Default type family instances
The following code does not compile in GHC 7.4, unless line (2) is uncommented. I thought line (1) would remove need for (2). If it does not, probably declaration (1) should cause an error.
{-# LANGUAGE TypeFamilies #-}
class R m where
type D m a :: *
type D m a = () -- (1)
f :: m a -> D m a -> ()
instance R Maybe where
-- type D Maybe a = () -- (2)
f = undefined
x = f (Nothing :: Maybe Int) ()
Trac metadata
Trac field | Value |
---|---|
Version | 7.4.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |