[Haskell-cafe] surprised by type class binding -- is this a bug?

[Greg Meredith]

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
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