From: Pierre Weis <weis@pauillac.inria.fr>
Message-Id: <199506081634.SAA29211@pauillac.inria.fr>
Subject: Re: boolean operators
To: calla@matpts.univ-poitiers.fr (CALLADINE Pierre)
Date: Thu, 8 Jun 1995 18:34:37 +0200 (MET DST)
In-Reply-To: <9506081217.AA02365@matpts.univ-poitiers.fr> from "CALLADINE Pierre" at Jun 8, 95 12:17:13 pm
> Is there a deep (or theoretical) reason for the particular status
> of boolean operators as "&" and "or"
> ( "prefix &" is not valid, for instance ?)
The problem is that there are no corresponding functions in Caml:
these syntactic constructs are equivalent to an if then else
expression, not to a function call to some primitive operation (as for
+ for example).
On the other hand, since Caml evaluation obeys call by value, there is
no means to define a function that does not evalute its
arguments. Hence you cannot define a function f such that
e1 & e2 could be equivalent to f e1 e2.
In effect, when e1 evaluates to false, the construct e1 & e2 returns
false and e2 is not evaluated. By contrast, the call f e1 e2 may
return some value but e2 will be evaluated before entering the body of
f.
Put it another way, & is a lazy construct: if e2 fails or does not
terminate then false & e2 evaluates to false, while any function call
with e2 as argument will fail or loop.
Pierre Weis
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