Joshua Ball wrote:
Here is how I am trying to solve the problem, using multi-parameter type classes.
class NPProblem inst cert where validates :: cert -> inst -> Bool certificates :: inst -> [cert] decide :: inst -> Bool decide i = any (\x -> x `validates` i) $ certificates i
Unfortunately, ghc throws the following type error:
NPProblem.hs:5:45 Could not deduce (NPProblem inst a) from the context (NPProblem inst cert) arising from use of `certificates' at NPProblem.hs:5:45-58 Possible fix: add (NPProblem inst a) to the class or instance method `decide' In the second argument of `($)', namely `certificates i' In the expression: (any (\ x -> x `validates` i)) $ (certificates i) In the definition of `decide': decide i = (any (\ x -> x `validates` i)) $ (certificates i)
Maybe something like?... class NPProblem inst cert where validates :: cert -> inst -> Bool certificates :: inst -> [cert] decide :: inst -> Bool decide i = any (\x -> x `validates` i) $ (certificates i :: [cert]) ...or a functional dependency of some sort... class NPProblem inst cert | inst -> cert where