expr :: Parser (AST' Text)
let_expr = Let
    <$> identifier
    <*  reservedOp "="
    <*> expr
    <*> expr

de
let_expr :: Parser (AST' Text)
let_expr = Let
    <$> identifier
    <*  reservedOp "="
    <*> expr
    <*> expr
let_expr = Let
    <$> identifier
    <*  reservedOp "="
    <*> expr
    <*> expr