🚀 go-pugleaf

In article <516@brazos.Rice.edu> preston@titan.rice.edu (Preston Briggs) writes:
>In article <14251@haddock.ima.isc.com> karl@haddock (Karl Heuer) writes:
>>which parses "a < x < b" as a ternary operator, 
>
>But you wouldn't want a single production to get a<b<c as a ternary operator;
>you really want to be able to handle a < b < c < ... < n

True; I was oversimplifying.  I'll cover the general case below.

>On the other hand, versions of LISP allow things like
>	(< a b c d)

That doesn't completely solve the problem; I want to be able to write
	0 <= i < N
without having to throw in +1/-1 on half of the expressions.  (I consider
a half-open interval to be the more natural endpoint-representation of a
range.)

Okay, then, here's one way to parse such constructs.  A comparison operator
like "a < b" returns an object of an internal type "cascadable boolean", whose
value is either {FALSE} or {TRUE, b}.  If a cascadable boolean is used in any
context other than the left operand of a compare, it immediately decays
into a normal boolean.  If it is the left operand of a compare, then
"{FALSE} < c" returns {FALSE} (but should probably still evaluate c for side
effects), while "{TRUE, b} < c" returns the same object as "b < c" (another
cascadable boolean).

This would apply to "==" and "!=" as well as "<" "<=" ">" ">=", and I would
make all six operators have the same precedence.  (The only reason for having
separate precedences for equality and relational operators is to allow
	a < b == c < d  /* do the two relations have the same truth-value? */
, which would still be legal if properly parenthesized -- enclosing a
cascadable boolean in parenthesis would also force the decay into a normal
boolean.)

(The above is an example of how to implement it, and perhaps how it would have
to be defined in the official language specs.  The language reference manual
can simply say that "expr relop expr relop ... relop expr" works the way a
mathematician might expect, and the whole construct returns a single boolean.)

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint