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
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