boolean operators

boolean operators

[CALLADINE Pierre]

      Is there a deep (or theoretical) reason for the particular status of boolean 
  operators as "&" and "or"  ( "prefix &" is not valid, for instance ?)

-- 
Pierre Calladine
Universite de Poitiers -        e-mail : calla@matpts.univ-poitiers.fr
Dept. Informatique		Tel: 49 45 37 01
40 Avenue du Recteur Pineau     Fax  49 45 40 10
86022 Poitiers Cedex     

Re: boolean operators

[Pierre Weis]

> 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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