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

data Arrow = Arrow (Int, Int)

instance Graph Arrow Int where

     source Arrow = fst Arrow

     target Arrow = snd Arrow

~
~
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

module Bonzo where

class Graph arrow node where

     source             :: arrow -> node

     target             :: arrow -> node

data Arrow = Arrow (Int, Int)

instance Graph Arrow Int where

     source Arrow = fst Arrow

     target Arrow = snd Arrow

~
~

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'

Regards,

Vasili