On Thu, Apr 02, 2009 at 08:18:27PM +0200, Peter Verswyvelen wrote:
> The type inferer seems to struggle to find the type of minBound and
> maxBound, and GHC asks to use a type annotation.
> To only way I see how to add a type annotation here is to use a GHC
> extension:
>
> {-# LANGUAGE ScopedTypeVariables #-}Just use 'asTypeOf'. It is defined as > asTypeOf :: a -> a -> a > asTypeOf = const so that @asTypeOf x y == x@ but both types are constrained to be equal. The above function would become > randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a > randomEnum = do > let min = minBound; max = maxBound > randVal <- getRandomR (fromEnum min, fromEnum max) > return $ toEnum randVal `asTypeOf` min `asTypeOf` max Note that I use the fact that 'return' is constrained to the type variable 'a' we want to constrain its argument, and the 'asTypeOf' constrains everything to be of the same type. HTH, -- Felipe. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
