I apologize if this question has been asked before...
It seems to me the following code should be legal:
type Thing m = m ()
type Const a b = a
f :: Thing m -> Thing m
f x = x
test :: Thing (Const Int) -> Thing (Const Int)
test = f
However, GHC gives the following error:
Couldn't match `Thing m' against `Thing (Const Int)'
Expected type: Thing m
Inferred type: Thing (Const Int)
Expected type: Thing (Const Int) -> Thing (Const Int)
Inferred type: Thing m -> Thing m
But if you change the definition of Const
newtype Const a b = MakeConst a
the code compiles fine.
Is this the desired behavior?
Is it the case that when you have quantification of a type variable of
kind * -> * (in this case, m), that variable may only become bound to a
type constructor?
Sam Moelius
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