I think what you're looking for is functional dependencies. Basically, the way you have the typeclass set up right now, "fn $ MyInt 5" could in theory turn into anything; if you wanted, you could declare an "instance MyClass MyInt Double" or anything else. You would like the compiler to be able to automatically determine the output type. Using functional dependencies: class Show b => MyClass a b | a -> b where You could also use type families for this, but I believe you cannot express the "Show" superclass: class MyClass a where type MyResult a fn :: a -> MyResult a instance MyClass MyInt where type MyResult MyInt = MyString fn (MyInt i) = MyString (show i) Michael On Thu, Jul 15, 2010 at 8:59 AM, Alex wrote:

Hi,

I'm trying to get the following code to compile:

======= Main.hs ==========

import IO

data MyInt = MyInt Int data MyString = MyString String deriving Show

class Show b => MyClass a b where fn :: a -> b

instance MyClass MyInt MyString where fn (MyInt i) = MyString (show i)

myprint :: (MyClass a b) => a -> IO () myprint a = putStrLn $ show (fn a)

main = myprint 3

======= Main.hs ==========

with ghc Main.hs -XMultiParamTypeClasses. However, the compiler cannot deduce the type of the b type variable (which in this case is MyString). How can I explicitly tell this information to the compiler?

Many thanks, Alex.

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