On Thu, Apr 2, 2009 at 9:51 PM, Felipe Lessa
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.
Interesting alternative. However, I think the ScopedTypeVariables looks a little bit cleaner. I'll keep the asTypeOf in mind for the future though. Michael