Ordering of declarations can cause typechecking to fail
The following will successfully typecheck:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeFamilies #-}
data CInst
data G (b :: ()) = G
class C a where
type family F a
class (C a) => C' a where
type family F' a (b :: F a)
-- data CInst
instance C CInst where
type F CInst = ()
instance C' CInst where
type F' CInst (b :: F CInst) = G b
But if the data CInst
declaration is moved to where it is currently commented out, typechecking fails with this error:
Test.hs:23:18: error:
• Expected kind ‘F CInst’, but ‘b’ has kind ‘()’
• In the second argument of ‘F'’, namely ‘(b :: F CInst)’
In the type instance declaration for ‘F'’
In the instance declaration for ‘C' CInst’
|
23 | type F' CInst (b :: F CInst) = G b
|
However, the data declaration //can// be in the lower position if the kind annotation for its argument is instead written as data G (b :: F CInst) = G
.
This behavior is also exhibited when G is a type family (I believe the sort of type family does not matter, but I know for sure closed and open type families).
I was using GHC 8.2.2 when I discovered this, but user erisco
on #haskell
confirmed for 8.2.1 as well.
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |