Confusing error message with PolyKinds (#9017) · Issues · Glasgow Haskell Compiler / GHC

Confusing error message with PolyKinds

Consider the following example code:

import Control.Arrow

foo :: a b (m b)
foo = arr return

if we typecheck this we rightly get two errors about missing instances (Arrow a, Monad m). However, if we enable PolyKinds we get the following error message:

T.hs:4:7:
    Kind incompatibility when matching types:
      a0 :: * -> * -> *
      a :: k1 -> k -> *
    Expected type: a b (m b)
      Actual type: a0 b0 (m0 b0)
    Relevant bindings include foo :: a b (m b) (bound at T.hs:4:1)
    In the expression: arr return
    In an equation for ‘foo’: foo = arr return

I can sort of see where this is coming from if I think really hard :), so perhaps it's not a bug per se, but it's definitely confusing; and it's a pity that a type error (forgotten type qualifier) is reported as a kind error.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

Consider the following example code:

```
import Control.Arrow

foo :: a b (m b)
foo = arr return
```

if we typecheck this we rightly get two errors about missing instances (Arrow a, Monad m). However, if we enable `PolyKinds` we get the following error message:

```
T.hs:4:7:
    Kind incompatibility when matching types:
      a0 :: * -> * -> *
      a :: k1 -> k -> *
    Expected type: a b (m b)
      Actual type: a0 b0 (m0 b0)
    Relevant bindings include foo :: a b (m b) (bound at T.hs:4:1)
    In the expression: arr return
    In an equation for ‘foo’: foo = arr return
```

I can *sort of* see where this is coming from if I think really hard :), so perhaps it's not a bug per se, but it's definitely confusing; and it's a pity that a type error (forgotten type qualifier) is reported as a kind error.

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 7.8.2        |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>