[Haskell-beginners] Class definition syntax

Shawn Willden shawn-haskell at willden.org
Sat Oct 31 23:42:24 EDT 2009

On Saturday 31 October 2009 08:55:56 pm Joe Fredette wrote:
> Well, I think the issue is you're thinking too OOPy...

I understand what you're saying, but I don't think I am.

> But let me answer the actual problem first, type classes are
> (basically) functions on types. So a type of "kind" `* -> * -> *`
> means it is a type which accepts two type variables. So:
> 	newtype Foo a b = Foo (a, b)

Okay, that makes sense.  What I'd read about kinds was considerably less 
clear.  Thanks.

> 	newtype Board = Board IArray ...
> means that _you can just use the IArray types_! Well, almost, really
> what you want is a type-synonym:
> 	type Board = IArray Location ...
> Now you can write functions like
> 	foo :: Board -> Int
> 	foo = Board !! (1,2)
> and it will "just work" because Board _is_ an "IArray".
> Hope that makes sense...

It does make sense, but it doesn't solve my problem.  See, Board isn't the 
only type I have (and, also, Board has to be a newtype rather than a type 
synonym because it's also an instance of another class -- well, unless I want 
to turn on the extension that allows instances of synonyms, and I'm not sure 
what the etiquette is there), and some of the others aren't just IArrays with 
an aliased name, they have other data elements as well.  For example:

data ScoredBoard = ScoredBoard {
    arry     :: (IArray Location String)
    score    :: Int
    maxScore :: Int

I would like to be able to use (!), (//), bound, range, etc., on those as 
well, and without having to say "range (arry sb)", or having to define a 
bunch of fooRange, barRange, bazRange, etc., functions.

Basically I want to take this set of common array operations and overload them 
for a bunch of different types.  As I understand it, classes are effectively 
the only way to overload in Haskell.

Perhaps it just isn't possible to do what I want?  If kind signatures must 
match, then that's a problem, because different types will have different 
numbers of construction parameters.

Thanks for the help,


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