Must useless expressions be evaluated?

[ark at rabbit.UUCP]

Barry Margolin points out that if foo is a reducible function,
then the compiler is allowed to generate code for

	if bool_var | foo(x) then ...

which does not evaluate foo if bool_var is true.  This statement
is correct, but slightly misleading -- the PL/I implementation
I have used (and, if I recall correctly, the ANSI standard) permits
the compiler to use short-circuit evaluation in this context even
if foo is not reducible.  In general, the compiler is only obliged
to evaluate enough of an expression to determine its result.