Hmmm.... this is an interesting way of doing it, but I would argue that
it's pointless: the fact that you're using MPTCs doesn't give you
anything extra that the original class. Furthermore, as I said earlier,
it doesn't make sense to constrain the label type just to make an
instance of a type class.
(Now, if we had other functions in there which _might_ depend on the
label types, this _would_ make sense; as it stands however, it doesn't.)
Jason Dagit
Perhaps this "working" example illustrates the change I want to make. Working in the sense that it type checks but it's a silly example just to illustrate the point:
\begin{code} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} module Graph where
import Data.Graph.Inductive.Graph hiding (Graph)
-- Defive some arbitrary class, and give it a 'boring' -- reason to use it. class Cls a where boring :: a
data Blah = Blah
-- Make sure we have at least one instance, but not really needed for this example instance Cls Blah where boring = Blah
data B a = B [a]
data GrB a b = GrB (B a)
-- Just copy the bits from FGL that are interesting here class Graph gr a b where empty :: gr a b -- | True if the given 'Graph' is empty. isEmpty :: gr a b -> Bool -- | Create a 'Graph' from the list of 'LNode's and 'LEdge's. mkGraph :: [LNode a] -> [LEdge b] -> gr a b -- | A list of all 'LNode's in the 'Graph'. labNodes :: gr a b -> [LNode a]
instance Cls a => Graph GrB a b where empty = GrB (B [boring]) isEmpty (GrB (B [])) = True isEmpty _ = False mkGraph _ _ = GrB (B []) labNodes _ = [] \end{code}
The Graph class is actually unchanged other than mentioning 'a' and 'b'. This mention of 'a' and 'b' allows instance writers to add contexts other than () when defining instances.
Jason
-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com