On Tuesday 25 May 2010 20:51:06, Eugeny N Dzhurinsky wrote:
Hello, all!
I'm trying to create set of polymorphic functions for working with custom containers. I decided to try out typeclass and define generic function, which uses the methods from the typeclass. The quick and naive solution is listed below:
As Stephen said, FunctionalDependencies and TypeFamilies are two options. The problem is that stList is unusable because there's no way to find out which instance to use (determine k and t) from a use of stList. Also, you can't determine a from a use of stExists, so that's unusable, too. You could make all class parameters depend on b (via FunDeps or TypeFamilies), or - make b a type constructor of kind (* -> *) and - move stList to its own class. class StList b where stList :: b a -> [a] class Storage k t b where stExists :: k -> t -> b a -> Bool stAdjust :: k -> t -> (a -> a) -> b a -> b a stInsert :: k -> t -> a -> b a -> b a
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Don't put language extensions in an OPTIONS_GHC pragma, use a LANGUAGE pragma instead.
{-# OPTIONS_GHC -XMultiParamTypeClasses -XTypeSynonymInstances #-} import Data.List as L
class Storage k t a b where stExists :: k -> t -> b -> Bool stAdjust :: k -> t -> ( a -> a ) -> b -> b stInsert :: k -> t -> a -> b -> b stList :: b -> [a]