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Re: [Caml-list] Lexicographic sort
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Date: -- (:)
From: Luc Maranget <luc.maranget@i...>
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



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