[REBOL] Re(2): 'parse trick and 'unset
From: arolls::bigpond::net::au at: 17-Dec-2000 3:34
Change 'id, so that the full code is:
line1: "Ju<li>e<><><<1234>"
line2: "Jules<1234>ffg<5678>"
digits: charset "0123456789"
id: ["<" 3 4 digits ">"]
rule: [a: some [id | [skip b:]] (print copy/part a b)]
parse line1 rule
parse line2 rule
> > Try this out:
> > line1: "Ju<li>e<><><<1234>"
> > id: [thru "<" 3 4 digits ">"]
> > rule: [a: some [id | [skip b:]] (print copy/part a b)]
> > parse line1 rule
>
> Nice trick, I would never have thought of it. It will really be useful for
> 'extract rule checking (see my last post). I was surprised by the
> results of
> the following test though:
>
> >> parse line1 rule
> Ju<li>e<><><
> == true
> >> line2: "Jules<1234>ffg<5678>"
> == "Jules<1234>ffg<5678>"
> >> parse line2 rule
> ** Script Error: Invalid /part count: <1234>.
> ** Where: print copy/part a b
>
> Weird. After some stuffing around I discovered this was because
> 'a and 'b were
> still defined from the last operation, so the values need to be
> cleared. This
> is apparently not as easy as it would seem:
That's not the reason.
The reason is because parsing line2 didn't ever set 'b.
'b was still set to point to line1, with the index
at "<1234>".
Why? Because in 'line2 we instantly found 2 id's and
got to the end in two steps. That's because id
began with [thru ">" ...], which means "jump through
all input until after the first ">".
This happens twice and we're finished already.
Remove the 'thru, and 'id is defined more precisely.
My mistake.
> Finally Anton, your trick always returns true, even if it didn't
> find the id
> !:
>
> line1: "Ju<li>e<><><<no-id-here>"
> parse line1 rule
> Ju<li>e<><><<no-id-here>
> == true
>
> So I'll have to redefine 'rule:
>
> parse line1 rule: [a: (success: false) some [id (success: true)|
> [skip b:]]
> (print copy/part a b)]
> if success [do something]
That's a good idea.
Anton.
