Type family checking oddity
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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.