[Haskell-cafe] Funny type signature question

[Peter Verswyvelen]

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 #-}

randomEnum :: forall a g. (Enum a, Bounded a, RandomGen g) => Rand g a
randomEnum = do
    randVal <- getRandomR (fromEnum (minBound::a), fromEnum (maxBound::a))
    return $ toEnum randVal


It is annoying when the type inferer encounters ambiguities - you also get
this all the time when using OpenGL e.g. GL.colour - but I don't know how to
solve this without adding type annotations


On Thu, Apr 2, 2009 at 8:03 PM, Michael Snoyman <michael at snoyman.com> wrote:

> I've butted into this problem multiple times, so I thought it's finally
> time to get a good solution. I don't even have the terminology to describe
> the issue, so I'll just post the code I'm annoyed with and hope someone
> understands what I mean.
>
> import Control.Monad.Random
> import System.Random
>
> data Marital = Single | Married | Divorced
>     deriving (Enum, Bounded, Show)
>
> randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a
> randomEnum = do
>     let minb = minBound
>         maxb = maxBound
>     randVal <- getRandomR (fromEnum minb, fromEnum maxb)
>     return $ head [toEnum randVal, minb, maxb] -- if I do the obvious thing
> (return $ toEnum randVal) I get funny errors
>
> main = do
>     stdGen <- newStdGen
>     let marital = evalRand randomEnum stdGen :: Marital
>     putStrLn $ "Random marital status: " ++ show marital
>
> Any help is appreciated. Thanks!
> Michael
>
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