skaller writes:
 > > > > I'm wondering what concepts Ocaml can't express/enforce? 
 > > > 
 > > > Iterators. 
 > > 
 > > Why can't you do this kind of in ocaml?  Returning something like a
 > > "next" function would seem to give you a very basic (but usable)
 > > iterator.  Which part of the iterator abstraction can't you do?
 > 
 > I suggest you try it. I don't know how to answer the question.

Perhaps I missed  something in this thread but it  is not so difficult
to  write iterators  in  ocaml (of  course it  can  not be  done in  a
systematic way --- may be that was John's point).

In ocamlgraph we provide depth-first and breadth-first graph traversal
both as HOF and as iterators. Iterators have the following signature:

======================================================================
  type iterator
  val start : G.t -> iterator
  val step : iterator -> iterator
  val get : iterator -> G.V.t
======================================================================

(where G.t is  the type of graphs).  It was a lot of  fun coding these
itearators,  and very  useful  to write  a  backtracking algorithm  to
colors graphs.  It was particularly easy as  iterators are implemented
as persistent  data structures:  thus there is  no `undo' part  in the
backtracking code.

I guess one cannot write  a backtracking algorithm using the usual HOF
fold function.

If one is interested the code is here: 
http://www.lri.fr/~filliatr/ocamlgraph/

-- 
Jean-Christophe Filliâtre

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