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[Caml-list] Design advice
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Date: 2002-09-30 (15:53)
From: Alessandro Baretta <alex@b...>
Subject: Re: [Caml-list] Design advice

Lauri Alanko wrote:
> This doesn't help you much, but you may be interested to know that in
> Haskell this wouldn't be much of a problem. You would do simply:
> data Suit = Spades | Hearts | Diamonds | Clubs
> 	deriving (Eq, Ord, Enum, Ix)
> ...
> Finally, "Ix" means that the compiler generates some functionality that
> is required for a data type to be usable as an array index. And this is
> probably the feature that you seem to need. In Haskell, you could have
> an array whose _index_ type is Suit. With such an array, there's no fear
> of overflowing, because there simply _aren't_ any index values of the
> proper type that weren't included in the domain of the array.
> Of course you can get the same safety in ocaml, too, but it just
> requires some more work on your part.
> This isn't an unconditional plug for Haskell, mind you. Both Haskell and
> ocaml are fine languages, and each has great features that the other
> lacks.
> Lauri Alanko

type tagged_type = Tag1 of foo | Tag2 of bar | ...

class ['a] tagged_array size =
   val map = Hashtbl.create size
   method set index value = Hashtbl.remove map index;
     Hashtbl.add map index value
   method get index = Hashtbl.find map index
   (* any other methods you might need *)

It's not really an array, but it's almost as fast and works 
very much like one. I think this approach might solve most 
of Dan's problems.


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