[Haskell-beginners] deducing type of multi-parameter type class

[Alex]

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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