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 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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On Thu, Apr 02, 2009 at 08:18:27PM +0200, 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 #-}

Just use 'asTypeOf'. It is defined as

asTypeOf :: a -> a -> a asTypeOf = const

so that @asTypeOf x y == x@ but both types are constrained to be equal. The above function would become

randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a randomEnum = do let min = minBound; max = maxBound randVal <- getRandomR (fromEnum min, fromEnum max) return $ toEnum randVal `asTypeOf` min `asTypeOf` max

Note that I use the fact that 'return' is constrained to the type variable 'a' we want to constrain its argument, and the 'asTypeOf' constrains everything to be of the same type. HTH, -- Felipe.

Ah, I did not know this asTypeOf function. But ScopedTypeVariables also allows you to give inner functions type signatures that reuse polymorphic type parameters of the parent scope, which makes code clearer I think. On Thu, Apr 2, 2009 at 8:54 PM, Michael Snoyman wrote:

On Thu, Apr 2, 2009 at 9:51 PM, Felipe Lessa wrote:

...

On Thu, Apr 02, 2009 at 08:18:27PM +0200, 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 #-}

Just use 'asTypeOf'. It is defined as

...

asTypeOf :: a -> a -> a asTypeOf = const

so that @asTypeOf x y == x@ but both types are constrained to be equal. The above function would become

...

randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a randomEnum = do let min = minBound; max = maxBound randVal <- getRandomR (fromEnum min, fromEnum max) return $ toEnum randVal `asTypeOf` min `asTypeOf` max

Note that I use the fact that 'return' is constrained to the type variable 'a' we want to constrain its argument, and the 'asTypeOf' constrains everything to be of the same type.

HTH,

-- Felipe.

Interesting alternative. However, I think the ScopedTypeVariables looks a little bit cleaner. I'll keep the asTypeOf in mind for the future though.

Michael

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