Compiler: shadowed variable can lead to runtime error in generated JS

[jvoigtlaender]

Replaces https://github.com/elm-lang/elm-compiler/issues/1223, has SSCCE.

This program:

import Html

bad param =
  identity <| case param of
    Just _ ->
      0

    Nothing ->
      let
        param = True
      in
        if param then 1 else 2

main = Html.text (toString (bad Nothing))

leads to a runtime error.

The 0.17 produced JS is:

var _user$project$Temp1461826735203948$bad = function (param) {
    return _elm_lang$core$Basics$identity(
        function () {
            var _p0 = param;
            if (_p0.ctor === 'Just') {
                return 0;
            } else {
                var param = true;
                return param ? 1 : 2;
            }
        }());
};

Why that is wrong is explained in https://github.com/elm-lang/elm-compiler/issues/1223#issuecomment-169645306.

There is also a test case PR open, as https://github.com/elm-lang/elm-compiler/pull/1253.

[evancz]

Tracking in #1377

[jvoigtlaender]

A related code gen bug, though with slightly different characteristics (no "function call around a case expression") was raised as https://github.com/elm-lang/elm-compiler/issues/1428.

[brianhempel]

Perhaps a slightly simpler example. Outputs "undefined":

import Html

main =
    let x = "outer" in
    (\_ -> 
        let shouldBeOuter = x in
        let x = "inner" in
        Html.text shouldBeOuter
    ) ()

var _user$project$Main$main = _elm_lang$virtual_dom$Native_VirtualDom.staticProgram(
    function () {
        var x = 'outer';
        return function (_p0) {
            var shouldBeOuter = x;
            var x = 'inner';
            return _elm_lang$html$Html$text(shouldBeOuter);
        }(
            {ctor: '_Tuple0'});
    }());

Javascript closure environments = 😞

[evancz]

Variable shadowing like this will be disallowed in 0.19. You can use the same name as things that appear in an exposing, but you will not be able to shadow variables within a local definition.

In other words, this will be resolved by the fact that the initial code will no longer be accepted by the compiler.