Hi all, For my own study, I've been playing around with various NP complete problems. Previuosly I was doing so in Java, but because I want to learn Haskell, I'm trying to port the algorithms. In Java, I had an abstract class called AbstractNPProblem which looked like this: public abstract class AbstractNPProblem implements NPProblem { public abstract boolean validates(Certificate c); public abstract List<Certificate> certificates(); public boolean decide() { for (Certificate c : certificates()) { if (validates(c)) { return true; } } return false; } } This has one problem, however: it is slightly dynamically typed. Anybody that implements the verify method must cast the object c to a particular type (could be a bitmask, a list of integers, a subgraph, etc.) I'd like to get rid of this problem in porting to Haskell. 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) Could somebody explain what is wrong with my intuitive approach? Also, is multi parameter type classes the way to go, or is there a better way?