Surprising type (family) type derived
Consider the following module
{-# LANGUAGE TypeFamilies #-}
module Bug5 where
class C a where
type T a
f :: T a -> T a -> T a
data D a = D { t :: T a }
g r = f (t r) (t r)
Now ask for the type of g
*Bug5> :t g
g :: (T a ~ T a1, C a1) => D a -> T a1
Why isn't the type
g :: (C a) => D a -> T a
?
The strange type is a minor nuisance, but it gets worse
{-# LANGUAGE TypeFamilies #-}
module Bug6 where
class C a where
type T a
type U a
f :: T a -> T a -> T a
x :: U a -> T a
data D a = D { t :: T a, u :: U a }
g r = f (t r) (x (u r))
This doesn't type check at all.
An even simpler example that fails:
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Bug7 where
class C a where
type T a
type U a
x :: U a -> T a
data D a = D (U a)
g :: (C a) => D a -> T a
g (D u) = x u
Trac metadata
Trac field | Value |
---|---|
Version | 6.10.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | lennart@augustsson.net |
Operating system | |
Architecture |