[Haskell-cafe] Question about kinds
derek.a.elkins at gmail.com
Fri Jun 6 22:43:25 EDT 2008
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?
> type Id a = a
> x :: Id Int
> x = undefined
> y :: (a Int) -> (a Int)
> y = undefined
> test = y x
> 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.
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