[Haskell-cafe] Inscrutable error message from PolyKinds (should it be in GHC202X?)

[J. Reinders]

More examples of GHC2021 inclusion of PolyKinds confusing people:

* https://discourse.haskell.org/t/different-typeable-constraints-behaviour-in-ghc2021-haskell2010/4606?u=jaror
* https://stackoverflow.com/q/72329476/15207568

Sometimes it even causes errors where there were none before.

Jaro

> On 23 Jan 2023, at 14:56, Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> 
> I love the idea of GHC2021, but I've just discovered that PolyKinds,
> which is part of it, leads to inscrutable error messages.  Firstly, a
> scrutable error message, without PolyKinds:
> 
>    {-# LANGUAGE GHC2021 #-}
>    {-# LANGUAGE NoPolyKinds #-}
> 
>    -- Error: • No instance for (Foldable t) arising from a use of
>    -- ‘length’
>    foo :: t a -> Int
>    foo = length
> 
> Good.  I know exactly how to fix that.  Now the inscrutable message:
> 
>    {-# LANGUAGE GHC2021 #-}
> 
>    -- • Couldn't match kind ‘k’ with ‘*’
>    --   When matching types
>    --     t0 :: * -> *
>    --     t :: k -> *
>    --   Expected: t a -> Int
>    --     Actual: t0 a0 -> Int
>    --   ‘k’ is a rigid type variable bound by
>    --     the type signature for:
>    --       foo :: forall {k} (t :: k -> *) (a :: k). t a -> Int
>    --     at /tmp/bar.hs:3:1-17
>    -- • In the expression: length
>    --   In an equation for ‘foo’: foo = length
>    -- • Relevant bindings include
>    --     foo :: t a -> Int (bound at /tmp/bar.hs:4:1)
>    foo :: t a -> Int
>    foo = length
> 
> What?  Non-expert users will experience much puzzlement.
> 
> (PolyKinds tries to generalise the kind of `a` and fails, because the
> use of `length` restricts it to `Type`.)
> 
> Should PolyKinds really be in GHC202X?
> 
> Tom
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