[Haskell-cafe] ImplicitParams in GHC 9.0.1

[☂Josh Chia (謝任中)]

I just file a feature request issue:

https://gitlab.haskell.org/ghc/ghc/-/issues/20818

On Tue, Dec 14, 2021 at 12:24 AM David Feuer <david.feuer at gmail.com> wrote:

> 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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