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


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