Let bindings overriding parameters

[jcollard]

I believe the following should result in 7 but results in 2000.

module Broken where

import Text (asText)


foo a =
    let x = a
        a = 2000
    in x

main = asText (foo 7)

[jcollard]

http://share-elm.com/sprout/546cb67ce4b00800031fee3d

[maxsnew]

This is because let is mutually recursive. If let acted the way you're saying we'd need a letrec or something to explicitly turn on the mutually recursive version, which needlessly complicates the language, IMO. If you want the behavior you expected you can add another nested let. I'd say this is not a bug.

foo a = let x = a in let a = 2000 in x
main = asText (foo 7)

unfortunately there's a codegen bug with variable shadowing so the code I pasted doesn't work in elm 0.13. Not sure if it's fixed in 0.14.

[jcollard]

Okay.

I just noticed this as it shows up as bug in the new Random library in core. I'll fix that so it isn't an issue in that library.

Some kind of warning when you do this might be useful. An elm-lint tool would be useful at some point.

[maxsnew]

A warning for variable shadowing? Yeah that would be nice, but it's not going to happen unless we expose ADTs and I don't think @evancz wants to be tied down by giving that a public interface.

[mgold]

@maxsnew By ADTs, did you mean ASTs? Because that would make more sense (to me). I think we had talked about AST export, I think in theory it's a good thing to do, but compiler complexity and schedule may well intervene.

[maxsnew]

Yeah ASTs, sorry.

[evancz]

I'll open an issue for "compiler warnings" as a general concept, which should cover this issue.