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