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
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
> simo...@microsoft.com will cease to work. Use simon.peytonjo...@gmail.com
> instead. (For now, it just forwards to simo...@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.hs#L166-174
>
>
>
>
> 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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