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

[Ivan Lazar Miljenovic]

On 13 September 2011 14:08, Vasili I. Galchin <vigalchin at gmail.com> 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 at gmail.com
IvanMiljenovic.wordpress.com