6 Jun
2008
6 Jun
'08
10:43 p.m.
On Fri, 2008-06-06 at 15:41 -0700, Klaus Ostermann wrote:
Why does the code below not pass the type checker?
If I could explictly parameterize y with the type constructor Id (as e.g. in System F), then 'y Id' should have the type Int -> Int and hence "y Id x" should be OK, but with Haskell's implicit type parameters it does not work.
So, how can I make this work?
Klaus ------------
type Id a = a
x :: Id Int x = undefined
y :: (a Int) -> (a Int) y = undefined
test = y x
Error: Couldn't match expected type `a Int' against inferred type `Id Int' In the first argument of `y', namely `x' In the expression: y x In the definition of `test': test = y x
Down this path is higher-order unification which is undecidable. You can use a newtype.