Type family checking oddity
Consider the following snipped
class C a where
type T a
data D a = D (T a)
If we now ask for the type of D we get
> :t D
D :: T a -> D a
I find this odd. The type T is in class C, so why is there no context on D saying so? It would be natural.
Now try some expression
> D True
<interactive>:1:2:
Couldn't match expected type `T a' against inferred type `Bool'
In the first argument of `D', namely `True'
In the expression: D True
In the definition of `it': it = D True
But this isn't really type incorrect, we just don't know yet. To remedy the problem we can use a context on the data declaration.
data (C a) => D a = D (T a)
And we try again
> :t D True
D True :: (Bool ~ T a, C a) => D a
This is what I would have expected in the first try as well; a context indicating what must hold for this to be type correct.
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 | |
Operating system | |
Architecture |