[Haskell-cafe] Funny type signature question

[Michael Snoyman]

Peter and Lennart,

Scoped type variables is exactly what I needed to know. Thanks for solving
this annoyance for me!

Michael

On Thu, Apr 2, 2009 at 9:18 PM, Peter Verswyvelen <bugfact at gmail.com> 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 #-}
>
> 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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>>
>>
>
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