Re: [Haskell-cafe] Class/Instance : what am I doing wrong in this example ?

david48 wrote: | I'm really inexperienced at this :

class Gadget g where fInit :: g -> a -> g

data FString = FString !Int !String deriving Show

instance Gadget FString where fInit (FString n _) s = FString n (take n s)

The types of:

fInit :: g -> a -> g

and:

take :: Int -> [a] -> [a]

cause the mismatched types error. You're trying to apply 'take n' to a value of type 'a' ('take n' requires [a]), moreover putting the value of 'take n s' into the FString further constrains its type to be [Char] == String. So either fix the Gadget g class, or fix the Gadget FString instance. Claude -- http://claudiusmaximus.goto10.org