[Haskell-cafe] Funny type signature question

[Ross Mellgren]

There's nothing connecting the Enum/Bounded used in fromEnum and min/ 
maxBound to the toEnum, as there's an Int in the middle. Annotated  
very explicitly, the type inferrer probably sees something like:

> randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a
> randomEnum = do
>     let minb = (minBound :: a1)
>         maxb = (maxBound :: a1)
>     randVal <- getRandomR (fromEnum minb, fromEnum maxb) -- a1 here
>     return $ head [toEnum randVal, minb, maxb] -- putting minb and  
> maxb in the list forces the unknown a1 to be a, because lists are  
> homogeneous

So you have to give it some clue what you really want.

-Ross


On Apr 2, 2009, at 2:18 PM, 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 #-}
>
> 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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