On Dec 20, 2007 5:26 PM, Tillmann Rendel
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
That was a very clear explanation of what's wrong, thanks a lot !
you should try one of
class Gadget g where fInit :: g -> String -> g
This will not do, I want instances of gadget that work with other values than string.
or class Gadget g a where fInit :: g -> a -> g
A bit of explanation. I want types of class "Gadget" to be able to hold values of some type. for example, some of the types in the class Gadget would hold strings, some would hold Integers. With a variable of a type belonging to the class Gadget, I want to be able to - Initialise it - Fetch the value it holds - Have the user input the value - Display the value To keep it simple, in this example I only kept the funtion to initialise. So it seems I have to use the multiparameter class version, but I'm stuck here as well : class Gadget g a where fInit :: g -> a -> g data FString = FString !Int !String deriving Show instance Gadget FString String where fInit (FString n _) s = FString n (take n s) fString :: Int -> FString fString n = FString n "" ---------------------- *Main> :r [1 of 1] Compiling Main ( Gadget.hs, interpreted ) Ok, modules loaded: Main. *Main> fString 5 "" <interactive>:1:0: Couldn't match expected type `[Char] -> t' against inferred type `FString' In the expression: fString 5 "" In the definition of `it': it = fString 5 "" *Main>