On 13 September 2011 14:08, Vasili I. Galchin wrote:

Hello,

      I am trying to model multigraphs ....but getting errors with ghci and can't figure out why.... I have a serious blind spot ....

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

module Bonzo where

class Graph arrow node where

     source             :: arrow -> node

     target             :: arrow -> node

You probably want a fundep there: "class Graph arrow node | arrow -> node where ..."

data Arrow = Arrow (Int, Int)

instance Graph Arrow Int where

     source Arrow = fst Arrow

     target Arrow = snd Arrow

Ummm.... that doesn't make sense. Consider this: newtype Arrow = Arrow { arrowPair :: (Int, Int) } instance Graph Arrow Int where source = fst . arrowPair target = snd . arrowPair (Implemented just to be similar to how you've done it; it's not how I would do it in actual code.) Alternatively, you could have kept the data definition as is and had the method instances look like "source (Arrow arr) = fst arr", etc.

ghci> :load junk1.hs [1 of 1] Compiling Bonzo            ( junk1.hs, interpreted )

junk1.hs:19:12:     Constructor `Arrow' should have 1 argument, but has been given 0     In the pattern: Arrow     In the definition of `source': source Arrow = fst Arrow     In the instance declaration for `Graph Arrow Int'

junk1.hs:21:12:     Constructor `Arrow' should have 1 argument, but has been given 0     In the pattern: Arrow     In the definition of `target': target Arrow = snd Arrow     In the instance declaration for `Graph Arrow Int'

This is just from the errors in your method instances. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com