Re: "Re: [Caml-list] A G'Caml question" + additional info

[Brian Rogoff <bpr@best.com>]

On Sat, 30 Jun 2001, Patrick M Doane wrote:
> On Sat, 30 Jun 2001, John Max Skaller wrote:
> > 	G'caml makes it easier to write readable algorithms,
> > and to do 'cut and paste' genericity. But it can't do what
> > you can do in C++. 
> I agree.  Although I'm not advocating that it do everything you can do in
> C++.

Less talk, more coding examples please. GCaml polymorphism is remarkably
easy to use. A while ago, when there was some discussion of overloading
on the list, Pierre Weis mentioned that he'd like a style of overloading
that would allow us to overload array access, string access, and hashtbl
access. How easy is that? Well, it writes itself from that sentence

generic get = case
  | string -> int -> char  => String.get
  | $a array -> int -> $a  => Array.get
  | $a list -> int -> $a => List.nth
  | ($a,$b) Hashtbl.t -> $a -> $b => Hashtbl.find 
  | ($a -> $b) -> $a -> $b => fun f -> f;;

OK, I'd like a bit of syntax sugar to make this .(), but that generic does
the trick, and then some (it works with a single argument function too). 

This extensional polymorphism is beautiful, as well as practical. One of
the nice things about the STL is the way that it allows you to write the
algorithms independently of the containers, using the iterator interfaces. 
You can write functions over generics like "get" to achieve the same
effect in GCaml. 

> > > The proposed 'include' feature makes it much easier to
> > > extend generic values, but doesn't help with derived generics. 

How doesn't it help? 

> > 	Objects can help, but really that isn't the answer.
> > The answer is more like: what can we do to make C++ like
> > generics properly parametric?
> > 
> > 	It is possible to make STL like algorithms in Ocaml now.
> > The problem is that you have to pass in a LOT of data, such as
> > functions to deref and increment the iterator.
> 
> I think that one can implement much of the STL algorithms in the Caml
> object system (avoiding the need to pass lots of data). 

While I think the STL is a fine design for a language like C++, I'm not
sure that STL emulation is high on my list of criteria for an OCaml 
overloading extension. 

Once that include feature is in *and* it is integrated with the rest of
the type system (modules and signatures, classes, polymorphic variants,
etc.) we'll have an enormous amount of power. 

-- Brian


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