At 13:41 -0800 03/03/31, Dave Long wrote:
Norm thinks about short-circuit operators in:
<http://www.cap-lore.com/Software/McDijk.html>
McCarthy is credited with observing that in a programming language it
is useful to require that a boolean operator between two predicate
expressions not evaluate the second operand if the operator result is
determined by the value of the first. Thus the C expression "(p && p->
field)" is safe even when the pointer p is NULL, for then the
indeterminate side effects of fetching the field are suppressed.
and among other things, comes up with:
(p && (a, 1) || (b, 0))
as an expression encoding an if-then-else.
However, given a short-circuit NAND operator,
we wouldn't need both AND and OR. Using the
Schaeffer stroke, |, for NAND, and taking it
to be a short-circuit operator (assuming the
commands implicitly succeed), we recover the
Algol68 brief notation:
(p|a|b)
as a shortcut for:
if p then a else b fi
Seems like a pun that requires knowledge of both Iriquois and Punjabi.
I had trouble swapping both idea sets in at once.
I think that if | is nand and is left associative then the boolean value of 'a'
determines whether 'b' is executed.
If right associative then neither a nor b is executed when p is false.
It is scary that it almost works.
By the way after years of withdrawal symptoms there is a new Algol68 compiler!
See <http://www.xs4all.nl/~jmvdveer>.
-Dave
--
Norman Hardy <http://cap-lore.com/>
