Re: [Ocaml-lib-devel] Request polymorphic set and map

[Michal M. <mal...@pl...>]

On Tue, Jun 17, 2003 at 04:41:58PM +0900, Nicolas Cannasse wrote:
> > What about:
> >
> > let s1 = ExtSet.make cmp1 in
> > let s2 = ExtSet.make cmp2 in
> > ...
> > let s = ExtSet.union s1 s2
> 
> Should raise something like "exception Comparator_differs" if not physical
> equality between s1.cmp and s2.cmp.

Physical equality of functions can be tricky. Example:

# let g () = fun x -> x;;
val g : unit -> 'a -> 'a = <fun>
# g () == g ();;
- : bool = false

It is possible that you define comparator function to be local (but not
to depend on environment), and your runtime check will fail.

let f () =
  let cmp a b = a.x - b.x in
  let s = ExtSet.make cmp in
  ...
  s

> in functor approach, you cannot actually do it since the comparator is bound
> to a type/module so errors are detected at compilation time and not at
> runtime. 

Well, I have to admit that I like the compile-time check a lot more.

> But that's a small tradeoff compared to the ability to define sets
> without having to define a module for every type you need.

Defining module isn't as painful :-) It can be in fact done in single
line:

module Iset = Set.Make(struct type t = int let compare = compare end)

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv
: When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h