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From: Pierre Weis <pierre.weis@inria.fr>
Message-Id: <200212261636.RAA01979@pauillac.inria.fr>
Subject: Re: [Caml-list] Guards vs. conditionals
In-Reply-To: <20021226091303.GA803@iliana> from Sven Luther at "Dec 26, 102 10:13:03 am"
To: luther@dpt-info.u-strasbg.fr (Sven Luther)
Date: Thu, 26 Dec 2002 17:36:06 +0100 (MET)
Cc: caml-list@inria.fr
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[...]
> > So, my question is, is there any objective reason to prefer the
> > pattern-match version over the conditional, or vice versa? Or is it just
> > a matter of coding style?
> 
> It is just a matter of coding style. I think that the if version is
> maybe easier to do prooves on or something such,
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
What do you mean ?

> and that guard version is easier to read and maybe better when there
> are more than one condition, but the compiler does not know when the
> guards cover all the cases, and may output a warning when non is
> needed :
> 
> consider :
> 
> let foo = function
> | i when i > 500 -> 1
> | i when i = 500 -> 2
> | i when i < 500 -> 3
> 
> Which will output a warning.
> 
> Friendly,
> 
> Sven Luther

The guard version has the additional good property of allowing
sequences in the clauses with no need for parens or begin end:

 | i when i > 500 -> print i; printe_newline ()
 | i -> ...

Concerning the conjunction of guards and partial matches, the problem
is covered in large in the FAQ of the language, under the question

* Partial match with guards ?

Have a look to:

http://pauillac.inria.fr/caml/FAQ/FAQ_EXPERT-eng.html#gardes_partielles

In short, you just have to remember that the last clause does not need
a guard: use comments and the warning desapears :). For instance,

 | i (* when i < 500 *) -> 3

Friendly,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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