On Thu, Apr 2, 2009 at 9:51 PM, Felipe Lessa
<felipe.lessa@gmail.com> wrote:
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.