[REBOL] Re: parse, again... (or rather: still!)
From: joel:neely:fedex at: 6-Nov-2001 0:33
Hallvard Ystad wrote:
> Just a sec... Why does parse "" [to end to end] yield 'true?
> What happens when 'parse has done the first [to end]? Where is
> 'parse at then? And wouldn't it be simple for rebol to know that
> once 'parse has reached 'end, all further parse instructions
> must fail or be ignored? (I just feel the infinite loop is
Please! NO! Let's not add one more special case that has to
be memorized because it doesn't follow the simple, consistent
Consider this example..
>> parse "x" [to "x" to "x" to "x" to "x" to "x" to "x" skip]
As I understand it (and I'm sure someone will tell me if I'm
off target ;-), the meaning of
is very simple:
Is it possible to move forward 0 or more characters and
arrive at foo?
(I'm thinking here only in terms of the result of the entire
PARSE expression, and ignoring the side-effects for the moment.)
Being able to move forward zero characters is highly useful in
parsing (generally speaking, not REBOL specific) and I'd certainly
not like to lose that ability.
Of course, the meaning of
Is it possible to move forward exactly one character?
and the final result of PARSE means
Did the supplied rule reach the end of the string?
With only these pieces of model, we can accurately predict the
behavior both of the previous example and this one:
>> parse "x" [to "x" to end to end to end to end to end to end]
Now, consider that
Can we match the sub-pattern FOO one or more times?
Match the sub-pattern FOO as many times (>= 1) as possible.
Then it's not surprising that
>> parse "x" [some [to "x"] skip]
will wander off into the weeds and never come back (and we can
get the same with ANY = "... zero or more ...").
I suspect that any programming language sufficienly powerful for
practical use allows one to write an infinite loop. Complicating
it with extra heuristics designed to protect me from myself seems
the road to ruin.
Just my $0.02...
The most important thing in the programming language is the name. A
language will not succeed without a good name. I have recently
invented a very good name and now I am looking for a suitable
-- Donald Knuth