def analyse(node, env, non_generic=None):
    """Computes the type of the expression given by node.

    The type of the node is computed in the context of the context of the
    supplied type environment env. Data types can be introduced into the
    language simply by having a predefined set of identifiers in the initial
    environment. Environment; this way there is no need to change the syntax
    or more importantly, the type-checking program when extending the language.

    Args:
        node: The root of the abstract syntax tree.
        env: The type environment is a mapping of expression identifier names
            to type assignments.
        non_generic: A set of non-generic variables, or None

    Returns:
        The computed type of the expression.

    Raises:
        InferenceError: The type of the expression could not be inferred,
        PythranTypeError: InferenceError with user friendly message + location
    """

    if non_generic is None:
        non_generic = set()

    # expr
    if isinstance(node, gast.Name):
        if isinstance(node.ctx, (gast.Store)):
            new_type = TypeVariable()
            non_generic.add(new_type)
            env[node.id] = new_type
        return get_type(node.id, env, non_generic)
    elif isinstance(node, gast.Num):
        if isinstance(node.n, int):
            return Integer()
        elif isinstance(node.n, float):
            return Float()
        elif isinstance(node.n, complex):
            return Complex()
        else:
            raise NotImplementedError
    elif isinstance(node, gast.Str):
        return Str()
    elif isinstance(node, gast.Compare):
        left_type = analyse(node.left, env, non_generic)
        comparators_type = [analyse(comparator, env, non_generic)
                            for comparator in node.comparators]
        ops_type = [analyse(op, env, non_generic)
                    for op in node.ops]
        prev_type = left_type
        result_type = TypeVariable()
        for op_type, comparator_type in zip(ops_type, comparators_type):
            try:
                unify(Function([prev_type, comparator_type], result_type),
                      op_type)
                prev_type = comparator_type
            except InferenceError:
                raise PythranTypeError(
                    "Invalid comparison, between `{}` and `{}`".format(
                        prev_type,
                        comparator_type
                    ),
                    node)
        return result_type
    elif isinstance(node, gast.Call):
        if is_getattr(node):
            self_type = analyse(node.args[0], env, non_generic)
            attr_name = node.args[1].s
            _, attr_signature = attributes[attr_name]
            attr_type = tr(attr_signature)
            result_type = TypeVariable()
            try:
                unify(Function([self_type], result_type), attr_type)
            except InferenceError:
                if isinstance(prune(attr_type), MultiType):
                    msg = 'no attribute found, tried:\n{}'.format(attr_type)
                else:
                    msg = 'tried {}'.format(attr_type)
                raise PythranTypeError(
                    "Invalid attribute for getattr call with self"
                    "of type `{}`, {}".format(self_type, msg), node)

        else:
            fun_type = analyse(node.func, env, non_generic)
            arg_types = [analyse(arg, env, non_generic) for arg in node.args]
            result_type = TypeVariable()
            try:
                unify(Function(arg_types, result_type), fun_type)
            except InferenceError:
                # recover original type
                fun_type = analyse(node.func, env, non_generic)
                if isinstance(prune(fun_type), MultiType):
                    msg = 'no overload found, tried:\n{}'.format(fun_type)
                else:
                    msg = 'tried {}'.format(fun_type)
                raise PythranTypeError(
                    "Invalid argument type for function call to "
                    "`Callable[[{}], ...]`, {}"
                    .format(', '.join('{}'.format(at) for at in arg_types),
                            msg),
                    node)
        return result_type

    elif isinstance(node, gast.IfExp):
        test_type = analyse(node.test, env, non_generic)
        unify(Function([test_type], Bool()),
              tr(MODULES['__builtin__']['bool_']))

        if is_test_is_none(node.test):
            none_id = node.test.left.id
            body_env = env.copy()
            body_env[none_id] = NoneType
        else:
            none_id = None
            body_env = env

        body_type = analyse(node.body, body_env, non_generic)

        if none_id:
            orelse_env = env.copy()
            if is_option_type(env[none_id]):
                orelse_env[none_id] = prune(env[none_id]).types[0]
            else:
                orelse_env[none_id] = TypeVariable()
        else:
            orelse_env = env

        orelse_type = analyse(node.orelse, orelse_env, non_generic)

        try:
            return merge_unify(body_type, orelse_type)
        except InferenceError:
            raise PythranTypeError(
                "Incompatible types from different branches:"
                "`{}` and `{}`".format(
                    body_type,
                    orelse_type
                ),
                node
            )
    elif isinstance(node, gast.UnaryOp):
        operand_type = analyse(node.operand, env, non_generic)
        op_type = analyse(node.op, env, non_generic)
        result_type = TypeVariable()
        try:
            unify(Function([operand_type], result_type), op_type)
            return result_type
        except InferenceError:
            raise PythranTypeError(
                "Invalid operand for `{}`: `{}`".format(
                    symbol_of[type(node.op)],
                    operand_type
                ),
                node
            )
    elif isinstance(node, gast.BinOp):
        left_type = analyse(node.left, env, non_generic)
        op_type = analyse(node.op, env, non_generic)
        right_type = analyse(node.right, env, non_generic)
        result_type = TypeVariable()
        try:
            unify(Function([left_type, right_type], result_type), op_type)
        except InferenceError:
            raise PythranTypeError(
                "Invalid operand for `{}`: `{}` and `{}`".format(
                    symbol_of[type(node.op)],
                    left_type,
                    right_type),
                node
            )
        return result_type
    elif isinstance(node, gast.Pow):
        return tr(MODULES['numpy']['power'])
    elif isinstance(node, gast.Sub):
        return tr(MODULES['operator_']['sub'])
    elif isinstance(node, (gast.USub, gast.UAdd)):
        return tr(MODULES['operator_']['pos'])
    elif isinstance(node, (gast.Eq, gast.NotEq, gast.Lt, gast.LtE, gast.Gt,
                           gast.GtE, gast.Is, gast.IsNot)):
        return tr(MODULES['operator_']['eq'])
    elif isinstance(node, (gast.In, gast.NotIn)):
        contains_sig = tr(MODULES['operator_']['contains'])
        contains_sig.types[:-1] = reversed(contains_sig.types[:-1])
        return contains_sig
    elif isinstance(node, gast.Add):
        return tr(MODULES['operator_']['add'])
    elif isinstance(node, gast.Mult):
        return tr(MODULES['operator_']['mul'])
    elif isinstance(node, gast.MatMult):
        return tr(MODULES['operator_']['matmul'])
    elif isinstance(node, (gast.Div, gast.FloorDiv)):
        return tr(MODULES['operator_']['floordiv'])
    elif isinstance(node, gast.Mod):
        return tr(MODULES['operator_']['mod'])
    elif isinstance(node, (gast.LShift, gast.RShift)):
        return tr(MODULES['operator_']['lshift'])
    elif isinstance(node, (gast.BitXor, gast.BitAnd, gast.BitOr)):
        return tr(MODULES['operator_']['lshift'])
    elif isinstance(node, gast.List):
        new_type = TypeVariable()
        for elt in node.elts:
            elt_type = analyse(elt, env, non_generic)
            try:
                unify(new_type, elt_type)
            except InferenceError:
                raise PythranTypeError(
                    "Incompatible list element type `{}` and `{}`".format(
                        new_type, elt_type),
                    node
                )
        return List(new_type)
    elif isinstance(node, gast.Set):
        new_type = TypeVariable()
        for elt in node.elts:
            elt_type = analyse(elt, env, non_generic)
            try:
                unify(new_type, elt_type)
            except InferenceError:
                raise PythranTypeError(
                    "Incompatible set element type `{}` and `{}`".format(
                        new_type, elt_type),
                    node
                )
        return Set(new_type)
    elif isinstance(node, gast.Dict):
        new_key_type = TypeVariable()
        for key in node.keys:
            key_type = analyse(key, env, non_generic)
            try:
                unify(new_key_type, key_type)
            except InferenceError:
                raise PythranTypeError(
                    "Incompatible dict key type `{}` and `{}`".format(
                        new_key_type, key_type),
                    node
                )
        new_value_type = TypeVariable()
        for value in node.values:
            value_type = analyse(value, env, non_generic)
            try:
                unify(new_value_type, value_type)
            except InferenceError:
                raise PythranTypeError(
                    "Incompatible dict value type `{}` and `{}`".format(
                        new_value_type, value_type),
                    node
                )
        return Dict(new_key_type, new_value_type)
    elif isinstance(node, gast.Tuple):
        return Tuple([analyse(elt, env, non_generic) for elt in node.elts])
    elif isinstance(node, gast.Index):
        return analyse(node.value, env, non_generic)
    elif isinstance(node, gast.Slice):
        def unify_int_or_none(t, name):
            try:
                unify(t, Integer())
            except InferenceError:
                try:
                    unify(t, NoneType)
                except InferenceError:
                    raise PythranTypeError(
                        "Invalid slice {} type `{}`, expecting int or None"
                        .format(name, t)
                    )
        if node.lower:
            lower_type = analyse(node.lower, env, non_generic)
            unify_int_or_none(lower_type, 'lower bound')
        else:
            lower_type = Integer()
        if node.upper:
            upper_type = analyse(node.upper, env, non_generic)
            unify_int_or_none(upper_type, 'upper bound')
        else:
            upper_type = Integer()
        if node.step:
            step_type = analyse(node.step, env, non_generic)
            unify_int_or_none(step_type, 'step')
        else:
            step_type = Integer()
        return Slice
    elif isinstance(node, gast.ExtSlice):
        return [analyse(dim, env, non_generic) for dim in node.dims]
    elif isinstance(node, gast.NameConstant):
        if node.value is None:
            return env['None']
    elif isinstance(node, gast.Subscript):
        new_type = TypeVariable()
        value_type = prune(analyse(node.value, env, non_generic))
        try:
            slice_type = prune(analyse(node.slice, env, non_generic))
        except PythranTypeError as e:
            raise PythranTypeError(e.msg, node)

        if isinstance(node.slice, gast.ExtSlice):
            nbslice = len(node.slice.dims)
            dtype = TypeVariable()
            try:
                unify(Array(dtype, nbslice), clone(value_type))
            except InferenceError:
                raise PythranTypeError(
                    "Dimension mismatch when slicing `{}`".format(value_type),
                    node)
            return TypeVariable()  # FIXME
        elif isinstance(node.slice, gast.Index):
            # handle tuples in a special way
            isnum = isinstance(node.slice.value, gast.Num)
            if isnum and is_tuple_type(value_type):
                try:
                    unify(prune(prune(value_type.types[0]).types[0])
                          .types[node.slice.value.n],
                          new_type)
                    return new_type
                except IndexError:
                    raise PythranTypeError(
                        "Invalid tuple indexing, "
                        "out-of-bound index `{}` for type `{}`".format(
                            node.slice.value.n,
                            value_type),
                        node)
        try:
            unify(tr(MODULES['operator_']['getitem']),
                  Function([value_type, slice_type], new_type))
        except InferenceError:
            raise PythranTypeError(
                "Invalid subscripting of `{}` by `{}`".format(
                    value_type,
                    slice_type),
                node)
        return new_type
        return new_type
    elif isinstance(node, gast.Attribute):
        from pythran.utils import attr_to_path
        obj, path = attr_to_path(node)
        if obj.signature is typing.Any:
            return TypeVariable()
        else:
            return tr(obj)

    # stmt
    elif isinstance(node, gast.Import):
        for alias in node.names:
            if alias.name not in MODULES:
                raise NotImplementedError("unknown module: %s " % alias.name)
            if alias.asname is None:
                target = alias.name
            else:
                target = alias.asname
            env[target] = tr(MODULES[alias.name])
        return env
    elif isinstance(node, gast.ImportFrom):
        if node.module not in MODULES:
            raise NotImplementedError("unknown module: %s" % node.module)
        for alias in node.names:
            if alias.name not in MODULES[node.module]:
                raise NotImplementedError(
                    "unknown function: %s in %s" % (alias.name, node.module))
            if alias.asname is None:
                target = alias.name
            else:
                target = alias.asname
            env[target] = tr(MODULES[node.module][alias.name])
        return env
    elif isinstance(node, gast.FunctionDef):
        ftypes = []
        for i in range(1 + len(node.args.defaults)):
            new_env = env.copy()
            new_non_generic = non_generic.copy()

            # reset return special variables
            new_env.pop('@ret', None)
            new_env.pop('@gen', None)

            hy = HasYield()
            for stmt in node.body:
                hy.visit(stmt)
            new_env['@gen'] = hy.has_yield

            arg_types = []
            istop = len(node.args.args) - i
            for arg in node.args.args[:istop]:
                arg_type = TypeVariable()
                new_env[arg.id] = arg_type
                new_non_generic.add(arg_type)
                arg_types.append(arg_type)
            for arg, expr in zip(node.args.args[istop:],
                                 node.args.defaults[-i:]):
                arg_type = analyse(expr, new_env, new_non_generic)
                new_env[arg.id] = arg_type

            analyse_body(node.body, new_env, new_non_generic)

            result_type = new_env.get('@ret', NoneType)

            if new_env['@gen']:
                result_type = Generator(result_type)

            ftype = Function(arg_types, result_type)
            ftypes.append(ftype)
        if len(ftypes) == 1:
            ftype = ftypes[0]
            env[node.name] = ftype
        else:
            env[node.name] = MultiType(ftypes)
        return env
    elif isinstance(node, gast.Module):
        analyse_body(node.body, env, non_generic)
        return env
    elif isinstance(node, (gast.Pass, gast.Break, gast.Continue)):
        return env
    elif isinstance(node, gast.Expr):
        analyse(node.value, env, non_generic)
        return env
    elif isinstance(node, gast.Delete):
        for target in node.targets:
            if isinstance(target, gast.Name):
                if target.id in env:
                    del env[target.id]
                else:
                    raise PythranTypeError(
                        "Invalid del: unbound identifier `{}`".format(
                            target.id),
                        node)
            else:
                analyse(target, env, non_generic)
        return env
    elif isinstance(node, gast.Print):
        if node.dest is not None:
            analyse(node.dest, env, non_generic)
        for value in node.values:
            analyse(value, env, non_generic)
        return env
    elif isinstance(node, gast.Assign):
        defn_type = analyse(node.value, env, non_generic)
        for target in node.targets:
            target_type = analyse(target, env, non_generic)
            try:
                unify(target_type, defn_type)
            except InferenceError:
                raise PythranTypeError(
                    "Invalid assignment from type `{}` to type `{}`".format(
                        target_type,
                        defn_type),
                    node)
        return env
    elif isinstance(node, gast.AugAssign):
        # FIMXE: not optimal: evaluates type of node.value twice
        fake_target = deepcopy(node.target)
        fake_target.ctx = gast.Load()
        fake_op = gast.BinOp(fake_target, node.op, node.value)
        gast.copy_location(fake_op, node)
        res_type = analyse(fake_op, env, non_generic)
        target_type = analyse(node.target, env, non_generic)

        try:
            unify(target_type, res_type)
        except InferenceError:
            raise PythranTypeError(
                "Invalid update operand for `{}`: `{}` and `{}`".format(
                    symbol_of[type(node.op)],
                    res_type,
                    target_type
                ),
                node
            )
        return env
    elif isinstance(node, gast.Raise):
        return env  # TODO
    elif isinstance(node, gast.Return):
        if env['@gen']:
            return env

        if node.value is None:
            ret_type = NoneType
        else:
            ret_type = analyse(node.value, env, non_generic)
        if '@ret' in env:
            try:
                ret_type = merge_unify(env['@ret'], ret_type)
            except InferenceError:
                raise PythranTypeError(
                    "function may returns with incompatible types "
                    "`{}` and `{}`".format(env['@ret'], ret_type),
                    node
                )

        env['@ret'] = ret_type
        return env
    elif isinstance(node, gast.Yield):
        assert env['@gen']
        assert node.value is not None

        if node.value is None:
            ret_type = NoneType
        else:
            ret_type = analyse(node.value, env, non_generic)
        if '@ret' in env:
            try:
                ret_type = merge_unify(env['@ret'], ret_type)
            except InferenceError:
                raise PythranTypeError(
                    "function may yields incompatible types "
                    "`{}` and `{}`".format(env['@ret'], ret_type),
                    node
                )

        env['@ret'] = ret_type
        return env
    elif isinstance(node, gast.For):
        iter_type = analyse(node.iter, env, non_generic)
        target_type = analyse(node.target, env, non_generic)
        unify(Collection(TypeVariable(), TypeVariable(), TypeVariable(),
                         target_type),
              iter_type)
        analyse_body(node.body, env, non_generic)
        analyse_body(node.orelse, env, non_generic)
        return env
    elif isinstance(node, gast.If):
        test_type = analyse(node.test, env, non_generic)
        unify(Function([test_type], Bool()),
              tr(MODULES['__builtin__']['bool_']))

        body_env = env.copy()
        body_non_generic = non_generic.copy()

        if is_test_is_none(node.test):
            none_id = node.test.left.id
            body_env[none_id] = NoneType
        else:
            none_id = None

        analyse_body(node.body, body_env, body_non_generic)

        orelse_env = env.copy()
        orelse_non_generic = non_generic.copy()

        if none_id:
            if is_option_type(env[none_id]):
                orelse_env[none_id] = prune(env[none_id]).types[0]
            else:
                orelse_env[none_id] = TypeVariable()
        analyse_body(node.orelse, orelse_env, orelse_non_generic)

        for var in body_env:
            if var not in env:
                if var in orelse_env:
                    try:
                        new_type = merge_unify(body_env[var], orelse_env[var])
                    except InferenceError:
                        raise PythranTypeError(
                            "Incompatible types from different branches for "
                            "`{}`: `{}` and `{}`".format(
                                var,
                                body_env[var],
                                orelse_env[var]
                            ),
                            node
                        )
                else:
                    new_type = body_env[var]
                env[var] = new_type

        for var in orelse_env:
            if var not in env:
                # may not be unified by the prev loop if a del occured
                if var in body_env:
                    new_type = merge_unify(orelse_env[var], body_env[var])
                else:
                    new_type = orelse_env[var]
                env[var] = new_type

        if none_id:
            try:
                new_type = merge_unify(body_env[none_id], orelse_env[none_id])
            except InferenceError:
                msg = ("Inconsistent types while merging values of `{}` from "
                       "conditional branches: `{}` and `{}`")
                err = msg.format(none_id,
                                 body_env[none_id],
                                 orelse_env[none_id])
                raise PythranTypeError(err, node)
            env[none_id] = new_type

        return env
    elif isinstance(node, gast.While):
        test_type = analyse(node.test, env, non_generic)
        unify(Function([test_type], Bool()),
              tr(MODULES['__builtin__']['bool_']))

        analyse_body(node.body, env, non_generic)
        analyse_body(node.orelse, env, non_generic)
        return env
    elif isinstance(node, gast.Try):
        analyse_body(node.body, env, non_generic)
        for handler in node.handlers:
            analyse(handler, env, non_generic)
        analyse_body(node.orelse, env, non_generic)
        analyse_body(node.finalbody, env, non_generic)
        return env
    elif isinstance(node, gast.ExceptHandler):
        if(node.name):
            new_type = ExceptionType
            non_generic.add(new_type)
            if node.name.id in env:
                unify(env[node.name.id], new_type)
            else:
                env[node.name.id] = new_type
        analyse_body(node.body, env, non_generic)
        return env
    elif isinstance(node, gast.Assert):
        if node.msg:
            analyse(node.msg, env, non_generic)
        analyse(node.test, env, non_generic)
        return env
    elif isinstance(node, gast.UnaryOp):
        operand_type = analyse(node.operand, env, non_generic)
        return_type = TypeVariable()
        op_type = analyse(node.op, env, non_generic)
        unify(Function([operand_type], return_type), op_type)
        return return_type
    elif isinstance(node, gast.Invert):
        return MultiType([Function([Bool()], Integer()),
                          Function([Integer()], Integer())])
    elif isinstance(node, gast.Not):
        return tr(MODULES['__builtin__']['bool_'])
    elif isinstance(node, gast.BoolOp):
        op_type = analyse(node.op, env, non_generic)
        value_types = [analyse(value, env, non_generic)
                       for value in node.values]

        for value_type in value_types:
            unify(Function([value_type], Bool()),
                  tr(MODULES['__builtin__']['bool_']))

        return_type = TypeVariable()
        prev_type = value_types[0]
        for value_type in value_types[1:]:
            unify(Function([prev_type, value_type], return_type), op_type)
            prev_type = value_type
        return return_type
    elif isinstance(node, (gast.And, gast.Or)):
        x_type = TypeVariable()
        return MultiType([
            Function([x_type, x_type], x_type),
            Function([TypeVariable(), TypeVariable()], TypeVariable()),
        ])

    raise RuntimeError("Unhandled syntax node {0}".format(type(node)))