Re: and then

[news.crd.ge.com!sunup!duncan@uunet.uu.net (Art Duncan)]

I think the question of "and then" was discussed in this group
about a year ago.  One question was whether or not an expression like

	if X /= 0 and Y/X > 3 then
	  ...
	end if;

should raise an exception if X = 0. If "and" is optimized to "and then"
it will not. However, the prevailing wisdom at that time was that the
above expression should raise an exception when X = 0.

There is also a matter of uniformity of the language. If B1, B2, and B3
are boolean vectors of the same dimension, the expression

	B1 and B2 and B3

will (and should) and them all together.

There is also the phrase " ... and the operands of an expression that does
not contain a short-circuit control form, are evaluated in SOME order that
is not defined by the language." This means that it is perfectly legal for
the compiler to generate code that evaluates

	X/Y > 3

before evaluating

	X /= 0

The term "some order" is used frequently in the LRM, with the implication
that any expression that depends, for its meaning, on the order of
evaluation is erroneous. Thus, if one wishes to force the order of
evaluation, the only linguistically correct method is to use the
short circuit forms.

Cheers,

	- Art Duncan
	  General Electric Company
	  Corporate Research and Development
	  Schenectady, NY 12301
	  duncan@crd.ge.com