Odd error message due to PolyKinds

[Richard Eisenberg]

Though there's still room for improvement, this one has gotten a lot better since RC2:

> Scratch.hs:24:39: error:
>     • Couldn't match type ‘k0 -> *’ with ‘*’
>       Expected type: T W
>         Actual type: T W
>       Use -fprint-explicit-kinds to see the kind arguments
>     • In the first argument of ‘undefined ::
>                                   EqSyn w => T w -> ()’, namely
>         ‘(undefined :: T W)’
>       In the expression:
>         (undefined :: EqSyn w => T w -> ()) (undefined :: T W)
>       In an equation for ‘x’:
>           x = (undefined :: EqSyn w => T w -> ()) (undefined :: T W)

Richard

On Apr 9, 2016, at 6:41 PM, Kosyrev Serge <_deepfire at feelingofgreen.ru> wrote:

> Good day folks,
> 
> Here is a test case that exposes a certain weakness in PolyKinds
> error messaging.  In the following snippet (T W) :: * is malkinded:
> 
>> {-# LANGUAGE PolyKinds #-}
>> {-# LANGUAGE UnicodeSyntax #-}
>> 
>> class Eq a ⇒ EqSyn a where
>> data T w
>> data W f
>> 
>> x = (undefined ∷ EqSyn w ⇒ T w → ()) (undefined ∷ T W)
> 
> 8.0.1rc2 yields:
> 
>> ../ghc-playground.hs:8:42: error:
>>    • Expected kind ‘T W’, but ‘undefined :: T W’ has kind ‘T W’
>>    • In the first argument of ‘undefined ::
>>                                  EqSyn w => T w -> ()’, namely
>>        ‘(undefined :: T W)’
>>      In the expression:
>>        (undefined :: EqSyn w => T w -> ()) (undefined :: T W)
>>      In an equation for ‘main’:
>>          main = (undefined :: EqSyn w => T w -> ()) (undefined :: T W)
> 
> ..which is.. not informative.
> 
> With PolyKinds removed:
> 
>> ../ghc-playground.hs:7:56: error:
>>    • Expecting one more argument to ‘W’
>>      Expected a type, but ‘W’ has kind ‘* -> *’
>>    • In the first argument of ‘T’, namely ‘W’
>>      In an expression type signature: T W
>>      In the first argument of ‘undefined ::
>>                                  EqSyn w => T w -> ()’, namely
>>        ‘(undefined :: T W)’
> 
> I'm not sure if this was reported, so I'm sorry for the noise if it was.
> 
> -- 
> с уважениeм / respectfully,
> Косырев Сергей
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