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.