[Haskell-beginners] Class definition syntax

Joe Fredette jfredett at gmail.com
Sat Oct 31 23:53:16 EDT 2009

Ahh, I see what you need, you want to "lift" the IArray functions into  
your type. Well, Rather than trying to instance the type- you could  
define your type like this:

	newtype Board = Board IArray.IArray ... whatever

	(!) :: Board -> Location -> Int
	(!) = IArray.(!)

That is, create synonyms manually for each function you _absolutely  
need, assuming they don't conflict elsewhere. You would have to  
manually import each -- I feel like there is probably a better way to  
do this, but this will definitely work. Though, I'm not sure why you'd  
need to be instancing another class with a type like this, it's a  
_very_ specific type, I imagine one or the other set of functions  
ought to be easy enough to define simply about the type (dodging the  
typeclass entirely). I imagine extensibility comes to play here.

One thing you might be able to do is

	class IArray a Location Int , OtherClass a ... => MyClass a ... where

Which would force you to have a type which is an IArray of Location ->  
Ints, and an OtherClass, etc. I don't know all the details of your  
implementation, so I don't know how well this would work, but I  
imagine thats probably the "better" solution I'm thinking of...


On Oct 31, 2009, at 11:42 PM, Shawn Willden wrote:

> 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,
> 	Shawn.

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