World: r3wp

in R2,  the best way is to set a word to the value you're evaluating, 
and conditional to that value's pass/fail, you switch the following 
rule to allow it to continue or track back, so it matches another 
rule.


here is a rather verbose example.  note that this can be tweaked 
to be a shorter rule, but it becomes hard to map out how each part 
of the rules relate.. here, each part is clearly layed out.

rebol []


pass-rule: [none]
fail-rule: [thru end]

condition-rule: pass-rule



parse [
	word-a
	"ere"
	835
	word-b
	15
	word
	86
	bullshit
	#doglieru3
	word-c
][
	any [
		
		[
			; this rule only matches words ending with "-?"
			set val word! 
			[
				[
					(
						val: to-string val
						either #"-" = pick (back back tail val) 1  [
							condition-rule: pass-rule
						][
							condition-rule: fail-rule
						]
					)
					condition-rule
					(print ["PATTERN WORD:" val])
				]
				|[
					(print ["Arbitrary word: " val])
				]
			]
		]
		
		| skip
	]
]

ask ""

in R3 there are some new Parse ops which allow to make this almost 
a one-liner

(I just don't have time to build you an example... :-p  )

R3 but obfuscated.

match: [
	  some [thru #"-"] skip end 	(print [w "end with -?"])
	| some [thru #"*"] end		(print [w "end with *"])
]

parse  [
	word-a 	"ere" 	835 	word-b 	15
	word* 	w86	bullshit*	#doglieru3 	word-c
][
	some [
 		  and change set w word! [(form w)]
		  change [do into match | skip] w
		| skip
	]
]

hmm.. thanks a lot guys!
so practically i can fail in R2 by trying to match 'none?

no, by trying to match  [thru end]

none    *never* fails.

oh, so u can't go THRU end only TO end ?

yep.

it going thru end would break space-time, so its not allowed by the 
interpreter    ;-)

going thru end would break space-time

 - it is allowed in R3 and there is no reason to break anything, in 
 fact. It is just about the implementation.

Ladislav: any other pass/fail technique in R2?

imean dynamic pass/fail

Did you check the


http://en.wikibooks.org/wiki/REBOL_Programming/Language_Features/Parse/Parse_expressions#Parse_idioms

article?

I guess, that the methods described in the idioms section can be 
used (but have not read the above discussion thoroughly).

[thru end] is not a good rule to use to fail. A much more reasonable 
rule is [end skip]

i just read carl article about the call for making that idiom page. 
but when i checked it didn't have much stuff yet. thx for the reminder!

Lad   [thru end] means *exactly* the same thing as  [end skip].  
 I don't know why R3 decided to change that, but I find that a regression.

It does not, Max.

[thru end] is supposed to mean:

[end | skip]

, i.e. it fails in R2 only because of the faulty implementation

Err, correcting myself

a: [thru end] is supposed to mean the same as a: [end | skip a]

And that should never fail

See the above section.

thru is supposed to move the cursor *past* a match,  you cannot go 
past the end, you can only be at the end (the same way you cannot 
go past the tail).

Where do you think the cursor is after matching the [end] rule?

Just try the idiom

a: [b | skip a]

and you will see, that it always means the same as

a: [thru b]

(no matter how many characters the B rule matches)

to doesn't match the end... it moves to it..  its different than 
simply putting  end in a rule.  thru is supposed to move PAST the 
result of a to.

>> parse/all "12345" [[to "5"] a: (probe a)]
5
== false
>> parse/all "12345" [[thru "5"] a: (probe a)]

== true

>> parse/all "12345" [[to end] a: (probe a)]

== true


so if I try to move past the end, its logical that it raises a failure, 
since it cannot advance one more character.

That "advance one more charactef" is where you are wrong. The THRU 
directive has to stop after matching the rule, not "advance one more 
character".

In this case there is no right or wrong, its a question of opinion. 
 There is no *after* the end, as far as I am concerned.

Sorry, but it is not a question of opinion.

There may be just a correct implementation or a bug.

The fact, that there is no "advance one character" is quite obvious. 
Every rule matching advances as many characters as the rule being 
matched prescribes. For example, when matching

    parse "aaaaa" [before: "aaa" after: to end]
    index? before ; == 1
    index? after ; == 4


the rule matches a three character string and, therefore, the correct 
position after the match is three characters past the before position 
(not one character, as you incorrectly stated)

Being at it, the following case reveals a PARSE implementation bug:

     parse "aaaaa" ["" to end] ; == false

, since the empty string should match.

first of all, your before after parse rule has nothing to do with 
the to/thru handling.


yes, this second rule shows an actual bug... "" and none rules are 
conceptually equivalent.

to/thru are not matching rules, they are skipping rules.  matching 
rules always return *past* the match. not *at* the match, like to 
will do.

OK, checking the situation of the NONE rule:

    parse "aaaaa" [before: none after: to end]
    index? before; == 1
    index? after ; == 1

So, the parse position before and after the match remained the same, 
not "one character past the match"

So, generally, the positions before the match and after the match 
can differ, optionally, but the difference is not prescribed to be 
exactly one character.

no the cursor did not advance.   there are two concepts at play here... 
the notion of index, and the notion of "slots"   a single slot has 
two positions, its start and end, but it has only one index.

a better name for slot, probably is segment... just like in video 
editing.

Now, check the PARSE documentation at:

http://www.rebol.com/r3/docs/concepts/parsing-summary.html

and then we can continue the discussion

a matching rule, will expand the segment's area, but not its index. 
 rules are stacked based on end-to segments.  if a rule has a segment 
of size 0 (as in the none rule) there is no index change in the next 
rule segment. i.e. it shares its index since its index is previous 
index + 0

to/thru are not matching rules

 - sorry, once again, an incorrect opinion. They can be used to match 
 the input like every other PARSE rule.

...and they can either succeed or fail

Exactly like the idiom a: [b | skip a], which you did not even try

well... to/thru are listed alongside  skip under "skipping input" 
in both r2 and r3 docs...  they cannot use sub rules, since they 
only do a find on the input.

Did you really read the documentation?

You could notice: "thru rule" "scan forward in input for matching 
rules, advance input to tail of the match "

(no purported "one character")

And, the fact, that the rules are listed under "skipping input" does 
not support your incorrect "are not matching rules".