Alpha-equivalence for recursive let-bindings

[Christiaan Baaij]

Alright, I opened an issue,
https://gitlab.haskell.org/ghc/ghc/-/issues/20641, and I'll make an MR
later.

Any objections if I change the implementation of

GHC.Core.Utils.eqExpr :: InScopeSet -> Expr -> Expr -> Bool
https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Core/Utils.hs#L2124

to

eqExpr _ e1 e2 = deBruijnize e1 == deBruijnize e2

and at the same time mark it deprecated telling people to use deBruijnize?
(I don't want to remove it since GHC API users might be using it)

`eqExpr` also does alpha-equivalence on CoreExpr, and has the same mistake
w.r.t. alpha-equivalence as the `Eq (DeBruijn CoreExpr)` instance

-- Christiaan


On Mon, 8 Nov 2021 at 13:02, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Huh!  Dead right!
>
>
>
> Would you like to:
>
>    - Open a ticket (you can use the text from this email)
>    - Submit a MR?
>
>
>
> On the MR,
>
>    - Add a Note that again gives your killer example; and mention why we
>    don’t need the check for NonRec
>    - Worth also pointing out that letrec { x = e1; y = e2 } in b is NOT
>    considered equal to letrec { y = e1; x = e1 } in b.   Nor are let x=e1 in
>    let y = e2 in b   considered equal to  let y = e1 in let x = e1 in b.
>    This is fine; but worth pointing out.
>
>
>
> Thanks for pointing this out!
>
>
>
> Simon
>
>
>
> PS: I am leaving Microsoft at the end of November 2021, at which point
> simonpj at microsoft.com will cease to work.  Use simon.peytonjones at gmail.com
> instead.  (For now, it just forwards to simonpj at microsoft.com.)
>
>
>
> *From:* ghc-devs <ghc-devs-bounces at haskell.org> *On Behalf Of *Christiaan
> Baaij
> *Sent:* 07 November 2021 21:08
> *To:* ghc-devs <ghc-devs at haskell.org>
> *Subject:* Alpha-equivalence for recursive let-bindings
>
>
>
> Hi list,
>
>
>
> I was looking at the `Eq (DeBruijn CoreExpr)` instance and I noticed that
> the types of recursive let-bindings aren't checked for alpha-equivalence:
>
>
>
>
> https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Core/Map/Expr.hs#L166-174
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fblob%2Fmaster%2Fcompiler%2FGHC%2FCore%2FMap%2FExpr.hs%23L166-174&data=04%7C01%7Csimonpj%40microsoft.com%7C4e4f1afdc4d64d66f2ef08d9a232bca8%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637719161836942634%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=6TGl3MHDSGLyUFjfMwce13K%2FVprCl7YRMQnRGrJj%2BAI%3D&reserved=0>
>
>
>
>     go (Let (Rec ps1) e1) (Let (Rec ps2) e2)
>       = equalLength ps1 ps2
>       && D env1' rs1 == D env2' rs2
>       && D env1' e1  == D env2' e2
>       where
>         (bs1,rs1) = unzip ps1
>         (bs2,rs2) = unzip ps2
>         env1' = extendCMEs env1 bs1
>         env2' = extendCMEs env2 bs2
>
>
>
> But doesn't that mean that:
>
> let (x :: Int) = x in x
>
> and
>
> let (y :: Bool) = y in y
>
> are considered alpha-equivalent?
>
> If that is the case, then I think that's wrong. Agree?
>
> I understand that you don't have to check types for non-recursive
> let-bindings: when the RHSs match, the types must be the same.
>
>
>
> -- Christiaan
>
>
>
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