[Haskell-beginners] Polymorphism question from an OO-speaking newbie

[Jan Jakubuv]

On Mon, May 04, 2009 at 10:34:09AM -0500, Joel Neely wrote:
> Short version: Is it possible/reasonable to define a single function
> that accepts a single string or a list of strings (in which case it
> maps the single-string "flavor" over the list)?
> 

Hi Joel,

you are looking for type classes (http://haskell.org/tutorial/classes.html).
They allow you to use the same function name for different types. This is
called ``ad-hoc'' polymorphism and you can define different implementation
of the overloaded function for different types (which is your case). Haskell
also support ``parametric'' polymorphism which can be used when you have the
same implementation for different types (like the function `head` which
returns the first member of a list independently on the type of elements).

Try the following:

    {-# OPTIONS -fglasgow-exts #-}

    quoteS s = "\"" ++ s ++ "\""
    quoteL l = map quoteS l

    class Quatable q where
        quote :: q -> q

    instance Quatable String where
        quote s = "\"" ++ s ++ "\""

    instance Quatable [String] where
        quote = map quote

Then you can use it as follows:

    *Main> "Quux said " ++ quote "foo" ++ " loudly"
    "Quux said \"foo\" loudly"
    
    *Main> quote ["foo", "baz", "bletch"]
    ["\"foo\"","\"baz\"","\"bletch\""]

Don't forget the line `{-# OPTIONS -fglasgow-exts #-}` which enables some
features of GHC you need for this example (but not necessarily for all
examples using type classes). Alternatively you can use the line

    {-# LANGUAGE TypeSynonymInstances,FlexibleInstances #-}

which lists necessary extensions explicitly.

Sincerely,
   Jan.



-- 
Heriot-Watt University is a Scottish charity
registered under charity number SC000278.