This site is updated infrequently. For up-to-date information, please visit the new OCaml website at ocaml.org.

Re: [Caml-list] Lexicographic sort
• Luc Maranget
[ Home ] [ Index: by date | by threads ]
[ Search: ]

[ Message by date: previous | next ] [ Message in thread: previous | next ] [ Thread: previous | next ]
 Date: -- (:) From: Luc Maranget Subject: Re: [Caml-list] Lexicographic sort
```>
> On Thu, 30 Jan 2003, Francois Pottier wrote:
>
> >
> > Hi,
> >
> > I just found a nice way of writing lexicographic sort in O'Caml
> > using Luc Maranget's recent extension of pattern syntax, which
> > allows a single identifier to be bound in several alternatives.
> >
> > For instance, here is code that sorts integer triples:
> >
> >   let compare (major1, middle1, minor1) (major2, middle2, minor2) =
> >     match major1 - major2, middle1 - middle2, minor1 - minor2 with
> >     | 0, 0, d
> >     | 0, d, _
> >     | d, _, _ ->
> > 	d
> >
> > Quite beautiful. Perhaps this is obvious to many, but I thought
> > I'd post it.
> >
>
> Warning, from the Caml manual,
> http://pauillac.inria.fr/ocaml/htmlman/manual014.html
> ``Or'' patterns :
> If both matchings succeed, it is undefined which set of bindings is
> selected
>
> eg. if (0,0,d) and (d,_,_) succeed, you don't hnow the result...
> yes in this case, it works, but in some other, it doesn't :
> let f = function (_::l) | l -> l
> in f [1;2]
>

Well, Francois's example is a good incentive to suppress this
undefined matching order.

In fact, matching is left-to-right, except when one of the
alternatives is a variable (or an underscore).  I have a loose agenda
to enforce left-to-right matching order, and Francois's code won't be
broken by future versions.

Again this undefined matching order in or-patterns is a mistake of mine
and it should be corrected.

--Luc

-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners

```