On 08/01/06, Brian Hulley
For example, suppose I'm writing a module M that deals with grammar, where the elements in a grammar rule are parameterised so that rules can be written using strings but processed as if we'd used ints instead:
data Element a = Terminal a | Nonterminal a | Action a
data Rule a = Rule (Element a) [[Element a]]
Now I want to convert elements and rules from a to Int, so at the moment I have to write:
convertElement :: Element a -> CM (Element Int) ...
convertRule :: Rule a -> CM (Rule Int)
for some appropriate monad CM. Whereas I would have much preferred to use just the word "convert" in both cases. It is tremendously annoying to have to suffix everything with the type name.
This is what typeclasses are for. class Convert c where convert :: c a -> CM (c Int) - Cale