Hello, I'm trying to write some code which boils down to this idea : I have a collection (Collection), and I want to play with some annotations (of any type) to the items of the collection. I will often have only one collection and many different annotation sets. What matters to me is to be able to have an arbitrary index type (or at least instances of Ix) for the collection, and use it with the annotations as well. I've come to this code so far : -------------------- {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} -- Just for the example data Thing = Foo | Bar deriving Show -- A collection -- i is the index type -- c is the collection type class Collection i c where getThing :: c -> i -> Thing -- A set of annotations on collection objects -- a is an annotation type constructor (a y is an annotation of type y) class Collection i c => Annotation i c a where collection :: a y -> c -- access the underlying Collection select :: a y -> i -> y -- select the annotation of an item given its index -- My custom collection data Things = Things [Thing] deriving Show -- An annotation set based on a list data MyAnnotation c y = Ann c [y] instance Collection Int Things where getThing (Things ts) i = ts !! i instance Annotation Int Things (MyAnnotation Things) where collection (Ann c _) = c select (Ann _ as) i = as !! i main = print $ select ann (2 :: Int) where ts = Things [Foo, Bar, Foo, Foo] ann = Ann ts ["a", "b", "c", "d"] -------------------- This does not work (No instance for (Annotation Int c0 (MyAnnotation Things)) arising from a use of `select'). I do not know how I can tell the type system that c0 is Things here, if it is actually possible. I'll appreciate any help or comments on how to rewrite this. Cheers, -- Olivier Iffrig