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

david48 wrote:

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

data FString = FString !Int !String deriving Show

instance Gadget FString where

at this point fInit has this type: FString -> a -> FString

fInit (FString n _) s = FString n (take n s)

but your implementation has this type FString -> String -> FString These types are incompatible, your fInit implementation should be able to work with whatever type a the caller has choosen to provide. Maybe you should try one of class Gadget g where fInit :: g -> String -> g or class Gadget g a where fInit :: g -> a -> g instead. Tillmann