"Kevin Quick"
Yes, I was hoping to use FGL directly (or it's replacement as I've scanned some of the recent Cafe discussions and seen that Ivan in particular is undertaking this).
FGL isn't really set up for this kind of "the data type _must_ be restricted" approach.
The key here is that the decorators for the Node are of type a, and I need that type a to be of (Cls a) because I use the methods in Cls a to implement the Graph functionality. I've attached a simple example below that attempts to demonstrate this need (and my numerous failures).
{-# LANGUAGE RankNTypes #-}
module Main where
import Data.Graph.Inductive.Graph
class Cls a where int :: a -> Int -- just to have something
data (Cls a) => B a = B [a]
-- The intent is that B is a collection of objects fulfilling the Cls -- class interface. It is also the intent to represent B as a Graph -- object. However, in order to create the Graph, the Cls operations -- are needed.
-- To make a Graph representation of B, I need to convert my -- univariant B datatype into a bivariant type. This is odd because: -- (1) I ignore/drop b because it's not needed, and (2) I have a -- constraint on a imposed by B.
data GrB a b = GrB (B a) -- data (Cls a) => GrB a b = GrB (B a) -- no difference in compilation errors
instance Graph GrB where -- instance (Cls a) => Graph GrB where -- error: ambiguous constraint, must mention type a -- instance (Cls a) => forall a. Graph GrB where -- error: malformed instance header -- instance (Cls a) Graph GrB | GrB -> a where -- error: parse error on | -- empty :: (Cls a) => GrB a b -- error: Misplaced type signature (can't redefine the type) empty = GrB (B []) -- error: could not deduce (Cls a) from context () for B
isEmpty (GrB (B l)) = null l
match _ g = (Nothing, g) -- Actually need Cls methods on 'a' type to generate the non-trivial case
mkGraph n e = GrB (B []) -- TBD labNodes g = [] -- TBD
Unless you have something else you haven't put here, I don't see any reason why you have to have the constraint on the datatype rather than on the actual functions (outside of the class instance) you need them for later on.
Thanks again for the advice and help. Sorry I was rude in not answering for so long: shortly after my original post I realized sleep was needed.
Yeah, that pesky sleep thing... -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com