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@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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