I think the problem is, that in the datatype Elemental the compiler can't tell, what 'a' is. Both of these compile: {-# LANGUAGE ExistentialQuantification #-} ...code... data Elemental = forall a. (Elements a) => Elemental { name :: String, move :: a } or ...code... data Elemental a = Elemental { name :: String, move :: a } For the first, Elemental is an existentially quantified type over a, which takes any 'a' that is in the typeclass Elements. The second is a polymorphic type with a parameter 'a'. On 8/20/10, Jonas Almström Duregård wrote:

Not sure if this helps, but when you have

move :: Elements a => a

It doesn't mean move will return some unknown Elements type, it means you can choose any type which is a member of elements and move will return that type.

This means that

Burn :: Elements a => a

implies

Burn :: Water

Which is wrong.

3 :: Double 3 :: Integer

are both correct on the other hand.

/J

On 20 August 2010 10:58, Michael Bradley wrote:

...

 Hello all!

I have been experimenting with data and class declarations and came across a problem. I have been trying to do this:

 Code:

data Fire = Burn | Ember data Water = Bubble | WaterGun

class Elements a

instance Elements Fire instance Elements Water

data Elemental = Elemental { name :: String,                              move :: (Elements a) => a                            }

So, the idea was that "move" in the data constructor "Elemental" would be able to take any value from either the type Fire or Water. However, the error message tells me this is an illegal polymorphic type.

Therefore, I tried creating a function that could read my value for me after "show" was applied to the move. Hence, the data declaration for Elemental could now assign "move" to a String. The function looked like this:

 Code:

getMove :: (Elements b) => String -> b getMove x = read x :: (Elements a) => a

This will not work either, as the function "read" complains of ambiguity in the letter a. I also tried this (amongst other attempts)

 Code:

getMove :: (Elements b) => String -> b getMove "Burn" = Burn getMove "Ember" = Ember getMove "Bubble" = Bubble getMove "WaterGun" = WaterGun getMove _ = error "Unknown move!"

The above caused the function to infer the type Fire, and then complain about the type Water. So, how can I either create a function that can return multiple types like I am trying to above, or is there a way to adjust the data declaration for Elemental?

Also, I have noticed that 3 :: (Num a) => a will work but Burn :: (Elements a) => a causes an ambiguity error. Why is this the case?

Please help!

Mike

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