Very unhelpful message resulting from kind mismatch
The following code:
module Foo where
import Data.Typeable
tyConOf :: Typeable a => Proxy a -> TyCon
tyConOf = typeRepTyCon . typeRep
tcList :: TyCon
tcList = tyConOf (Proxy :: Proxy [])fails because -XPolyKinds is not enabled. But the error message that you get is quite different on GHC 7.10 and 8.0.
On GHC 7.10.3:
[1 of 1] Compiling Foo ( Foo.hs, Foo.o )
Foo.hs:9:19:
Couldn't match kind ‘* -> *’ with ‘*’
Expected type: Proxy a0
Actual type: Proxy []
In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’
In the expression: tyConOf (Proxy :: Proxy [])
In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])But on GHC 8.0.1-rc1:
Foo.hs:9:19: error:
• Expected kind ‘Proxy []’,
but ‘Data.Proxy.Proxy :: Proxy []’ has kind ‘Proxy []’
• In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’
In the expression: tyConOf (Proxy :: Proxy [])
In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1-rc1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |