        end
      in
      Eat.semicolon env;
      label
    ) env in
    if label = None && not (in_loop env || in_switch env)
    then error_at env (loc, Error.IllegalBreak);
    loc, Statement.Break { Statement.Break.label }

  and continue env =
    let loc, label = with_loc (fun env ->
      Expect.token env T_CONTINUE;
      let label =
        if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
        then None
        else begin
          let (_, name) as label =
            Parse.identifier env in
          if not (SSet.mem name (labels env))
          then error env (Error.UnknownLabel name);
          Some label
        end in
      Eat.semicolon env;
      label
    ) env in
    if not (in_loop env) then error_at env (loc, Error.IllegalContinue);
    loc, Statement.Continue { Statement.Continue.label }

  and debugger = with_loc (fun env ->
    Expect.token env T_DEBUGGER;
    Eat.semicolon env;
    Statement.Debugger
  )

  and do_while = with_loc (fun env ->
    Expect.token env T_DO;
    let body = Parse.statement (env |> with_in_loop true) in

  Annex B allows labelled FunctionDeclarations ( see 
        sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    if not (in_strict_mode env) && is_labelled_function body
    then function_as_statement_error_at env (fst body);

    Expect.token env T_WHILE;
    Expect.token env T_LPAREN;
    let test = Parse.expression env in
    Expect.token env T_RPAREN;
  The rules of automatic semicolon insertion in ES5 do n't mention this , 
      * but the semicolon after a do - while loop is optional . This is properly 
      * specified in ES6
     * but the semicolon after a do-while loop is optional. This is properly
     * specified in ES6 *)
    if Peek.token env = T_SEMICOLON
    then Eat.semicolon env;
    Statement.DoWhile { Statement.DoWhile.
      body;
      test;
    }
  )

  and for_ =
    let assert_can_be_forin_or_forof env err = function
      | loc, { Statement.VariableDeclaration.declarations; _; } ->
  Only a single declarator is allowed , without an init . So 
            * something like 
            * 
            * for ( var x in y ) { } 
            * 
            * is allowed , but we disallow 
            * 
            * for ( var x , y in z ) { } 
            * for ( var x = 42 in y ) { } 
           
           * something like
           *
           * for (var x in y) {}
           *
           * is allowed, but we disallow
           *
           * for (var x, y in z) {}
           * for (var x = 42 in y) {}
           *)
          (match declarations with
          | [ (_, { Statement.VariableDeclaration.Declarator.init = None; _; }) ] -> ()
          | _ -> error_at env (loc, err))
    in

  Annex B allows labelled FunctionDeclarations ( see 
        sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    let assert_not_labelled_function env body =
      if not (in_strict_mode env) && is_labelled_function body
      then function_as_statement_error_at env (fst body)
      else ()
    in

    with_loc (fun env ->
      Expect.token env T_FOR;
      let async = allow_await env && Expect.maybe env T_AWAIT in
      Expect.token env T_LPAREN;

      let init, errs =
        let env = env |> with_no_in true in
        match Peek.token env with
        | T_SEMICOLON -> None, []
        | T_LET ->
            let loc, (decl, errs) = with_loc Declaration.let_ env in
            Some (For_declaration (loc, decl)), errs
        | T_CONST ->
            let loc, (decl, errs) = with_loc Declaration.const env in
            Some (For_declaration (loc, decl)), errs
        | T_VAR ->
            let loc, (decl, errs) = with_loc Declaration.var env in
            Some (For_declaration (loc, decl)), errs
        | _ ->
            let expr = Parse.expression_or_pattern (env |> with_no_let true) in
            Some (For_expression expr), []
      in

      match Peek.token env with
      (* If `async` is true, this must be a for-await-of loop. *)
      | t when t = T_OF || async ->
          let left = Statement.(match init with
          | Some (For_declaration decl) ->
            assert_can_be_forin_or_forof env Error.InvalidLHSInForOf decl;
            ForOf.LeftDeclaration decl
          | Some (For_expression expr) ->
  # sec - for - in - and - for - of - statements - static - semantics - early - errors
            let patt = Pattern_cover.as_pattern ~err:Error.InvalidLHSInForOf env expr in
            ForOf.LeftPattern patt
          | None -> assert false) in
          (* This is a for of loop *)
          Expect.token env T_OF;
          let right = Parse.assignment env in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.ForOf { Statement.ForOf.
            left;
            right;
            body;
            async;
          }
      | T_IN ->
          let left = match init with
          | Some (For_declaration decl) ->
            assert_can_be_forin_or_forof env Error.InvalidLHSInForIn decl;
            Statement.ForIn.LeftDeclaration decl
          | Some (For_expression expr) ->
  # sec - for - in - and - for - of - statements - static - semantics - early - errors
            let patt = Pattern_cover.as_pattern ~err:Error.InvalidLHSInForIn env expr in
            Statement.ForIn.LeftPattern patt
          | None -> assert false in
          (* This is a for in loop *)
          Expect.token env T_IN;
          let right = Parse.expression env in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.ForIn { Statement.ForIn.
            left;
            right;
            body;
            each = false;
          }
      | _ ->
          (* This is a for loop *)
          errs |> List.iter (error_at env);
          Expect.token env T_SEMICOLON;
          let init = match init with
          | Some (For_declaration decl) -> Some (Statement.For.InitDeclaration decl)
          | Some (For_expression expr) ->
            Some (Statement.For.InitExpression (Pattern_cover.as_expression env expr))
          | None -> None in
          let test = match Peek.token env with
          | T_SEMICOLON -> None
          | _ -> Some (Parse.expression env) in
          Expect.token env T_SEMICOLON;
          let update = match Peek.token env with
          | T_RPAREN -> None
          | _ -> Some (Parse.expression env) in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.For { Statement.For.
            init;
            test;
            update;
            body;
          }
    )

  and if_ =
    (**
     * Either the consequent or alternate of an if statement
     *)
    let if_branch env =
  Normally this would just be a Statement , but Annex B allows 
          FunctionDeclarations in non - strict mode . See 
          sec - functiondeclarations - in - ifstatement - statement - clauses
         FunctionDeclarations in non-strict mode. See
         sec-functiondeclarations-in-ifstatement-statement-clauses *)
      let stmt =
        if Peek.is_function env then
          function_as_statement env
        else
          Parse.statement env
      in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
          ( see sec - labelled - function - declarations ) , but not in IfStatement 
          ( see sec - if - statement - static - semantics - early - errors ) .
         (see sec-labelled-function-declarations), but not in IfStatement
         (see sec-if-statement-static-semantics-early-errors). *)
      if not (in_strict_mode env) && is_labelled_function stmt
      then function_as_statement_error_at env (fst stmt);

      stmt
    in

    with_loc (fun env ->
      Expect.token env T_IF;
      Expect.token env T_LPAREN;
      let test = Parse.expression env in
      Expect.token env T_RPAREN;
      let consequent = if_branch env in
      let alternate = if Peek.token env = T_ELSE
      then begin
        Expect.token env T_ELSE;
        Some (if_branch env)
      end else None in
      Statement.If { Statement.If.
        test;
        consequent;
        alternate;
      }
    )

  and return = with_loc (fun env ->
    if not (in_function env)
    then error env Error.IllegalReturn;
    Expect.token env T_RETURN;
    let argument =
      if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
      then None
      else Some (Parse.expression env) in
    Eat.semicolon env;
    Statement.Return { Statement.Return.
      argument;
    }
  )

  and switch =
    let rec case_list env (seen_default, acc) =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> List.rev acc
      | _ ->
        let start_loc = Peek.loc env in
        let test = match Peek.token env with
        | T_DEFAULT ->
            if seen_default
            then error env Error.MultipleDefaultsInSwitch;
            Expect.token env T_DEFAULT; None
        | _ ->
            Expect.token env T_CASE;
            Some (Parse.expression env) in
        let seen_default = seen_default || test = None in
        let end_loc = Peek.loc env in
        Expect.token env T_COLON;
        let term_fn = function
        | T_RCURLY | T_DEFAULT | T_CASE -> true
        | _ -> false in
        let consequent =
          Parse.statement_list ~term_fn (env |> with_in_switch true) in
        let end_loc = match List.rev consequent with
        | last_stmt::_ -> fst last_stmt
        | _ -> end_loc in
        let acc = (Loc.btwn start_loc end_loc, Statement.Switch.Case.({
          test;
          consequent;
        }))::acc in
        case_list env (seen_default, acc)

    in with_loc (fun env ->
      Expect.token env T_SWITCH;
      Expect.token env T_LPAREN;
      let discriminant = Parse.expression env in
      Expect.token env T_RPAREN;
      Expect.token env T_LCURLY;
      let cases = case_list env (false, []) in
      Expect.token env T_RCURLY;
      Statement.Switch { Statement.Switch.
        discriminant;
        cases;
      }
    )

  and throw = with_loc (fun env ->
    let start_loc = Peek.loc env in
    Expect.token env T_THROW;
    if Peek.is_line_terminator env
    then error_at env (start_loc, Error.NewlineAfterThrow);
    let argument = Parse.expression env in
    Eat.semicolon env;
    Statement.(Throw { Throw.argument; })
  )

  and try_ = with_loc (fun env ->
    Expect.token env T_TRY;
    let block = Parse.block_body env in
    let handler = match Peek.token env with
    | T_CATCH ->
        let catch = with_loc (fun env ->
          Expect.token env T_CATCH;
          let param = if Peek.token env = T_LPAREN
          then begin
            Expect.token env T_LPAREN;
            let p = Some (Parse.pattern env Error.StrictCatchVariable) in
            Expect.token env T_RPAREN;
            p
          end else
            None
          in
          let body = Parse.block_body env in
          { Ast.Statement.Try.CatchClause.
            param;
            body;
          }
        ) env in
        Some catch
    | _ -> None in
    let finalizer = match Peek.token env with
    | T_FINALLY ->
        Expect.token env T_FINALLY;
        Some (Parse.block_body env)
    | _ -> None in

    (* No catch or finally? That's an error! *)
    if handler = None && finalizer = None then
      error_at env (fst block, Error.NoCatchOrFinally);

    Statement.Try { Statement.Try.
      block;
      handler;
      finalizer;
    }
  )

  and var = with_loc (fun env ->
    let declaration, errs = Declaration.var env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and const = with_loc (fun env ->
    let declaration, errs = Declaration.const env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and let_ = with_loc (fun env ->
    let declaration, errs = Declaration.let_ env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and while_ = with_loc (fun env ->
    Expect.token env T_WHILE;
    Expect.token env T_LPAREN;
    let test = Parse.expression env in
    Expect.token env T_RPAREN;
    let body = Parse.statement (env |> with_in_loop true) in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
        ( see sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       (see sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    if not (in_strict_mode env) && is_labelled_function body
    then function_as_statement_error_at env (fst body);

    Statement.While { Statement.While.
      test;
      body;
    }
  )

  and with_ env =
    let loc, stmt = with_loc (fun env ->
      Expect.token env T_WITH;
      Expect.token env T_LPAREN;
      let _object = Parse.expression env in
      Expect.token env T_RPAREN;
      let body = Parse.statement env in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
          ( see sec - labelled - function - declarations ) , but not in WithStatement 
          ( see sec - with - statement - static - semantics - early - errors ) .
         (see sec-labelled-function-declarations), but not in WithStatement
         (see sec-with-statement-static-semantics-early-errors). *)
      if not (in_strict_mode env) && is_labelled_function body
      then function_as_statement_error_at env (fst body);

      Statement.With { Statement.With.
        _object;
        body;
      }
    ) env in
    strict_error_at env (loc, Error.StrictModeWith);
    loc, stmt

  and block env =
    let loc, block = Parse.block_body env in
    loc, Statement.Block block

  and maybe_labeled = with_loc (fun env ->
    match (Parse.expression env, Peek.token env) with
    | ((loc, Ast.Expression.Identifier label), T_COLON) ->
        let _, name = label in
        Expect.token env T_COLON;
        if SSet.mem name (labels env)
        then error_at env (loc, Error.Redeclaration ("Label", name));
        let env = add_label env name in
        let body =
  labelled FunctionDeclarations are allowed in non - strict mode 
              ( see # sec - labelled - function - declarations )
             (see #sec-labelled-function-declarations) *)
          if Peek.is_function env then function_as_statement env
          else Parse.statement env
        in
        Statement.Labeled { Statement.Labeled.label; body; }
    | expression, _ ->
        Eat.semicolon env;
        Statement.(Expression { Expression.expression; directive = None; })
  )

  and expression = with_loc (fun env ->
    let expression = Parse.expression env in
    Eat.semicolon env;
    let directive = if allow_directive env
      then match expression with
      | _, Ast.Expression.Literal { Ast.Literal.
          value = Ast.Literal.String _;
          raw;
          _
        } -> Some (String.sub raw 1 (String.length raw - 2))
      | _ -> None
      else None
    in
    Statement.Expression { Statement.Expression.
      expression;
      directive;
    }
  )

  and type_alias_helper env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeAlias;
    Expect.token env T_TYPE;
    Eat.push_lex_mode env Lex_mode.TYPE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    Expect.token env T_ASSIGN;
    let right = Type._type env in
    Eat.semicolon env;
    Eat.pop_lex_mode env;
    Statement.TypeAlias.({
      id;
      tparams;
      right;
    })

  and declare_type_alias env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let type_alias = type_alias_helper env in
    Statement.DeclareTypeAlias type_alias
  ) env

  and type_alias env =
    if Peek.ith_is_identifier ~i:1 env
    then
      let loc, type_alias = with_loc type_alias_helper env in
      loc, Statement.TypeAlias type_alias
    else
      Parse.statement env

  and opaque_type_helper ?(declare=false) env =
    if not (should_parse_types env)
    then error env Error.UnexpectedOpaqueTypeAlias;
    Expect.token env T_OPAQUE;
    Expect.token env T_TYPE;
    Eat.push_lex_mode env Lex_mode.TYPE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    let supertype = match Peek.token env with
    | T_COLON ->
        Expect.token env T_COLON;
        Some (Type._type env)
    | _ -> None in
    let impltype =
      if not declare then
        (Expect.token env T_ASSIGN;
        Some (Type._type env))
      else None in
    Eat.semicolon env;
    Eat.pop_lex_mode env;
    Statement.OpaqueType.({
      id;
      tparams;
      impltype;
      supertype;
    })

  and declare_opaque_type env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let opaque_t = opaque_type_helper ~declare:true env in
    Statement.DeclareOpaqueType opaque_t
  ) env

  and opaque_type env =
    match Peek.ith_token ~i:1 env with
      T_TYPE ->
        let loc, opaque_t = with_loc (opaque_type_helper ~declare:false) env in
        loc, Statement.OpaqueType opaque_t
    | _ -> Parse.statement env

  and interface_helper env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeInterface;
    Expect.token env T_INTERFACE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    let { Ast.Type.Interface.extends; body } = Type.interface_helper env in
    Statement.Interface.({
      id;
      tparams;
      body;
      extends;
    })

  and declare_interface env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let iface = interface_helper env in
    Statement.DeclareInterface iface
  ) env

  and interface env =
  disambiguate between a value named ` interface ` , like ` var interface = 1 ; interface++ ` , 
        and an interface declaration like ` interface  { } ` . `
       and an interface declaration like `interface Foo {}`.` *)
    if Peek.ith_is_identifier_name ~i:1 env
    then
      let loc, iface = with_loc interface_helper env in
      loc, Statement.InterfaceDeclaration iface
    else expression env

  and declare_class =
    let rec mixins env acc =
      let super = Type.generic env in
      let acc = super::acc in
      match Peek.token env with
      | T_COMMA ->
        Expect.token env T_COMMA;
        mixins env acc
      | _ -> List.rev acc

    (* This is identical to `interface`, except that mixins are allowed *)
    in fun env ->
      let env = env |> with_strict true in
      Expect.token env T_CLASS;
      let id = Parse.identifier env in
      let tparams = Type.type_parameter_declaration_with_defaults env in
      let extends = if Expect.maybe env T_EXTENDS then Some (Type.generic env) else None in
      let mixins = match Peek.token env with
      | T_IDENTIFIER { raw = "mixins"; _ } -> Eat.token env; mixins env []
      | _ -> []
      in
      let implements = match Peek.token env with
      | T_IMPLEMENTS -> Eat.token env; Object.class_implements env []
      | _ -> []
      in
      let body = Type._object ~is_class:true env in
      Statement.DeclareClass.({
        id;
        tparams;
        body;
        extends;
        mixins;
        implements;
      })

  and declare_class_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let fn = declare_class env in
    Statement.DeclareClass fn
  ) env

  and declare_function env =
    Expect.token env T_FUNCTION;
    let id = Parse.identifier env in
    let start_sig_loc = Peek.loc env in
    let tparams = Type.type_parameter_declaration env in
    let params = Type.function_param_list env in
    Expect.token env T_COLON;
    let return = Type._type env in
    let end_loc = fst return in
    let loc = Loc.btwn start_sig_loc end_loc in
    let annot = loc, Ast.Type.(Function {Function.
      params;
      return;
      tparams;
    }) in
    let annot = fst annot, annot in
    let predicate = Type.predicate_opt env in
    Eat.semicolon env;
    Statement.DeclareFunction.({
      id;
      annot;
      predicate;
    })

  and declare_function_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    begin match Peek.token env with
    | T_ASYNC ->
      error env Error.DeclareAsync;
      Expect.token env T_ASYNC
    | _ -> ()
    end;
    let fn = declare_function env in
    Statement.DeclareFunction fn
  ) env

  and declare_var env =
    Expect.token env T_VAR;
    let _loc, { Pattern.Identifier.name; annot; _; } =
      Parse.identifier_with_type env ~no_optional:true Error.StrictVarName in
    Eat.semicolon env;
    Statement.DeclareVariable.({ id=name; annot; })

  and declare_var_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let var = declare_var env in
    Statement.DeclareVariable var
  ) env

  and declare_module =
    let rec module_items env ~module_kind acc =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> (module_kind, List.rev acc)
      | _ ->
        let stmt = declare ~in_module:true env in
        (* TODO: This is a semantic analysis and shouldn't be in the parser *)
        let module_kind = Statement.(
          let (loc, stmt) = stmt in
          match (module_kind, stmt) with
* 
            * The first time we see either a ` declare export ` or a 
            * ` declare module.exports ` , we lock in the kind of the module . 
            * 
            * ` declare export type ` and ` declare export interface ` are the two 
            * exceptions to this rule because they are valid in both CommonJS 
            * and ES modules ( and thus do not indicate an intent for either ) . 
           
           * The first time we see either a `declare export` or a
           * `declare module.exports`, we lock in the kind of the module.
           *
           * `declare export type` and `declare export interface` are the two
           * exceptions to this rule because they are valid in both CommonJS
           * and ES modules (and thus do not indicate an intent for either).
           *)
          | None, DeclareModuleExports _ -> Some (DeclareModule.CommonJS loc)
          | None, DeclareExportDeclaration {
              DeclareExportDeclaration.declaration;
              _;
            } ->
            (match declaration with
              | Some (DeclareExportDeclaration.NamedType _)
              | Some (DeclareExportDeclaration.Interface _)
                -> module_kind
              | _ -> Some (DeclareModule.ES loc)
            )

* 
            * There should never be more than one ` declare module.exports ` 
            * statement
           * There should never be more than one `declare module.exports`
           * statement *)
          | Some (DeclareModule.CommonJS _), DeclareModuleExports _ ->
            error env Parse_error.DuplicateDeclareModuleExports;
            module_kind

* 
            * It 's never ok to mix and match ` declare export ` and 
            * ` declare module.exports ` in the same module because it leaves the 
            * kind of the module ( CommonJS vs ES ) ambiguous . 
            * 
            * The 1 exception to this rule is that ` export type / interface ` are 
            * both ok in CommonJS modules . 
           
           * It's never ok to mix and match `declare export` and
           * `declare module.exports` in the same module because it leaves the
           * kind of the module (CommonJS vs ES) ambiguous.
           *
           * The 1 exception to this rule is that `export type/interface` are
           * both ok in CommonJS modules.
           *)
          | Some (DeclareModule.ES _), DeclareModuleExports _ ->
            error env Parse_error.AmbiguousDeclareModuleKind;
            module_kind
          | Some (DeclareModule.CommonJS _), DeclareExportDeclaration {
              DeclareExportDeclaration.declaration;
              _;
            } ->
              (match declaration with
                | Some (DeclareExportDeclaration.NamedType _)
                | Some (DeclareExportDeclaration.Interface _)
                  -> ()
                | _ -> error env Parse_error.AmbiguousDeclareModuleKind
              );
              module_kind

          | _ -> module_kind
        ) in
        module_items env ~module_kind (stmt::acc)
    in

    let declare_module_ env start_loc =
      let id = match Peek.token env with
      | T_STRING (loc, value, raw, octal) ->
          if octal then strict_error env Error.StrictOctalLiteral;
          Expect.token env (T_STRING (loc, value, raw, octal));
          Statement.DeclareModule.Literal (loc, { StringLiteral.value; raw; })
      | _ ->
          Statement.DeclareModule.Identifier (Parse.identifier env) in
      let body_loc, (module_kind, body) = with_loc (fun env ->
        Expect.token env T_LCURLY;
        let res = module_items env ~module_kind:None [] in
        Expect.token env T_RCURLY;
        res
      ) env in
      let body = body_loc, { Statement.Block.body; } in
      let loc = Loc.btwn start_loc body_loc in
      let kind =
        match module_kind with
        | Some k -> k
        | None -> Statement.DeclareModule.CommonJS loc
      in
      loc,
      Statement.(DeclareModule DeclareModule.({ id; body; kind; }))
    in
    fun ?(in_module=false) env ->
      let start_loc = Peek.loc env in
      Expect.token env T_DECLARE;
      Expect.identifier env "module";
      if in_module || Peek.token env = T_PERIOD
      then
        let loc, exports = with_loc declare_module_exports env in
        Loc.btwn start_loc loc, exports
      else
        declare_module_ env start_loc

  and declare_module_exports env =
    Expect.token env T_PERIOD;
    Expect.identifier env "exports";
    let type_annot = Type.annotation env in
    Eat.semicolon env;
    Statement.DeclareModuleExports type_annot

  and declare ?(in_module=false) env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeDeclaration;
  eventually , just emit a wrapper AST node
    (match Peek.ith_token ~i:1 env with
      | T_CLASS ->
          declare_class_statement env
      | T_INTERFACE ->
          declare_interface env
      | T_TYPE -> (
          match Peek.token env with
          | T_IMPORT when in_module ->
            import_declaration env
          | _ ->
            declare_type_alias env
        )
      | T_OPAQUE ->
          declare_opaque_type env
      | T_TYPEOF when (Peek.token env) = T_IMPORT ->
          import_declaration env
      | T_FUNCTION
      | T_ASYNC ->
          declare_function_statement env
      | T_VAR ->
          declare_var_statement env
      | T_EXPORT when in_module ->
          declare_export_declaration ~allow_export_type:in_module env
      | T_IDENTIFIER { raw = "module"; _ } ->
          declare_module ~in_module env
      | _ when in_module -> (
          match Peek.token env with
          | T_IMPORT ->
            error env Error.InvalidNonTypeImportInDeclareModule;
            Parse.statement env
          | _ ->
            (* Oh boy, found some bad stuff in a declare module. Let's just
              * pretend it's a declare var (arbitrary choice) *)
            declare_var_statement env
        )
      | _ ->
          Parse.statement env
    )

  and export_source env =
    Expect.identifier env "from";
    match Peek.token env with
    | T_STRING (loc, value, raw, octal) ->
        if octal then strict_error env Error.StrictOctalLiteral;
        Expect.token env (T_STRING (loc, value, raw, octal));
        loc, { StringLiteral.value; raw; }
    | _ ->
        (* Just make up a string for the error case *)
        let ret = Peek.loc env, { StringLiteral.value = ""; raw = ""; } in
        error_unexpected env;
        ret

  and extract_pattern_binding_names =
    let rec fold acc = Pattern.(function
      | (_, Object {Object.properties; _;}) ->
        List.fold_left (fun acc prop ->
          match prop with
          | Object.Property (_, {Object.Property.pattern; _;})
          | Object.RestProperty (_, {Object.RestProperty.argument = pattern;})
            -> fold acc pattern
        ) acc properties
      | (_, Array {Array.elements; _;}) ->
        List.fold_left Array.(fun acc elem ->
          match elem with
          | Some (Element (_, {Element.argument = pattern; default = _}))
          | Some (RestElement (_, {RestElement.argument = pattern;}))
            -> fold acc pattern
          | None -> acc
        ) acc elements
      | (_, Identifier {Pattern.Identifier.name; _; }) ->
        name::acc
      | (_, Expression _) ->
        failwith "Parser error: No such thing as an expression pattern!"
    ) in
    List.fold_left fold

  and extract_ident_name (_, name) = name

  and export_specifiers ?(preceding_comma=true) env specifiers =
    match Peek.token env with
    | T_EOF
    | T_RCURLY ->
        List.rev specifiers
    | _ ->
        if not preceding_comma then error env Error.ExportSpecifierMissingComma;
        let specifier = with_loc (fun env ->
          let local = identifier_name env in
          let exported =
            match Peek.token env with
            | T_IDENTIFIER { raw = "as"; _ } ->
              Eat.token env;
              let exported = identifier_name env in
              record_export env exported;
              Some exported
            | _ ->
              record_export env local;
              None
          in
          { Statement.ExportNamedDeclaration.ExportSpecifier.local; exported; }
        ) env in
        let preceding_comma = Expect.maybe env T_COMMA in
        export_specifiers ~preceding_comma env (specifier::specifiers)

  and assert_export_specifier_identifiers env specifiers =
    let open Statement.ExportNamedDeclaration.ExportSpecifier in
    List.iter (function
      | _, { local = id; exported = None; } ->
        Parse.assert_identifier_name_is_identifier
          ~restricted_error:Parse_error.StrictVarName env id
      | _ ->
        ()
    ) specifiers

  and export_declaration ~decorators = with_loc (fun env ->
    let env = env |> with_strict true |> with_in_export true in
    let start_loc = Peek.loc env in
    Expect.token env T_EXPORT;
    match Peek.token env with
    | T_DEFAULT ->
        (* export default ... *)
        let open Statement.ExportDefaultDeclaration in
        let default, () = with_loc (fun env ->
          Expect.token env T_DEFAULT
        ) env in
        record_export env (Loc.btwn start_loc (Peek.loc env), "default");
        let declaration =
          if Peek.is_function env then
            (* export default [async] function [foo] (...) { ... } *)
            let fn = Declaration._function env in
            Declaration fn
          else if Peek.is_class env then
            (* export default class foo { ... } *)
            let _class = Object.class_declaration env decorators in
            Declaration _class
          else
            (* export default [assignment expression]; *)
            let expr = Parse.assignment env in
            Eat.semicolon env;
            Expression expr
        in
        Statement.ExportDefaultDeclaration {
          default;
          declaration;
        }
    | T_TYPE when (Peek.ith_token ~i:1 env) <> T_LCURLY ->
        (* export type ... *)
        let open Statement.ExportNamedDeclaration in
        if not (should_parse_types env)
        then error env Error.UnexpectedTypeExport;
        (match Peek.ith_token ~i:1 env with
        | T_MULT ->
          Expect.token env T_TYPE;
          let specifier_loc = Peek.loc env in
          Expect.token env T_MULT;
          let source = export_source env in
          Eat.semicolon env;
          Statement.ExportNamedDeclaration {
            declaration = None;
            specifiers = Some (ExportBatchSpecifier (specifier_loc, None));
            source = Some source;
            exportKind = Statement.ExportType;
          }
        | _ ->
          let loc, type_alias = with_loc type_alias_helper env in
          record_export env (loc, extract_ident_name type_alias.Statement.TypeAlias.id);
          let type_alias = (loc, Statement.TypeAlias type_alias) in
          Statement.ExportNamedDeclaration {
            declaration = Some type_alias;
            specifiers = None;
            source = None;
            exportKind = Statement.ExportType;
          }
        )
    | T_OPAQUE ->
        (* export opaque type ... *)
        let open Statement.ExportNamedDeclaration in
        let loc, opaque_t = with_loc opaque_type_helper env in
        record_export env (loc, extract_ident_name opaque_t.Statement.OpaqueType.id);
        let opaque_t = (loc, Statement.OpaqueType opaque_t) in
        Statement.ExportNamedDeclaration {
          declaration = Some opaque_t;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportType;
        }
    | T_INTERFACE ->
        (* export interface I { ... } *)
        let open Statement.ExportNamedDeclaration in
        if not (should_parse_types env)
        then error env Error.UnexpectedTypeExport;
        let interface = interface env in
        (match interface with
          | (loc, Statement.InterfaceDeclaration {Statement.Interface.id; _;}) ->
            record_export env (loc, extract_ident_name id)
          | _ -> failwith (
              "Internal Flow Error! Parsed `export interface` into something " ^
              "other than an interface declaration!"
            )
        );
        Statement.ExportNamedDeclaration {
          declaration = Some interface;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportType;
        }
    | T_LET
    | T_CONST
    | T_VAR
    (* not using Peek.is_class here because it would guard all of the
      * cases *)
    | T_AT
    | T_CLASS
    (* not using Peek.is_function here because it would guard all of the
      * cases *)
    | T_ASYNC
    | T_FUNCTION ->
        let open Statement.ExportNamedDeclaration in
        let stmt = Parse.statement_list_item env ~decorators:decorators in
        let names = Statement.(
          match stmt with
          | (_, VariableDeclaration { VariableDeclaration.declarations; _; }) ->
            List.fold_left (fun names (_, declaration) ->
              let id = declaration.VariableDeclaration.Declarator.id in
              extract_pattern_binding_names names [id]
            ) [] declarations
          | (loc, ClassDeclaration { Class.id = Some id; _; })
          | (loc, FunctionDeclaration { Function.id = Some id; _; })
            -> [(loc, extract_ident_name id)]
          | (loc, ClassDeclaration { Class.id = None; _; }) ->
            error_at env (loc, Error.ExportNamelessClass);
            []
          | (loc, FunctionDeclaration { Function.id = None; _; }) ->
            error_at env (loc, Error.ExportNamelessFunction);
            []
          | _ -> failwith "Internal Flow Error! Unexpected export statement declaration!"
        ) in
        List.iter (record_export env) names;
        Statement.ExportNamedDeclaration {
          declaration = Some stmt;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportValue;
        }
    | T_MULT ->
        let open Statement.ExportNamedDeclaration in
        let loc = Peek.loc env in
        Expect.token env T_MULT;
        let local_name =
          let parse_export_star_as =
            (parse_options env).esproposal_export_star_as
          in
          match Peek.token env with
          | T_IDENTIFIER { raw = "as"; _ } ->
            Eat.token env;
            if parse_export_star_as
            then Some (Parse.identifier env)
            else (error env Error.UnexpectedTypeDeclaration; None)
          | _ ->
            None
        in
        let specifiers =
          Some (ExportBatchSpecifier (loc, local_name))
        in
        let source = export_source env in
        let source = Some source in
        Eat.semicolon env;
        Statement.ExportNamedDeclaration {
          declaration = None;
          specifiers;
          source;
          exportKind = Statement.ExportValue;
        }
    | _ ->
        let open Statement.ExportNamedDeclaration in
        let exportKind = (
          match Peek.token env with
          | T_TYPE -> Eat.token env; Statement.ExportType
          | _ -> Statement.ExportValue
        ) in
        Expect.token env T_LCURLY;
        let specifiers = export_specifiers env [] in
        Expect.token env T_RCURLY;
        let source =
          match Peek.token env with
          | T_IDENTIFIER { raw = "from"; _ } ->
            Some (export_source env)
          | _ ->
            assert_export_specifier_identifiers env specifiers;
            None
        in
        Eat.semicolon env;
        Statement.ExportNamedDeclaration {
          declaration = None;
          specifiers = Some (ExportSpecifiers specifiers);
          source;
          exportKind;
        }
  )

  and declare_export_declaration ?(allow_export_type=false) = with_loc (fun env ->
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeDeclaration;
    Expect.token env T_DECLARE;

    let env = env |> with_strict true |> with_in_export true in
    Expect.token env T_EXPORT;
    Statement.DeclareExportDeclaration.(match Peek.token env with
    | T_DEFAULT ->
        (* declare export default ... *)
        let default, () = with_loc (fun env ->
          Expect.token env T_DEFAULT
        ) env in
        let declaration = match Peek.token env with
        | T_FUNCTION ->
            (* declare export default function foo (...): ...  *)
            let fn = with_loc declare_function env in
            Some (Function fn)
        | T_CLASS ->
            (* declare export default class foo { ... } *)
            let class_ = with_loc declare_class env in
            Some (Class class_)
        | _ ->
            (* declare export default [type]; *)
            let type_ = Type._type env in
            Eat.semicolon env;
            Some (DefaultType type_)
          in
        Statement.DeclareExportDeclaration {
          default = Some default;
          declaration;
          specifiers = None;
          source = None;
        }
    | T_LET
    | T_CONST
    | T_VAR
    | T_CLASS
    | T_FUNCTION ->
        let declaration = match Peek.token env with
        | T_FUNCTION ->
            (* declare export function foo (...): ...  *)
            let fn = with_loc declare_function env in
            Some (Function fn)
        | T_CLASS ->
            (* declare export class foo { ... } *)
            let class_ = with_loc declare_class env in
            Some (Class class_)
        | T_LET
        | T_CONST
        | T_VAR as token ->
            (match token with
            | T_LET -> error env Error.DeclareExportLet
            | T_CONST -> error env Error.DeclareExportConst
            | _ -> ());
            (* declare export var foo: ... *)
            let var = with_loc declare_var env in
            Some (Variable var)
        | _ -> assert false in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration;
          specifiers = None;
          source = None;
        }
    | T_MULT ->
        (* declare export * from 'foo' *)
        let loc = Peek.loc env in
        Expect.token env T_MULT;
        let parse_export_star_as =
          (parse_options env).esproposal_export_star_as
        in
        let local_name =
          match Peek.token env with
          | T_IDENTIFIER { raw = "as"; _ } ->
            Eat.token env;
            if parse_export_star_as
            then Some (Parse.identifier env)
            else (error env Error.UnexpectedTypeDeclaration; None)
          | _ ->
            None
        in
        let specifiers = Statement.ExportNamedDeclaration.(
          Some (ExportBatchSpecifier (loc, local_name))
        ) in
        let source = export_source env in
        Eat.semicolon env;
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = None;
          specifiers;
          source = Some source;
        }
    | T_TYPE when allow_export_type ->
        (* declare export type = ... *)
        let alias = with_loc type_alias_helper env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (NamedType alias);
          specifiers = None;
          source = None;
        }
    | T_OPAQUE ->
        (* declare export opaque type = ... *)
        let opaque = with_loc (opaque_type_helper ~declare:true) env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (NamedOpaqueType opaque);
          specifiers = None;
          source = None;
        }
    | T_INTERFACE when allow_export_type ->
        (* declare export interface ... *)
        let iface = with_loc interface_helper env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (Interface iface);
          specifiers = None;
          source = None;
        }
    | _ ->
        (match Peek.token env with
          | T_TYPE -> error env Error.DeclareExportType
          | T_INTERFACE -> error env Error.DeclareExportInterface
          | _ -> ()
        );
        Expect.token env T_LCURLY;
        let specifiers = export_specifiers env [] in
        Expect.token env T_RCURLY;
        let source =
          match Peek.token env with
          | T_IDENTIFIER { raw = "from"; _ } ->
            Some (export_source env)
          | _ ->
            assert_export_specifier_identifiers env specifiers;
            None
        in
        Eat.semicolon env;
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = None;
          specifiers = Some (Statement.ExportNamedDeclaration.ExportSpecifiers specifiers);
          source;
        }
    )
  )

  and import_declaration =
    let open Statement.ImportDeclaration in

    let source env =
      Expect.identifier env "from";
      match Peek.token env with
      | T_STRING (loc, value, raw, octal) ->
          if octal then strict_error env Error.StrictOctalLiteral;
          Expect.token env (T_STRING (loc, value, raw, octal));
          loc, { StringLiteral.value; raw; }
      | _ ->
          (* Just make up a string for the error case *)
          let ret = Peek.loc env, { StringLiteral.value = ""; raw = ""; } in
          error_unexpected env;
          ret

    in let is_type_import = function
      | T_TYPE
      | T_TYPEOF -> true
      | _ -> false

    (* `x` or `x as y` in a specifier *)
    in let with_maybe_as ~for_type ?error_if_type env =
      let identifier env =
        if for_type then Type.type_identifier env else Parse.identifier env
      in
      match Peek.ith_token ~i:1 env with
      | T_IDENTIFIER { raw = "as"; _ } ->
        let remote = identifier_name env in
        Eat.token env; (* as *)
        let local = Some (identifier env) in
        remote, local
      | T_EOF
      | T_COMMA
      | T_RCURLY ->
        identifier env, None
      | _ ->
        begin match error_if_type, Peek.token env with
        | Some error_if_type, T_TYPE
        | Some error_if_type, T_TYPEOF ->
          error env error_if_type;
          Eat.token env; (* consume `type` or `typeof` *)
          Type.type_identifier env, None
        | _ ->
          identifier env, None
        end


       ImportSpecifier[Type ] : 
         [ ~Type ] ImportedBinding 
         [ ~Type ] IdentifierName ImportedTypeBinding 
         [ ~Type ] IdentifierName IdentifierName ImportedBinding 
         [ ~Type ] IdentifierName IdentifierName IdentifierName ImportedTypeBinding 
         [ + Type ] ImportedTypeBinding 
         [ + Type ] IdentifierName IdentifierName ImportedTypeBinding 

       Static Semantics : 

       ` IdentifierName ImportedTypeBinding ` : 
       - It is a Syntax Error if IdentifierName 's StringValue is not " type " or " typeof " 

       ` IdentifierName IdentifierName   : 
       - It is a Syntax Error if the second IdentifierName 's StringValue is not " as " 

       ` IdentifierName IdentifierName IdentifierName   ImportedTypeBinding ` : 
       - It is a Syntax Error if the first IdentifierName 's StringValue is not " type " 
         or " typeof " , and the third IdentifierName 's StringValue is not " as " 
    
      ImportSpecifier[Type]:
        [~Type] ImportedBinding
        [~Type] IdentifierName ImportedTypeBinding
        [~Type] IdentifierName IdentifierName ImportedBinding
        [~Type] IdentifierName IdentifierName IdentifierName ImportedTypeBinding
        [+Type] ImportedTypeBinding
        [+Type] IdentifierName IdentifierName ImportedTypeBinding

      Static Semantics:

      `IdentifierName ImportedTypeBinding`:
      - It is a Syntax Error if IdentifierName's StringValue is not "type" or "typeof"

      `IdentifierName IdentifierName ImportedBinding`:
      - It is a Syntax Error if the second IdentifierName's StringValue is not "as"

      `IdentifierName IdentifierName IdentifierName  ImportedTypeBinding`:
      - It is a Syntax Error if the first IdentifierName's StringValue is not "type"
        or "typeof", and the third IdentifierName's StringValue is not "as"
    *)
    in let specifier env =
      let kind = match Peek.token env with
      | T_TYPE -> Some ImportType
      | T_TYPEOF -> Some ImportTypeof
      | _ -> None
      in

      if is_type_import (Peek.token env) then begin
        (* consume `type`, but we don't know yet whether this is `type foo` or
           `type as foo`. *)
        let type_keyword_or_remote = identifier_name env in
        match Peek.token env with
        (* `type` (a value) *)
        | T_EOF
        | T_RCURLY
        | T_COMMA ->
          let remote = type_keyword_or_remote in (* `type` becomes a value *)
          Parse.assert_identifier_name_is_identifier env remote;
          { remote; local = None; kind = None }

        (* `type as foo` (value named `type`) or `type as,` (type named `as`) *)
        | T_IDENTIFIER { raw = "as"; _ } ->
          begin match Peek.ith_token ~i:1 env with
          | T_EOF
          | T_RCURLY
          | T_COMMA ->
            (* `type as` *)
            { remote = Type.type_identifier env; local = None; kind }
          | T_IDENTIFIER { raw = "as"; _ } ->
            (* `type as as foo` *)
            let remote = identifier_name env in (* first `as` *)
            Eat.token env; (* second `as` *)
            let local = Some (Type.type_identifier env) in (* `foo` *)
            { remote; local; kind }
          | _ ->
            (* `type as foo` *)
            let remote = type_keyword_or_remote in (* `type` becomes a value *)
            Parse.assert_identifier_name_is_identifier env remote;
            Eat.token env; (* `as` *)
            let local = Some (Parse.identifier env) in
            { remote; local; kind = None }
          end

        (* `type x`, or `type x as y` *)
        | _ ->
          let remote, local = with_maybe_as ~for_type:true env in
          { remote; local; kind }
      end else
        (* standard `x` or `x as y` *)
        let remote, local = with_maybe_as ~for_type:false env in
        { remote; local; kind = None }

    (* specifier in an `import type { ... }` *)
    in let type_specifier env =
      let remote, local = with_maybe_as env
        ~for_type:true
        ~error_if_type:Error.ImportTypeShorthandOnlyInPureImport
      in
      { remote; local; kind = None }

    (* specifier in an `import typeof { ... }` *)
    in let typeof_specifier env =
      let remote, local = with_maybe_as env
        ~for_type:true
        ~error_if_type:Error.ImportTypeShorthandOnlyInPureImport
      in
      { remote; local; kind = None }

    in let rec specifier_list ?(preceding_comma=true) env statement_kind acc =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> List.rev acc
      | _ ->
        if not preceding_comma then error env Error.ImportSpecifierMissingComma;
        let specifier = match statement_kind with
        | ImportType -> type_specifier env
        | ImportTypeof -> typeof_specifier env
        | ImportValue -> specifier env
        in
        let preceding_comma = Expect.maybe env T_COMMA in
        specifier_list ~preceding_comma env statement_kind (specifier::acc)

    in let named_or_namespace_specifier env import_kind =
      let start_loc = Peek.loc env in
      match Peek.token env with
      | T_MULT ->
          Expect.token env T_MULT;
          Expect.identifier env "as";
          let id = match import_kind with
          | ImportType
          | ImportTypeof -> Type.type_identifier env
          | ImportValue -> Parse.identifier env
          in
          ImportNamespaceSpecifier (Loc.btwn start_loc (fst id), id)
      | _ ->
          Expect.token env T_LCURLY;
          let specifiers = specifier_list env import_kind [] in
          Expect.token env T_RCURLY;
          ImportNamedSpecifiers specifiers

    in let with_specifiers importKind env =
      let specifiers = Some (named_or_namespace_specifier env importKind) in
      let source = source env in
      Eat.semicolon env;
      Statement.ImportDeclaration {
        importKind;
        source;
        specifiers;
        default = None;
      }

    in let with_default importKind env =
      let default_specifier = match importKind with
      | ImportType
      | ImportTypeof -> Type.type_identifier env
      | ImportValue -> Parse.identifier env
      in

      let additional_specifiers =
        match Peek.token env with
  ` import  , ... `
            Expect.token env T_COMMA;
            Some (named_or_namespace_specifier env importKind)
        | _ -> None
      in

      let source = source env in
      Eat.semicolon env;
      Statement.ImportDeclaration {
        importKind;
        source;
        specifiers = additional_specifiers;
        default = Some default_specifier;
      }

    in with_loc (fun env ->
      let env = env |> with_strict true in
      Expect.token env T_IMPORT;

      match Peek.token env with
  ` import * as ns from " ModuleName " ; `
      | T_MULT ->
        with_specifiers ImportValue env

  ` import { ... } from " ModuleName " ; `
      | T_LCURLY ->
        with_specifiers ImportValue env

  ` import " ModuleName " ; `
      | T_STRING (str_loc, value, raw, octal) ->
        if octal then strict_error env Error.StrictOctalLiteral;
        Expect.token env (T_STRING (str_loc, value, raw, octal));
        let source = (str_loc, { StringLiteral.value; raw; }) in
        Eat.semicolon env;
        Statement.ImportDeclaration {
          importKind = ImportValue;
          source;
          specifiers = None;
          default = None;
        }

  ` import type [ ... ] from " ModuleName " ; ` 
          note that if [ ... ] is missing , we 're importing a value named ` type ` !
         note that if [...] is missing, we're importing a value named `type`! *)
      | T_TYPE when should_parse_types env ->
        begin match Peek.ith_token ~i:1 env with
  ` import type , { other , names } from " ModuleName " ; `
        | T_COMMA
  ` import type from " ModuleName " ; `
        | T_IDENTIFIER { raw = "from"; _ } ->
          with_default ImportValue env
        | T_MULT ->
          (* `import type *` is invalid, since the namespace can't be a type *)
          Eat.token env; (* consume `type` *)
          error_unexpected env; (* unexpected `*` *)
          with_specifiers ImportType env
        | T_LCURLY ->
          Eat.token env; (* consume `type` *)
          with_specifiers ImportType env
        | _ ->
          Eat.token env; (* consume `type` *)
          with_default ImportType env
        end

  ` import typeof ... from " ModuleName " ; `
      | T_TYPEOF when should_parse_types env ->
        Expect.token env T_TYPEOF;
        begin match Peek.token env with
        | T_MULT
        | T_LCURLY -> with_specifiers ImportTypeof env
        | _ -> with_default ImportTypeof env
        end

  import  from " ModuleName " ;
      | _ ->
        with_default ImportValue env
    )
end

 If `async` is true, this must be a for-await-of loop. 
 This is a for of loop 
 This is a for in loop 
 This is a for loop 
*
     * Either the consequent or alternate of an if statement
     
 No catch or finally? That's an error! 
 This is identical to `interface`, except that mixins are allowed 
 TODO: This is a semantic analysis and shouldn't be in the parser 
 Oh boy, found some bad stuff in a declare module. Let's just
              * pretend it's a declare var (arbitrary choice) 
 Just make up a string for the error case 
 export default ... 
 export default [async] function [foo] (...) { ... } 
 export default class foo { ... } 
 export default [assignment expression]; 
 export type ... 
 export opaque type ... 
 export interface I { ... } 
 not using Peek.is_class here because it would guard all of the
      * cases 
 not using Peek.is_function here because it would guard all of the
      * cases 
 declare export default ... 
 declare export default function foo (...): ...  
 declare export default class foo { ... } 
 declare export default [type]; 
 declare export function foo (...): ...  
 declare export class foo { ... } 
 declare export var foo: ... 
 declare export * from 'foo' 
 declare export type = ... 
 declare export opaque type = ... 
 declare export interface ... 
 Just make up a string for the error case 
 `x` or `x as y` in a specifier 
 as 
 consume `type` or `typeof` 
 consume `type`, but we don't know yet whether this is `type foo` or
           `type as foo`. 
 `type` (a value) 
 `type` becomes a value 
 `type as foo` (value named `type`) or `type as,` (type named `as`) 
 `type as` 
 `type as as foo` 
 first `as` 
 second `as` 
 `foo` 
 `type as foo` 
 `type` becomes a value 
 `as` 
 `type x`, or `type x as y` 
 standard `x` or `x as y` 
 specifier in an `import type { ... }` 
 specifier in an `import typeof { ... }` 
 `import type *` is invalid, since the namespace can't be a type 
 consume `type` 
 unexpected `*` 
 consume `type` 
 consume `type` 
* 
  * Copyright ( c ) Facebook , Inc. and its affiliates . 
  * 
  * This source code is licensed under the MIT license found in the 
  * LICENSE file in the root directory of this source tree . 
 
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *)

module Ast = Flow_ast

open Token
open Parser_env
open Flow_ast
module Error = Parse_error
module SSet = Set.Make(String)

open Parser_common

module type STATEMENT = sig
 val for_: env -> (Loc.t, Loc.t) Statement.t
 val if_: env -> (Loc.t, Loc.t) Statement.t
 val let_: env -> (Loc.t, Loc.t) Statement.t
 val try_: env -> (Loc.t, Loc.t) Statement.t
 val while_: env -> (Loc.t, Loc.t) Statement.t
 val with_: env -> (Loc.t, Loc.t) Statement.t
 val block: env -> (Loc.t, Loc.t) Statement.t
 val break: env -> (Loc.t, Loc.t) Statement.t
 val continue: env -> (Loc.t, Loc.t) Statement.t
 val debugger: env -> (Loc.t, Loc.t) Statement.t
 val declare: ?in_module:bool -> env -> (Loc.t, Loc.t) Statement.t
 val declare_export_declaration: ?allow_export_type:bool -> env -> (Loc.t, Loc.t) Statement.t
 val declare_opaque_type : env -> (Loc.t, Loc.t) Statement.t
 val do_while: env -> (Loc.t, Loc.t) Statement.t
 val empty: env -> (Loc.t, Loc.t) Statement.t
 val export_declaration: decorators:(Loc.t, Loc.t) Class.Decorator.t list -> env -> (Loc.t, Loc.t) Statement.t
 val expression: env -> (Loc.t, Loc.t) Statement.t
 val import_declaration: env -> (Loc.t, Loc.t) Statement.t
 val interface: env -> (Loc.t, Loc.t) Statement.t
 val maybe_labeled: env -> (Loc.t, Loc.t) Statement.t
 val opaque_type : env -> (Loc.t, Loc.t) Statement.t
 val return: env -> (Loc.t, Loc.t) Statement.t
 val switch: env -> (Loc.t, Loc.t) Statement.t
 val throw: env -> (Loc.t, Loc.t) Statement.t
 val type_alias: env -> (Loc.t, Loc.t) Statement.t
 val var: env -> (Loc.t, Loc.t) Statement.t
 val const: env -> (Loc.t, Loc.t) Statement.t
end

module Statement
  (Parse: PARSER)
  (Type: Type_parser.TYPE)
  (Declaration: Declaration_parser.DECLARATION)
  (Object: Object_parser.OBJECT)
  (Pattern_cover : Pattern_cover.COVER)
: STATEMENT = struct
  type for_lhs =
    | For_expression of pattern_cover
    | For_declaration of (Loc.t * (Loc.t, Loc.t) Ast.Statement.VariableDeclaration.t)

  FunctionDeclaration is not a valid Statement , but Annex B sometimes allows it . 
      However , AsyncFunctionDeclaration and  are never 
      allowed as statements . We still parse them as statements ( and raise an error ) to 
      recover gracefully .
     However, AsyncFunctionDeclaration and GeneratorFunctionDeclaration are never
     allowed as statements. We still parse them as statements (and raise an error) to
     recover gracefully. *)
  let function_as_statement env =
    let func = Declaration._function env in
    if in_strict_mode env then
      function_as_statement_error_at env (fst func)
    else begin
      let open Ast.Statement in
      match func with
      | loc, FunctionDeclaration { Ast.Function.async = true; _ } ->
        error_at env (loc, Parse_error.AsyncFunctionAsStatement)
      | loc, FunctionDeclaration { Ast.Function.generator = true; _ } ->
        error_at env (loc, Parse_error.GeneratorFunctionAsStatement)
      | _ -> ()
    end;
    func

  let rec empty env =
    let loc = Peek.loc env in
    Expect.token env T_SEMICOLON;
    loc, Statement.Empty

  and break env =
    let loc, label = with_loc (fun env ->
      Expect.token env T_BREAK;
      let label =
        if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
        then None
        else begin
          let (_, name) as label =
            Parse.identifier env in
          if not (SSet.mem name (labels env))
          then error env (Error.UnknownLabel name);
          Some label
        end
      in
      Eat.semicolon env;
      label
    ) env in
    if label = None && not (in_loop env || in_switch env)
    then error_at env (loc, Error.IllegalBreak);
    loc, Statement.Break { Statement.Break.label }

  and continue env =
    let loc, label = with_loc (fun env ->
      Expect.token env T_CONTINUE;
      let label =
        if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
        then None
        else begin
          let (_, name) as label =
            Parse.identifier env in
          if not (SSet.mem name (labels env))
          then error env (Error.UnknownLabel name);
          Some label
        end in
      Eat.semicolon env;
      label
    ) env in
    if not (in_loop env) then error_at env (loc, Error.IllegalContinue);
    loc, Statement.Continue { Statement.Continue.label }

  and debugger = with_loc (fun env ->
    Expect.token env T_DEBUGGER;
    Eat.semicolon env;
    Statement.Debugger
  )

  and do_while = with_loc (fun env ->
    Expect.token env T_DO;
    let body = Parse.statement (env |> with_in_loop true) in

  Annex B allows labelled FunctionDeclarations ( see 
        sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    if not (in_strict_mode env) && is_labelled_function body
    then function_as_statement_error_at env (fst body);

    Expect.token env T_WHILE;
    Expect.token env T_LPAREN;
    let test = Parse.expression env in
    Expect.token env T_RPAREN;
  The rules of automatic semicolon insertion in ES5 do n't mention this , 
      * but the semicolon after a do - while loop is optional . This is properly 
      * specified in ES6
     * but the semicolon after a do-while loop is optional. This is properly
     * specified in ES6 *)
    if Peek.token env = T_SEMICOLON
    then Eat.semicolon env;
    Statement.DoWhile { Statement.DoWhile.
      body;
      test;
    }
  )

  and for_ =
    let assert_can_be_forin_or_forof env err = function
      | loc, { Statement.VariableDeclaration.declarations; _; } ->
  Only a single declarator is allowed , without an init . So 
            * something like 
            * 
            * for ( var x in y ) { } 
            * 
            * is allowed , but we disallow 
            * 
            * for ( var x , y in z ) { } 
            * for ( var x = 42 in y ) { } 
           
           * something like
           *
           * for (var x in y) {}
           *
           * is allowed, but we disallow
           *
           * for (var x, y in z) {}
           * for (var x = 42 in y) {}
           *)
          (match declarations with
          | [ (_, { Statement.VariableDeclaration.Declarator.init = None; _; }) ] -> ()
          | _ -> error_at env (loc, err))
    in

  Annex B allows labelled FunctionDeclarations ( see 
        sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    let assert_not_labelled_function env body =
      if not (in_strict_mode env) && is_labelled_function body
      then function_as_statement_error_at env (fst body)
      else ()
    in

    with_loc (fun env ->
      Expect.token env T_FOR;
      let async = allow_await env && Expect.maybe env T_AWAIT in
      Expect.token env T_LPAREN;

      let init, errs =
        let env = env |> with_no_in true in
        match Peek.token env with
        | T_SEMICOLON -> None, []
        | T_LET ->
            let loc, (decl, errs) = with_loc Declaration.let_ env in
            Some (For_declaration (loc, decl)), errs
        | T_CONST ->
            let loc, (decl, errs) = with_loc Declaration.const env in
            Some (For_declaration (loc, decl)), errs
        | T_VAR ->
            let loc, (decl, errs) = with_loc Declaration.var env in
            Some (For_declaration (loc, decl)), errs
        | _ ->
            let expr = Parse.expression_or_pattern (env |> with_no_let true) in
            Some (For_expression expr), []
      in

      match Peek.token env with
      | t when t = T_OF || async ->
          let left = Statement.(match init with
          | Some (For_declaration decl) ->
            assert_can_be_forin_or_forof env Error.InvalidLHSInForOf decl;
            ForOf.LeftDeclaration decl
          | Some (For_expression expr) ->
  # sec - for - in - and - for - of - statements - static - semantics - early - errors
            let patt = Pattern_cover.as_pattern ~err:Error.InvalidLHSInForOf env expr in
            ForOf.LeftPattern patt
          | None -> assert false) in
          Expect.token env T_OF;
          let right = Parse.assignment env in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.ForOf { Statement.ForOf.
            left;
            right;
            body;
            async;
          }
      | T_IN ->
          let left = match init with
          | Some (For_declaration decl) ->
            assert_can_be_forin_or_forof env Error.InvalidLHSInForIn decl;
            Statement.ForIn.LeftDeclaration decl
          | Some (For_expression expr) ->
  # sec - for - in - and - for - of - statements - static - semantics - early - errors
            let patt = Pattern_cover.as_pattern ~err:Error.InvalidLHSInForIn env expr in
            Statement.ForIn.LeftPattern patt
          | None -> assert false in
          Expect.token env T_IN;
          let right = Parse.expression env in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.ForIn { Statement.ForIn.
            left;
            right;
            body;
            each = false;
          }
      | _ ->
          errs |> List.iter (error_at env);
          Expect.token env T_SEMICOLON;
          let init = match init with
          | Some (For_declaration decl) -> Some (Statement.For.InitDeclaration decl)
          | Some (For_expression expr) ->
            Some (Statement.For.InitExpression (Pattern_cover.as_expression env expr))
          | None -> None in
          let test = match Peek.token env with
          | T_SEMICOLON -> None
          | _ -> Some (Parse.expression env) in
          Expect.token env T_SEMICOLON;
          let update = match Peek.token env with
          | T_RPAREN -> None
          | _ -> Some (Parse.expression env) in
          Expect.token env T_RPAREN;
          let body = Parse.statement (env |> with_in_loop true) in
          assert_not_labelled_function env body;
          Statement.For { Statement.For.
            init;
            test;
            update;
            body;
          }
    )

  and if_ =
    let if_branch env =
  Normally this would just be a Statement , but Annex B allows 
          FunctionDeclarations in non - strict mode . See 
          sec - functiondeclarations - in - ifstatement - statement - clauses
         FunctionDeclarations in non-strict mode. See
         sec-functiondeclarations-in-ifstatement-statement-clauses *)
      let stmt =
        if Peek.is_function env then
          function_as_statement env
        else
          Parse.statement env
      in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
          ( see sec - labelled - function - declarations ) , but not in IfStatement 
          ( see sec - if - statement - static - semantics - early - errors ) .
         (see sec-labelled-function-declarations), but not in IfStatement
         (see sec-if-statement-static-semantics-early-errors). *)
      if not (in_strict_mode env) && is_labelled_function stmt
      then function_as_statement_error_at env (fst stmt);

      stmt
    in

    with_loc (fun env ->
      Expect.token env T_IF;
      Expect.token env T_LPAREN;
      let test = Parse.expression env in
      Expect.token env T_RPAREN;
      let consequent = if_branch env in
      let alternate = if Peek.token env = T_ELSE
      then begin
        Expect.token env T_ELSE;
        Some (if_branch env)
      end else None in
      Statement.If { Statement.If.
        test;
        consequent;
        alternate;
      }
    )

  and return = with_loc (fun env ->
    if not (in_function env)
    then error env Error.IllegalReturn;
    Expect.token env T_RETURN;
    let argument =
      if Peek.token env = T_SEMICOLON || Peek.is_implicit_semicolon env
      then None
      else Some (Parse.expression env) in
    Eat.semicolon env;
    Statement.Return { Statement.Return.
      argument;
    }
  )

  and switch =
    let rec case_list env (seen_default, acc) =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> List.rev acc
      | _ ->
        let start_loc = Peek.loc env in
        let test = match Peek.token env with
        | T_DEFAULT ->
            if seen_default
            then error env Error.MultipleDefaultsInSwitch;
            Expect.token env T_DEFAULT; None
        | _ ->
            Expect.token env T_CASE;
            Some (Parse.expression env) in
        let seen_default = seen_default || test = None in
        let end_loc = Peek.loc env in
        Expect.token env T_COLON;
        let term_fn = function
        | T_RCURLY | T_DEFAULT | T_CASE -> true
        | _ -> false in
        let consequent =
          Parse.statement_list ~term_fn (env |> with_in_switch true) in
        let end_loc = match List.rev consequent with
        | last_stmt::_ -> fst last_stmt
        | _ -> end_loc in
        let acc = (Loc.btwn start_loc end_loc, Statement.Switch.Case.({
          test;
          consequent;
        }))::acc in
        case_list env (seen_default, acc)

    in with_loc (fun env ->
      Expect.token env T_SWITCH;
      Expect.token env T_LPAREN;
      let discriminant = Parse.expression env in
      Expect.token env T_RPAREN;
      Expect.token env T_LCURLY;
      let cases = case_list env (false, []) in
      Expect.token env T_RCURLY;
      Statement.Switch { Statement.Switch.
        discriminant;
        cases;
      }
    )

  and throw = with_loc (fun env ->
    let start_loc = Peek.loc env in
    Expect.token env T_THROW;
    if Peek.is_line_terminator env
    then error_at env (start_loc, Error.NewlineAfterThrow);
    let argument = Parse.expression env in
    Eat.semicolon env;
    Statement.(Throw { Throw.argument; })
  )

  and try_ = with_loc (fun env ->
    Expect.token env T_TRY;
    let block = Parse.block_body env in
    let handler = match Peek.token env with
    | T_CATCH ->
        let catch = with_loc (fun env ->
          Expect.token env T_CATCH;
          let param = if Peek.token env = T_LPAREN
          then begin
            Expect.token env T_LPAREN;
            let p = Some (Parse.pattern env Error.StrictCatchVariable) in
            Expect.token env T_RPAREN;
            p
          end else
            None
          in
          let body = Parse.block_body env in
          { Ast.Statement.Try.CatchClause.
            param;
            body;
          }
        ) env in
        Some catch
    | _ -> None in
    let finalizer = match Peek.token env with
    | T_FINALLY ->
        Expect.token env T_FINALLY;
        Some (Parse.block_body env)
    | _ -> None in

    if handler = None && finalizer = None then
      error_at env (fst block, Error.NoCatchOrFinally);

    Statement.Try { Statement.Try.
      block;
      handler;
      finalizer;
    }
  )

  and var = with_loc (fun env ->
    let declaration, errs = Declaration.var env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and const = with_loc (fun env ->
    let declaration, errs = Declaration.const env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and let_ = with_loc (fun env ->
    let declaration, errs = Declaration.let_ env in
    Eat.semicolon env;
    errs |> List.iter (error_at env);
    Statement.VariableDeclaration declaration
  )

  and while_ = with_loc (fun env ->
    Expect.token env T_WHILE;
    Expect.token env T_LPAREN;
    let test = Parse.expression env in
    Expect.token env T_RPAREN;
    let body = Parse.statement (env |> with_in_loop true) in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
        ( see sec - labelled - function - declarations ) , but not in IterationStatement 
        ( see sec - semantics - static - semantics - early - errors ) .
       (see sec-labelled-function-declarations), but not in IterationStatement
       (see sec-semantics-static-semantics-early-errors). *)
    if not (in_strict_mode env) && is_labelled_function body
    then function_as_statement_error_at env (fst body);

    Statement.While { Statement.While.
      test;
      body;
    }
  )

  and with_ env =
    let loc, stmt = with_loc (fun env ->
      Expect.token env T_WITH;
      Expect.token env T_LPAREN;
      let _object = Parse.expression env in
      Expect.token env T_RPAREN;
      let body = Parse.statement env in

  Annex B allows labelled FunctionDeclarations in non - strict mode 
          ( see sec - labelled - function - declarations ) , but not in WithStatement 
          ( see sec - with - statement - static - semantics - early - errors ) .
         (see sec-labelled-function-declarations), but not in WithStatement
         (see sec-with-statement-static-semantics-early-errors). *)
      if not (in_strict_mode env) && is_labelled_function body
      then function_as_statement_error_at env (fst body);

      Statement.With { Statement.With.
        _object;
        body;
      }
    ) env in
    strict_error_at env (loc, Error.StrictModeWith);
    loc, stmt

  and block env =
    let loc, block = Parse.block_body env in
    loc, Statement.Block block

  and maybe_labeled = with_loc (fun env ->
    match (Parse.expression env, Peek.token env) with
    | ((loc, Ast.Expression.Identifier label), T_COLON) ->
        let _, name = label in
        Expect.token env T_COLON;
        if SSet.mem name (labels env)
        then error_at env (loc, Error.Redeclaration ("Label", name));
        let env = add_label env name in
        let body =
  labelled FunctionDeclarations are allowed in non - strict mode 
              ( see # sec - labelled - function - declarations )
             (see #sec-labelled-function-declarations) *)
          if Peek.is_function env then function_as_statement env
          else Parse.statement env
        in
        Statement.Labeled { Statement.Labeled.label; body; }
    | expression, _ ->
        Eat.semicolon env;
        Statement.(Expression { Expression.expression; directive = None; })
  )

  and expression = with_loc (fun env ->
    let expression = Parse.expression env in
    Eat.semicolon env;
    let directive = if allow_directive env
      then match expression with
      | _, Ast.Expression.Literal { Ast.Literal.
          value = Ast.Literal.String _;
          raw;
          _
        } -> Some (String.sub raw 1 (String.length raw - 2))
      | _ -> None
      else None
    in
    Statement.Expression { Statement.Expression.
      expression;
      directive;
    }
  )

  and type_alias_helper env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeAlias;
    Expect.token env T_TYPE;
    Eat.push_lex_mode env Lex_mode.TYPE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    Expect.token env T_ASSIGN;
    let right = Type._type env in
    Eat.semicolon env;
    Eat.pop_lex_mode env;
    Statement.TypeAlias.({
      id;
      tparams;
      right;
    })

  and declare_type_alias env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let type_alias = type_alias_helper env in
    Statement.DeclareTypeAlias type_alias
  ) env

  and type_alias env =
    if Peek.ith_is_identifier ~i:1 env
    then
      let loc, type_alias = with_loc type_alias_helper env in
      loc, Statement.TypeAlias type_alias
    else
      Parse.statement env

  and opaque_type_helper ?(declare=false) env =
    if not (should_parse_types env)
    then error env Error.UnexpectedOpaqueTypeAlias;
    Expect.token env T_OPAQUE;
    Expect.token env T_TYPE;
    Eat.push_lex_mode env Lex_mode.TYPE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    let supertype = match Peek.token env with
    | T_COLON ->
        Expect.token env T_COLON;
        Some (Type._type env)
    | _ -> None in
    let impltype =
      if not declare then
        (Expect.token env T_ASSIGN;
        Some (Type._type env))
      else None in
    Eat.semicolon env;
    Eat.pop_lex_mode env;
    Statement.OpaqueType.({
      id;
      tparams;
      impltype;
      supertype;
    })

  and declare_opaque_type env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let opaque_t = opaque_type_helper ~declare:true env in
    Statement.DeclareOpaqueType opaque_t
  ) env

  and opaque_type env =
    match Peek.ith_token ~i:1 env with
      T_TYPE ->
        let loc, opaque_t = with_loc (opaque_type_helper ~declare:false) env in
        loc, Statement.OpaqueType opaque_t
    | _ -> Parse.statement env

  and interface_helper env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeInterface;
    Expect.token env T_INTERFACE;
    let id = Type.type_identifier env in
    let tparams = Type.type_parameter_declaration_with_defaults env in
    let { Ast.Type.Interface.extends; body } = Type.interface_helper env in
    Statement.Interface.({
      id;
      tparams;
      body;
      extends;
    })

  and declare_interface env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let iface = interface_helper env in
    Statement.DeclareInterface iface
  ) env

  and interface env =
  disambiguate between a value named ` interface ` , like ` var interface = 1 ; interface++ ` , 
        and an interface declaration like ` interface  { } ` . `
       and an interface declaration like `interface Foo {}`.` *)
    if Peek.ith_is_identifier_name ~i:1 env
    then
      let loc, iface = with_loc interface_helper env in
      loc, Statement.InterfaceDeclaration iface
    else expression env

  and declare_class =
    let rec mixins env acc =
      let super = Type.generic env in
      let acc = super::acc in
      match Peek.token env with
      | T_COMMA ->
        Expect.token env T_COMMA;
        mixins env acc
      | _ -> List.rev acc

    in fun env ->
      let env = env |> with_strict true in
      Expect.token env T_CLASS;
      let id = Parse.identifier env in
      let tparams = Type.type_parameter_declaration_with_defaults env in
      let extends = if Expect.maybe env T_EXTENDS then Some (Type.generic env) else None in
      let mixins = match Peek.token env with
      | T_IDENTIFIER { raw = "mixins"; _ } -> Eat.token env; mixins env []
      | _ -> []
      in
      let implements = match Peek.token env with
      | T_IMPLEMENTS -> Eat.token env; Object.class_implements env []
      | _ -> []
      in
      let body = Type._object ~is_class:true env in
      Statement.DeclareClass.({
        id;
        tparams;
        body;
        extends;
        mixins;
        implements;
      })

  and declare_class_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let fn = declare_class env in
    Statement.DeclareClass fn
  ) env

  and declare_function env =
    Expect.token env T_FUNCTION;
    let id = Parse.identifier env in
    let start_sig_loc = Peek.loc env in
    let tparams = Type.type_parameter_declaration env in
    let params = Type.function_param_list env in
    Expect.token env T_COLON;
    let return = Type._type env in
    let end_loc = fst return in
    let loc = Loc.btwn start_sig_loc end_loc in
    let annot = loc, Ast.Type.(Function {Function.
      params;
      return;
      tparams;
    }) in
    let annot = fst annot, annot in
    let predicate = Type.predicate_opt env in
    Eat.semicolon env;
    Statement.DeclareFunction.({
      id;
      annot;
      predicate;
    })

  and declare_function_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    begin match Peek.token env with
    | T_ASYNC ->
      error env Error.DeclareAsync;
      Expect.token env T_ASYNC
    | _ -> ()
    end;
    let fn = declare_function env in
    Statement.DeclareFunction fn
  ) env

  and declare_var env =
    Expect.token env T_VAR;
    let _loc, { Pattern.Identifier.name; annot; _; } =
      Parse.identifier_with_type env ~no_optional:true Error.StrictVarName in
    Eat.semicolon env;
    Statement.DeclareVariable.({ id=name; annot; })

  and declare_var_statement env = with_loc (fun env ->
    Expect.token env T_DECLARE;
    let var = declare_var env in
    Statement.DeclareVariable var
  ) env

  and declare_module =
    let rec module_items env ~module_kind acc =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> (module_kind, List.rev acc)
      | _ ->
        let stmt = declare ~in_module:true env in
        let module_kind = Statement.(
          let (loc, stmt) = stmt in
          match (module_kind, stmt) with
* 
            * The first time we see either a ` declare export ` or a 
            * ` declare module.exports ` , we lock in the kind of the module . 
            * 
            * ` declare export type ` and ` declare export interface ` are the two 
            * exceptions to this rule because they are valid in both CommonJS 
            * and ES modules ( and thus do not indicate an intent for either ) . 
           
           * The first time we see either a `declare export` or a
           * `declare module.exports`, we lock in the kind of the module.
           *
           * `declare export type` and `declare export interface` are the two
           * exceptions to this rule because they are valid in both CommonJS
           * and ES modules (and thus do not indicate an intent for either).
           *)
          | None, DeclareModuleExports _ -> Some (DeclareModule.CommonJS loc)
          | None, DeclareExportDeclaration {
              DeclareExportDeclaration.declaration;
              _;
            } ->
            (match declaration with
              | Some (DeclareExportDeclaration.NamedType _)
              | Some (DeclareExportDeclaration.Interface _)
                -> module_kind
              | _ -> Some (DeclareModule.ES loc)
            )

* 
            * There should never be more than one ` declare module.exports ` 
            * statement
           * There should never be more than one `declare module.exports`
           * statement *)
          | Some (DeclareModule.CommonJS _), DeclareModuleExports _ ->
            error env Parse_error.DuplicateDeclareModuleExports;
            module_kind

* 
            * It 's never ok to mix and match ` declare export ` and 
            * ` declare module.exports ` in the same module because it leaves the 
            * kind of the module ( CommonJS vs ES ) ambiguous . 
            * 
            * The 1 exception to this rule is that ` export type / interface ` are 
            * both ok in CommonJS modules . 
           
           * It's never ok to mix and match `declare export` and
           * `declare module.exports` in the same module because it leaves the
           * kind of the module (CommonJS vs ES) ambiguous.
           *
           * The 1 exception to this rule is that `export type/interface` are
           * both ok in CommonJS modules.
           *)
          | Some (DeclareModule.ES _), DeclareModuleExports _ ->
            error env Parse_error.AmbiguousDeclareModuleKind;
            module_kind
          | Some (DeclareModule.CommonJS _), DeclareExportDeclaration {
              DeclareExportDeclaration.declaration;
              _;
            } ->
              (match declaration with
                | Some (DeclareExportDeclaration.NamedType _)
                | Some (DeclareExportDeclaration.Interface _)
                  -> ()
                | _ -> error env Parse_error.AmbiguousDeclareModuleKind
              );
              module_kind

          | _ -> module_kind
        ) in
        module_items env ~module_kind (stmt::acc)
    in

    let declare_module_ env start_loc =
      let id = match Peek.token env with
      | T_STRING (loc, value, raw, octal) ->
          if octal then strict_error env Error.StrictOctalLiteral;
          Expect.token env (T_STRING (loc, value, raw, octal));
          Statement.DeclareModule.Literal (loc, { StringLiteral.value; raw; })
      | _ ->
          Statement.DeclareModule.Identifier (Parse.identifier env) in
      let body_loc, (module_kind, body) = with_loc (fun env ->
        Expect.token env T_LCURLY;
        let res = module_items env ~module_kind:None [] in
        Expect.token env T_RCURLY;
        res
      ) env in
      let body = body_loc, { Statement.Block.body; } in
      let loc = Loc.btwn start_loc body_loc in
      let kind =
        match module_kind with
        | Some k -> k
        | None -> Statement.DeclareModule.CommonJS loc
      in
      loc,
      Statement.(DeclareModule DeclareModule.({ id; body; kind; }))
    in
    fun ?(in_module=false) env ->
      let start_loc = Peek.loc env in
      Expect.token env T_DECLARE;
      Expect.identifier env "module";
      if in_module || Peek.token env = T_PERIOD
      then
        let loc, exports = with_loc declare_module_exports env in
        Loc.btwn start_loc loc, exports
      else
        declare_module_ env start_loc

  and declare_module_exports env =
    Expect.token env T_PERIOD;
    Expect.identifier env "exports";
    let type_annot = Type.annotation env in
    Eat.semicolon env;
    Statement.DeclareModuleExports type_annot

  and declare ?(in_module=false) env =
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeDeclaration;
  eventually , just emit a wrapper AST node
    (match Peek.ith_token ~i:1 env with
      | T_CLASS ->
          declare_class_statement env
      | T_INTERFACE ->
          declare_interface env
      | T_TYPE -> (
          match Peek.token env with
          | T_IMPORT when in_module ->
            import_declaration env
          | _ ->
            declare_type_alias env
        )
      | T_OPAQUE ->
          declare_opaque_type env
      | T_TYPEOF when (Peek.token env) = T_IMPORT ->
          import_declaration env
      | T_FUNCTION
      | T_ASYNC ->
          declare_function_statement env
      | T_VAR ->
          declare_var_statement env
      | T_EXPORT when in_module ->
          declare_export_declaration ~allow_export_type:in_module env
      | T_IDENTIFIER { raw = "module"; _ } ->
          declare_module ~in_module env
      | _ when in_module -> (
          match Peek.token env with
          | T_IMPORT ->
            error env Error.InvalidNonTypeImportInDeclareModule;
            Parse.statement env
          | _ ->
            declare_var_statement env
        )
      | _ ->
          Parse.statement env
    )

  and export_source env =
    Expect.identifier env "from";
    match Peek.token env with
    | T_STRING (loc, value, raw, octal) ->
        if octal then strict_error env Error.StrictOctalLiteral;
        Expect.token env (T_STRING (loc, value, raw, octal));
        loc, { StringLiteral.value; raw; }
    | _ ->
        let ret = Peek.loc env, { StringLiteral.value = ""; raw = ""; } in
        error_unexpected env;
        ret

  and extract_pattern_binding_names =
    let rec fold acc = Pattern.(function
      | (_, Object {Object.properties; _;}) ->
        List.fold_left (fun acc prop ->
          match prop with
          | Object.Property (_, {Object.Property.pattern; _;})
          | Object.RestProperty (_, {Object.RestProperty.argument = pattern;})
            -> fold acc pattern
        ) acc properties
      | (_, Array {Array.elements; _;}) ->
        List.fold_left Array.(fun acc elem ->
          match elem with
          | Some (Element (_, {Element.argument = pattern; default = _}))
          | Some (RestElement (_, {RestElement.argument = pattern;}))
            -> fold acc pattern
          | None -> acc
        ) acc elements
      | (_, Identifier {Pattern.Identifier.name; _; }) ->
        name::acc
      | (_, Expression _) ->
        failwith "Parser error: No such thing as an expression pattern!"
    ) in
    List.fold_left fold

  and extract_ident_name (_, name) = name

  and export_specifiers ?(preceding_comma=true) env specifiers =
    match Peek.token env with
    | T_EOF
    | T_RCURLY ->
        List.rev specifiers
    | _ ->
        if not preceding_comma then error env Error.ExportSpecifierMissingComma;
        let specifier = with_loc (fun env ->
          let local = identifier_name env in
          let exported =
            match Peek.token env with
            | T_IDENTIFIER { raw = "as"; _ } ->
              Eat.token env;
              let exported = identifier_name env in
              record_export env exported;
              Some exported
            | _ ->
              record_export env local;
              None
          in
          { Statement.ExportNamedDeclaration.ExportSpecifier.local; exported; }
        ) env in
        let preceding_comma = Expect.maybe env T_COMMA in
        export_specifiers ~preceding_comma env (specifier::specifiers)

  and assert_export_specifier_identifiers env specifiers =
    let open Statement.ExportNamedDeclaration.ExportSpecifier in
    List.iter (function
      | _, { local = id; exported = None; } ->
        Parse.assert_identifier_name_is_identifier
          ~restricted_error:Parse_error.StrictVarName env id
      | _ ->
        ()
    ) specifiers

  and export_declaration ~decorators = with_loc (fun env ->
    let env = env |> with_strict true |> with_in_export true in
    let start_loc = Peek.loc env in
    Expect.token env T_EXPORT;
    match Peek.token env with
    | T_DEFAULT ->
        let open Statement.ExportDefaultDeclaration in
        let default, () = with_loc (fun env ->
          Expect.token env T_DEFAULT
        ) env in
        record_export env (Loc.btwn start_loc (Peek.loc env), "default");
        let declaration =
          if Peek.is_function env then
            let fn = Declaration._function env in
            Declaration fn
          else if Peek.is_class env then
            let _class = Object.class_declaration env decorators in
            Declaration _class
          else
            let expr = Parse.assignment env in
            Eat.semicolon env;
            Expression expr
        in
        Statement.ExportDefaultDeclaration {
          default;
          declaration;
        }
    | T_TYPE when (Peek.ith_token ~i:1 env) <> T_LCURLY ->
        let open Statement.ExportNamedDeclaration in
        if not (should_parse_types env)
        then error env Error.UnexpectedTypeExport;
        (match Peek.ith_token ~i:1 env with
        | T_MULT ->
          Expect.token env T_TYPE;
          let specifier_loc = Peek.loc env in
          Expect.token env T_MULT;
          let source = export_source env in
          Eat.semicolon env;
          Statement.ExportNamedDeclaration {
            declaration = None;
            specifiers = Some (ExportBatchSpecifier (specifier_loc, None));
            source = Some source;
            exportKind = Statement.ExportType;
          }
        | _ ->
          let loc, type_alias = with_loc type_alias_helper env in
          record_export env (loc, extract_ident_name type_alias.Statement.TypeAlias.id);
          let type_alias = (loc, Statement.TypeAlias type_alias) in
          Statement.ExportNamedDeclaration {
            declaration = Some type_alias;
            specifiers = None;
            source = None;
            exportKind = Statement.ExportType;
          }
        )
    | T_OPAQUE ->
        let open Statement.ExportNamedDeclaration in
        let loc, opaque_t = with_loc opaque_type_helper env in
        record_export env (loc, extract_ident_name opaque_t.Statement.OpaqueType.id);
        let opaque_t = (loc, Statement.OpaqueType opaque_t) in
        Statement.ExportNamedDeclaration {
          declaration = Some opaque_t;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportType;
        }
    | T_INTERFACE ->
        let open Statement.ExportNamedDeclaration in
        if not (should_parse_types env)
        then error env Error.UnexpectedTypeExport;
        let interface = interface env in
        (match interface with
          | (loc, Statement.InterfaceDeclaration {Statement.Interface.id; _;}) ->
            record_export env (loc, extract_ident_name id)
          | _ -> failwith (
              "Internal Flow Error! Parsed `export interface` into something " ^
              "other than an interface declaration!"
            )
        );
        Statement.ExportNamedDeclaration {
          declaration = Some interface;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportType;
        }
    | T_LET
    | T_CONST
    | T_VAR
    | T_AT
    | T_CLASS
    | T_ASYNC
    | T_FUNCTION ->
        let open Statement.ExportNamedDeclaration in
        let stmt = Parse.statement_list_item env ~decorators:decorators in
        let names = Statement.(
          match stmt with
          | (_, VariableDeclaration { VariableDeclaration.declarations; _; }) ->
            List.fold_left (fun names (_, declaration) ->
              let id = declaration.VariableDeclaration.Declarator.id in
              extract_pattern_binding_names names [id]
            ) [] declarations
          | (loc, ClassDeclaration { Class.id = Some id; _; })
          | (loc, FunctionDeclaration { Function.id = Some id; _; })
            -> [(loc, extract_ident_name id)]
          | (loc, ClassDeclaration { Class.id = None; _; }) ->
            error_at env (loc, Error.ExportNamelessClass);
            []
          | (loc, FunctionDeclaration { Function.id = None; _; }) ->
            error_at env (loc, Error.ExportNamelessFunction);
            []
          | _ -> failwith "Internal Flow Error! Unexpected export statement declaration!"
        ) in
        List.iter (record_export env) names;
        Statement.ExportNamedDeclaration {
          declaration = Some stmt;
          specifiers = None;
          source = None;
          exportKind = Statement.ExportValue;
        }
    | T_MULT ->
        let open Statement.ExportNamedDeclaration in
        let loc = Peek.loc env in
        Expect.token env T_MULT;
        let local_name =
          let parse_export_star_as =
            (parse_options env).esproposal_export_star_as
          in
          match Peek.token env with
          | T_IDENTIFIER { raw = "as"; _ } ->
            Eat.token env;
            if parse_export_star_as
            then Some (Parse.identifier env)
            else (error env Error.UnexpectedTypeDeclaration; None)
          | _ ->
            None
        in
        let specifiers =
          Some (ExportBatchSpecifier (loc, local_name))
        in
        let source = export_source env in
        let source = Some source in
        Eat.semicolon env;
        Statement.ExportNamedDeclaration {
          declaration = None;
          specifiers;
          source;
          exportKind = Statement.ExportValue;
        }
    | _ ->
        let open Statement.ExportNamedDeclaration in
        let exportKind = (
          match Peek.token env with
          | T_TYPE -> Eat.token env; Statement.ExportType
          | _ -> Statement.ExportValue
        ) in
        Expect.token env T_LCURLY;
        let specifiers = export_specifiers env [] in
        Expect.token env T_RCURLY;
        let source =
          match Peek.token env with
          | T_IDENTIFIER { raw = "from"; _ } ->
            Some (export_source env)
          | _ ->
            assert_export_specifier_identifiers env specifiers;
            None
        in
        Eat.semicolon env;
        Statement.ExportNamedDeclaration {
          declaration = None;
          specifiers = Some (ExportSpecifiers specifiers);
          source;
          exportKind;
        }
  )

  and declare_export_declaration ?(allow_export_type=false) = with_loc (fun env ->
    if not (should_parse_types env)
    then error env Error.UnexpectedTypeDeclaration;
    Expect.token env T_DECLARE;

    let env = env |> with_strict true |> with_in_export true in
    Expect.token env T_EXPORT;
    Statement.DeclareExportDeclaration.(match Peek.token env with
    | T_DEFAULT ->
        let default, () = with_loc (fun env ->
          Expect.token env T_DEFAULT
        ) env in
        let declaration = match Peek.token env with
        | T_FUNCTION ->
            let fn = with_loc declare_function env in
            Some (Function fn)
        | T_CLASS ->
            let class_ = with_loc declare_class env in
            Some (Class class_)
        | _ ->
            let type_ = Type._type env in
            Eat.semicolon env;
            Some (DefaultType type_)
          in
        Statement.DeclareExportDeclaration {
          default = Some default;
          declaration;
          specifiers = None;
          source = None;
        }
    | T_LET
    | T_CONST
    | T_VAR
    | T_CLASS
    | T_FUNCTION ->
        let declaration = match Peek.token env with
        | T_FUNCTION ->
            let fn = with_loc declare_function env in
            Some (Function fn)
        | T_CLASS ->
            let class_ = with_loc declare_class env in
            Some (Class class_)
        | T_LET
        | T_CONST
        | T_VAR as token ->
            (match token with
            | T_LET -> error env Error.DeclareExportLet
            | T_CONST -> error env Error.DeclareExportConst
            | _ -> ());
            let var = with_loc declare_var env in
            Some (Variable var)
        | _ -> assert false in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration;
          specifiers = None;
          source = None;
        }
    | T_MULT ->
        let loc = Peek.loc env in
        Expect.token env T_MULT;
        let parse_export_star_as =
          (parse_options env).esproposal_export_star_as
        in
        let local_name =
          match Peek.token env with
          | T_IDENTIFIER { raw = "as"; _ } ->
            Eat.token env;
            if parse_export_star_as
            then Some (Parse.identifier env)
            else (error env Error.UnexpectedTypeDeclaration; None)
          | _ ->
            None
        in
        let specifiers = Statement.ExportNamedDeclaration.(
          Some (ExportBatchSpecifier (loc, local_name))
        ) in
        let source = export_source env in
        Eat.semicolon env;
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = None;
          specifiers;
          source = Some source;
        }
    | T_TYPE when allow_export_type ->
        let alias = with_loc type_alias_helper env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (NamedType alias);
          specifiers = None;
          source = None;
        }
    | T_OPAQUE ->
        let opaque = with_loc (opaque_type_helper ~declare:true) env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (NamedOpaqueType opaque);
          specifiers = None;
          source = None;
        }
    | T_INTERFACE when allow_export_type ->
        let iface = with_loc interface_helper env in
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = Some (Interface iface);
          specifiers = None;
          source = None;
        }
    | _ ->
        (match Peek.token env with
          | T_TYPE -> error env Error.DeclareExportType
          | T_INTERFACE -> error env Error.DeclareExportInterface
          | _ -> ()
        );
        Expect.token env T_LCURLY;
        let specifiers = export_specifiers env [] in
        Expect.token env T_RCURLY;
        let source =
          match Peek.token env with
          | T_IDENTIFIER { raw = "from"; _ } ->
            Some (export_source env)
          | _ ->
            assert_export_specifier_identifiers env specifiers;
            None
        in
        Eat.semicolon env;
        Statement.DeclareExportDeclaration {
          default = None;
          declaration = None;
          specifiers = Some (Statement.ExportNamedDeclaration.ExportSpecifiers specifiers);
          source;
        }
    )
  )

  and import_declaration =
    let open Statement.ImportDeclaration in

    let source env =
      Expect.identifier env "from";
      match Peek.token env with
      | T_STRING (loc, value, raw, octal) ->
          if octal then strict_error env Error.StrictOctalLiteral;
          Expect.token env (T_STRING (loc, value, raw, octal));
          loc, { StringLiteral.value; raw; }
      | _ ->
          let ret = Peek.loc env, { StringLiteral.value = ""; raw = ""; } in
          error_unexpected env;
          ret

    in let is_type_import = function
      | T_TYPE
      | T_TYPEOF -> true
      | _ -> false

    in let with_maybe_as ~for_type ?error_if_type env =
      let identifier env =
        if for_type then Type.type_identifier env else Parse.identifier env
      in
      match Peek.ith_token ~i:1 env with
      | T_IDENTIFIER { raw = "as"; _ } ->
        let remote = identifier_name env in
        let local = Some (identifier env) in
        remote, local
      | T_EOF
      | T_COMMA
      | T_RCURLY ->
        identifier env, None
      | _ ->
        begin match error_if_type, Peek.token env with
        | Some error_if_type, T_TYPE
        | Some error_if_type, T_TYPEOF ->
          error env error_if_type;
          Type.type_identifier env, None
        | _ ->
          identifier env, None
        end


       ImportSpecifier[Type ] : 
         [ ~Type ] ImportedBinding 
         [ ~Type ] IdentifierName ImportedTypeBinding 
         [ ~Type ] IdentifierName IdentifierName ImportedBinding 
         [ ~Type ] IdentifierName IdentifierName IdentifierName ImportedTypeBinding 
         [ + Type ] ImportedTypeBinding 
         [ + Type ] IdentifierName IdentifierName ImportedTypeBinding 

       Static Semantics : 

       ` IdentifierName ImportedTypeBinding ` : 
       - It is a Syntax Error if IdentifierName 's StringValue is not " type " or " typeof " 

       ` IdentifierName IdentifierName   : 
       - It is a Syntax Error if the second IdentifierName 's StringValue is not " as " 

       ` IdentifierName IdentifierName IdentifierName   ImportedTypeBinding ` : 
       - It is a Syntax Error if the first IdentifierName 's StringValue is not " type " 
         or " typeof " , and the third IdentifierName 's StringValue is not " as " 
    
      ImportSpecifier[Type]:
        [~Type] ImportedBinding
        [~Type] IdentifierName ImportedTypeBinding
        [~Type] IdentifierName IdentifierName ImportedBinding
        [~Type] IdentifierName IdentifierName IdentifierName ImportedTypeBinding
        [+Type] ImportedTypeBinding
        [+Type] IdentifierName IdentifierName ImportedTypeBinding

      Static Semantics:

      `IdentifierName ImportedTypeBinding`:
      - It is a Syntax Error if IdentifierName's StringValue is not "type" or "typeof"

      `IdentifierName IdentifierName ImportedBinding`:
      - It is a Syntax Error if the second IdentifierName's StringValue is not "as"

      `IdentifierName IdentifierName IdentifierName  ImportedTypeBinding`:
      - It is a Syntax Error if the first IdentifierName's StringValue is not "type"
        or "typeof", and the third IdentifierName's StringValue is not "as"
    *)
    in let specifier env =
      let kind = match Peek.token env with
      | T_TYPE -> Some ImportType
      | T_TYPEOF -> Some ImportTypeof
      | _ -> None
      in

      if is_type_import (Peek.token env) then begin
        let type_keyword_or_remote = identifier_name env in
        match Peek.token env with
        | T_EOF
        | T_RCURLY
        | T_COMMA ->
          Parse.assert_identifier_name_is_identifier env remote;
          { remote; local = None; kind = None }

        | T_IDENTIFIER { raw = "as"; _ } ->
          begin match Peek.ith_token ~i:1 env with
          | T_EOF
          | T_RCURLY
          | T_COMMA ->
            { remote = Type.type_identifier env; local = None; kind }
          | T_IDENTIFIER { raw = "as"; _ } ->
            { remote; local; kind }
          | _ ->
            Parse.assert_identifier_name_is_identifier env remote;
            let local = Some (Parse.identifier env) in
            { remote; local; kind = None }
          end

        | _ ->
          let remote, local = with_maybe_as ~for_type:true env in
          { remote; local; kind }
      end else
        let remote, local = with_maybe_as ~for_type:false env in
        { remote; local; kind = None }

    in let type_specifier env =
      let remote, local = with_maybe_as env
        ~for_type:true
        ~error_if_type:Error.ImportTypeShorthandOnlyInPureImport
      in
      { remote; local; kind = None }

    in let typeof_specifier env =
      let remote, local = with_maybe_as env
        ~for_type:true
        ~error_if_type:Error.ImportTypeShorthandOnlyInPureImport
      in
      { remote; local; kind = None }

    in let rec specifier_list ?(preceding_comma=true) env statement_kind acc =
      match Peek.token env with
      | T_EOF
      | T_RCURLY -> List.rev acc
      | _ ->
        if not preceding_comma then error env Error.ImportSpecifierMissingComma;
        let specifier = match statement_kind with
        | ImportType -> type_specifier env
        | ImportTypeof -> typeof_specifier env
        | ImportValue -> specifier env
        in
        let preceding_comma = Expect.maybe env T_COMMA in
        specifier_list ~preceding_comma env statement_kind (specifier::acc)

    in let named_or_namespace_specifier env import_kind =
      let start_loc = Peek.loc env in
      match Peek.token env with
      | T_MULT ->
          Expect.token env T_MULT;
          Expect.identifier env "as";
          let id = match import_kind with
          | ImportType
          | ImportTypeof -> Type.type_identifier env
          | ImportValue -> Parse.identifier env
          in
          ImportNamespaceSpecifier (Loc.btwn start_loc (fst id), id)
      | _ ->
          Expect.token env T_LCURLY;
          let specifiers = specifier_list env import_kind [] in
          Expect.token env T_RCURLY;
          ImportNamedSpecifiers specifiers

    in let with_specifiers importKind env =
      let specifiers = Some (named_or_namespace_specifier env importKind) in
      let source = source env in
      Eat.semicolon env;
      Statement.ImportDeclaration {
        importKind;
        source;
        specifiers;
        default = None;
      }

    in let with_default importKind env =
      let default_specifier = match importKind with
      | ImportType
      | ImportTypeof -> Type.type_identifier env
      | ImportValue -> Parse.identifier env
      in

      let additional_specifiers =
        match Peek.token env with
  ` import  , ... `
            Expect.token env T_COMMA;
            Some (named_or_namespace_specifier env importKind)
        | _ -> None
      in

      let source = source env in
      Eat.semicolon env;
      Statement.ImportDeclaration {
        importKind;
        source;
        specifiers = additional_specifiers;
        default = Some default_specifier;
      }

    in with_loc (fun env ->
      let env = env |> with_strict true in
      Expect.token env T_IMPORT;

      match Peek.token env with
  ` import * as ns from " ModuleName " ; `
      | T_MULT ->
        with_specifiers ImportValue env

  ` import { ... } from " ModuleName " ; `
      | T_LCURLY ->
        with_specifiers ImportValue env

  ` import " ModuleName " ; `
      | T_STRING (str_loc, value, raw, octal) ->
        if octal then strict_error env Error.StrictOctalLiteral;
        Expect.token env (T_STRING (str_loc, value, raw, octal));
        let source = (str_loc, { StringLiteral.value; raw; }) in
        Eat.semicolon env;
        Statement.ImportDeclaration {
          importKind = ImportValue;
          source;
          specifiers = None;
          default = None;
        }

  ` import type [ ... ] from " ModuleName " ; ` 
          note that if [ ... ] is missing , we 're importing a value named ` type ` !
         note that if [...] is missing, we're importing a value named `type`! *)
      | T_TYPE when should_parse_types env ->
        begin match Peek.ith_token ~i:1 env with
  ` import type , { other , names } from " ModuleName " ; `
        | T_COMMA
  ` import type from " ModuleName " ; `
        | T_IDENTIFIER { raw = "from"; _ } ->
          with_default ImportValue env
        | T_MULT ->
          with_specifiers ImportType env
        | T_LCURLY ->
          with_specifiers ImportType env
        | _ ->
          with_default ImportType env
        end

  ` import typeof ... from " ModuleName " ; `
      | T_TYPEOF when should_parse_types env ->
        Expect.token env T_TYPEOF;
        begin match Peek.token env with
        | T_MULT
        | T_LCURLY -> with_specifiers ImportTypeof env
        | _ -> with_default ImportTypeof env
        end

  import  from " ModuleName " ;
      | _ ->
        with_default ImportValue env
    )
end
