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#... 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 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@microsoft.com will cease to work. Use simon.peytonjones@gmail.com instead. (For now, it just forwards to simonpj@microsoft.com.)

*From:* ghc-devs *On Behalf Of *Christiaan Baaij *Sent:* 07 November 2021 21:08 *To:* ghc-devs *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.... 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