Is pattern matching not allowed for types to be matched as in the following example?
printit :: Int -> IO ()
printit i = print $ "Int i = " ++ show i
printit :: [Char] -> IO ()
printit s = print $ "[char] s = " ++ show s
main :: IO ()
main = do
printit 2
printit "two"
Type classes provide something similar to that:
class Printable a where printit :: a -> IO ()
instance Printable Int where printit i = print $ "Int i = " ++ show i
instance Printable [Char] where printit s = print $ "[char] s = " ++ show s
You probably want putStrLn instead of print in both implementations. You may also like the Typeable class; one could write
printWithType :: (Show a, Typeable a) => a -> IO ()
printWithType v = putStrLn $ show v ++ " :: " ++ show (typeOf v)
...which behaves thus:
> printWithType 3
3 :: Integer
> printWithType "foo"
"foo" :: [Char]
[char] s = is incorrect, it's really char s =. Implementing a Printable instance for lists, or at least for [Char], would be tricky, I suppose.
FlexibleInstances extension. There are more complicated approaches that avoid extensions; e.g. see showList and its history for the core idea.Printable that prints "[foo] = ..." if called with an argument of type [foo], without just defining an instance for every supported case (via FlexibleInstances).printWithType function does it. You can replicate and specialize the core ideas of printWithType to make a leaner typeclass than Typeable, but the basic idea is the same.instance Foo a => Foo [a] to handle all types of lists in one go (provided the element types implement some type class). I'm not too sure, though; maybe you would like to open a fresh question with the details of what you're wondering?