[Haskell-cafe] problem with collection (container) class

[Daniel Fischer]

Am Donnerstag, 7. Februar 2008 08:58 schrieb Leandro Demarco Vedelago:
> but when I try to load it in WinHugs I get the following error message:
>
> - Instance is more general than a dependency allows
> *** Instance         : Container Abb a b
> *** For class        : Container a b c
> *** Under dependency : a -> b
>
> and as I have stated above my knowledge about dependencies is almost null,
> not to say null, so I don´t even have an idea where the error is.

Maybe ghci's error message is more helpful:
dafis at linux:~/Documents/haskell/move> ghci Leandro
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Leandro          ( Leandro.hs, interpreted )

Leandro.hs:16:19:
    `Abb' is not applied to enough type arguments
    Expected kind `*', but `Abb' has kind `* -> * -> *'
    In the instance declaration for `Container Abb a b'
Failed, modules loaded: none.

There are a couple of other things, mainly that you have the wrong type for 
'add' and you need an Ord constraint for 'search'.
This works:
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{- # LANGUAGE FlexibleInstances # -}
module Leandro where

data Abb a b = Branch a b (Abb a b) (Abb a b) | Leaf

data ListAssoc a b = Node a b (ListAssoc a b) | Empty

class Container c a b |c -> a, c -> b where
    empty :: c
    add :: c -> a -> b -> c
    search :: c -> a -> Maybe b
    del :: c -> a -> c
    toListPair :: c -> [(a,b)]

instance (Ord a) => Container (Abb a b) a b where
    empty = Leaf
    add Leaf x y = Branch x y Leaf Leaf
    add arb@(Branch ni nd ri rd) x y
        |x == ni = arb
        |x > ni = Branch ni nd ri (add rd x y)
        |otherwise = Branch ni nd (add ri x y) rd
    search Leaf x = Nothing
    search (Branch ni nd ri rd) x
        |x == ni = Just nd
        |x > ni = search rd x
        |x < ni = search ri x


Note: The FlexibleInstances Language pragma is required by GHC 6.8.1 and 
6.8.2, but not by GHC 6.6.1 or hugs, I think that's a bug in 6.8.*

> A suggestion that I've received was to change the type of Abb for
>
> data Abb (a,b) = Branch a b (Abb (a,b)) (Abb (a,b)) | Leaf
>
> and declare container class with just two parameters like I found in all
> pages I visited. I have not tried this yet, as I still have hope that what
> I intend to do is possible.
>
> Well if you have any suggestions I'd appreciate you send it to me and sorry
> for bothering you and my english, but i'm "spanish-speaker".

Cheers,
Daniel