[Haskell-cafe] ImplicitParams in GHC 9.0.1

[Georgi Lyubenov]

Hey!

I'd say 'yes' seeing as how eta expanding the function works:

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE RankNTypes #-}

module Lib where

foo :: Int -> ((?self :: Int) => Char) -> Char
foo x y = undefined

bar :: Int -> (Char -> Char)
bar x = (let ?self = x in (\u v -> foo u v)) x

baz :: Int -> (Char -> Char)
baz x c = let ?self = x in foo x c

However, I think it's still worth issuing a ghc bug report for this, as it
seems to really hurt implicit param usability in this case

Cheers,

Georgi

On Mon, Dec 13, 2021 at 5:57 PM ☂Josh Chia (謝任中) <joshchia at gmail.com> wrote:

> Hi,
>
> I have the following code that builds successfully with "stack build"
> under GHC 8.10.7 but fails to build under GHC 9.0.1:
>
> https://github.com/jchia/ip-bug
>
> The compiler is switched by commenting/uncommenting the compiler line in
> stack.yaml.
>
> GHC 9.0.1 gives me the error I paste at the end. Is this expected (maybe
> as part of "simplified subsumtion" in
> https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#compiler-changes)?
> If so, how can the code be fixed? If not, is it a compiler bug?
>
> (This problem came up while using gi-gtk-3.0.38, where implicit params are
> used a lot).
>
> Josh
>
> /home/jchia/gh/ip-bug/src/Lib.hs:10:28: error:
>     • Couldn't match type ‘(?self::Int) => Char’ with ‘Char’
>       Expected: Int -> Char -> Char
>         Actual: Int -> ((?self::Int) => Char) -> Char
>     • In the expression: foo
>       In the expression: (let ?self = x in foo) :: Int -> Char -> Char
>       In the expression:
>         ((let ?self = x in foo) :: Int -> Char -> Char) x
>    |
> 10 | bar x = ((let ?self = x in foo) :: Int -> Char -> Char) x
>    |                            ^^^
>
> /home/jchia/gh/ip-bug/src/Lib.hs:13:26: error:
>     • Couldn't match type ‘(?self::Int) => Char’ with ‘Char’
>       Expected: Char -> Char
>         Actual: ((?self::Int) => Char) -> Char
>     • In the expression: foo x
>       In the expression: let ?self = x in foo x
>       In an equation for ‘baz’: baz x = let ?self = x in foo x
>    |
> 13 | baz x = let ?self = x in foo x
>    |                          ^^^^^
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