[Haskell-cafe] Haskell 101 on classes .... duh ..... :^)

[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


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
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