Haskellians, i'm sure i don't understand type classes, yet. Still, i was surprised at ghci's response to the code below. Clues gratefully accepted. Best wishes, --greg -- transcript -- Prelude> :l grn -- [1 of 1] Compiling GeneticRegulatoryNetwork ( grn.hs, interpreted ) -- grn.hs:33:35: -- Couldn't match expected type `b1' (a rigid variable) -- against inferred type `b' (a rigid variable) -- `b1' is bound by the type signature for `sequence' at grn.hs:25:36 -- `b' is bound by the instance declaration at grn.hs:31:0 -- Expected type: [b1] -- Inferred type: [b] -- In the expression: molecules -- In the definition of `sequence': -- sequence (Site l1 molecules) = molecules -- Failed, modules loaded: none. -- Prelude> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} -- -*- mode: Haskell;-*- -- Filename: grn.hs -- Authors: lgm -- Creation: Thu Dec 6 15:38:26 2007 -- Copyright: Not supplied -- Description: -- ---------------------------------------------------------------- module GeneticRegulatoryNetwork where data Segment p = Nil | Section [p] (Segment p) deriving (Eq, Show) class BindingMolecule b l | b -> l where name :: b -> l complement :: b -> b complements :: b -> Bool class Locale s l1 l2 | s -> l1 l2 where label :: s -> l1 sequence :: (BindingMolecule b l2) => s -> [b] provides :: (BindingMolecule b l2) => s -> [b] -> Bool matches :: (BindingMolecule b l2) => s -> [b] -> Bool data (BindingMolecule b l2) => Site b l1 l2 = Site l1 [b] deriving (Eq, Show) instance (BindingMolecule b l2) => Locale (Site b l1 l2) l1 l2 where label (Site l1 _) = l1 sequence (Site l1 molecules) = molecules provides (Site l1 molecules) molecules' = False -- tbd matches (Site l1 molecules) molecules' = False -- tbd -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com