Absolutely misleading error message on kind error

module Bug where

import Data.Proxy

fun :: Proxy a -> ()
fun Proxy = ()

bug :: ()
bug = fun (Proxy :: Proxy Maybe)

[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:9:12: error:
    • Expected kind ‘Proxy Maybe’,
        but ‘Data.Proxy.Proxy :: Proxy Maybe’ has kind ‘Proxy Maybe’
    • In the first argument of ‘fun’, namely ‘(Proxy :: Proxy Maybe)’
      In the expression: fun (Proxy :: Proxy Maybe)
      In an equation for ‘bug’: bug = fun (Proxy :: Proxy Maybe)

1. 10 gives the appropriate Couldn't match kind ‘* -> *’ with ‘*’ message.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 7

added Tbug Trac import labels

Note that if you pass -fprint-explicit-kinds the error is a bit better (although arguably still not as clear as 7.10's error),

Bug.hs:9:12: error:
    • Expected kind ‘Proxy * Maybe’,
        but ‘Data.Proxy.Proxy :: Proxy Maybe’ has kind ‘Proxy
                                                          (* -> *) Maybe’
    • In the first argument of ‘fun’, namely ‘(Proxy :: Proxy Maybe)’
      In the expression: fun (Proxy :: Proxy Maybe)
      In an equation for ‘bug’: bug = fun (Proxy :: Proxy Maybe)

Of course, this isn't a great situation; surely the compiler can see that the types as printed are indistinguishable and it should therefore providing kinds as well. I'm not certain of this, but I suspect Richard is already aware of this issue.

Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

Is Proxy Maybe even a kind? Unless I'm missing something, in this context it is a type, not a kind.

A fair point. Yes, "kind" does seem a bit misleading here.

assigned to @rae

Yes, I'm already aware of this issue, but another test case is good. Thanks!

Possibly a duplicate of #11485 (closed)?

added TypeInType label

For reference, here is the current error message in GHC 8.0.1/HEAD:

    • Couldn't match type ‘* -> *’ with ‘*’
      Expected type: Proxy Maybe
        Actual type: Proxy Maybe
      Use -fprint-explicit-kinds to see the kind arguments
    • In the first argument of ‘fun’, namely ‘(Proxy :: Proxy Maybe)’
      In the expression: fun (Proxy :: Proxy Maybe)
      In an equation for ‘bug’: bug = fun (Proxy :: Proxy Maybe)

I think this is much clearer, and doesn't incorrectly label the type as a kind.

closed

changed milestone to %8.0.1

This is fixed in 8.0.1.

Trac metadata

Trac field	Value
Version	8.1 → 8.0.1-rc1
Resolution	Unresolved → ResolvedFixed

added error messages label

added Phigh label