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
<mich...@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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
