[Haskell-cafe] ImplicitParams in GHC 9.0.1

[David Feuer]

It's also really bad for the (new!) LinearTypes extension, since we can't
pass x %1-> y where it expects x -> y. We have to eta expand. Ugh.

On Mon, Dec 13, 2021, 11:14 AM Georgi Lyubenov <godzbanebane at gmail.com>
wrote:

> 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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