[Haskell-beginners] Class definition syntax

Shawn Willden shawn-haskell at willden.org
Sat Oct 31 22:36:26 EDT 2009

On Saturday 31 October 2009 10:50:10 am Daniel Fischer wrote:
> Or perhaps he should look at the class IArray from Data.Array.IArray, maybe
> he can just declare instances of IArray for his datatypes.
> Without more information, I can't tell which way to go.

Looking into the idea of declaring my types as IArray instances, there's one 
immediate problem:  IArray's only method is "bounds".  All of the functions 
that I want as methods of my class are functions in the IArray module (if I'm 
reading it correctly).

So, it seems like what I want to do is to subclass IArray and add the 
additional methods.  Then I can declare instances for my various types and 
define the methods appropriately.

So, I wrote this:

import Data.Ix (Ix, inRange)
import qualified Data.Array.IArray (IArray, 

listArray   = Data.Array.IArray.listArray
array       = Data.Array.IArray.array              

class (Data.Array.IArray.IArray a e) => MyArray a e where
    bounds :: Ix i => a i e -> (i,i)
    range  :: Ix i => a i e -> [i]
    (!)    :: Ix i => a i e -> i -> e
    (//)   :: Ix i => a i e -> [(i,e)]

type Location = (Int, Int)
newtype Board = Board (Data.Array.IArray.Array Location Int)

instance MyArray Board where
    bounds = Data.Array.IArray.bounds
    (!)    = (Data.Array.IArray.!)

However, the instance declaration gives me a "kind mis-match" error.  It says 
that it expects kind '* -> * -> *', but Board has kind '*'.

So, I tried:

instance MyArray (Board Data.Array.IArray.Array Location Int) where

and other variations on that, but they all give me "Board is applied to too 
many type arguments".

How should this be written?



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