World: r3wp

What is the difference between BREAK and ACCEPT? Both "break" out 
of the rule, both with success (IMO).

Carl made a distinction in R3 blog, but they currently work the same, 
as far as I can tell, so, the only difference I see is, that ACCEPT 
is more self-explanatory.

Right: synonyms.

I'm still running into some problems with PARSE... mainly from the 
expectation of what ANY and SOME should do.

For example:
>> parse "" [any [copy tmp to end]]
>> tmp
== ""

what do you expect in this case ?

In the rewrite of DECODE-CGI, that behavior of ANY forces me to write:

parse "" [any [end break | copy tmp to end]]


This seems wrong to me if we define ANY as a MATCHing function, not 
as a LOOP function. This topic has been debated a bit between a few 
of us, but I think it deserves more attention.

In other words, is ANY smart about the input?  If there is no input, 
why should it even try?


Of course, in the past we've used ANY a bit like WHILE -- as a LOOPing 
method, not really as a MATCHing method.

It's a small thing, and maybe too late to change. I wanted to point 
it out.

We have so much alternatives that i don't see this as a burden

any [and skip copy tmp to end]
any [copy tmp [skip to end]]
etc...

There are a few ways to do it, but that is not my point.

I see your point, but what if the ANY block contains production rules 
?

parse "" [any [and skip copy tmp to end break | insert "1" and insert 
"2"]]

(i know, stupid example)

We have some cool new parse enhancements; really, really nice some 
of them. What I think will add the most value to PARSE--and maybe 
this is just me--are practical examples, idioms, and best practices.

For example


- Parsing an input that has nested structures, and how to collect 
the values you want.
- Showing the user where the parse failed.
- How to avoid infinite parse loops.
- How to safely modify the input stream.

More advanced examples would be great too of course.

Carl - first "error" in parse rewrite with some/any is the auto protection 
for non advancing input. It is like writting in BASIC

10 Print "Hello"
20 goto 10


... and not expecting it to run forever, because some magical internal 
mechanism kicks-in. If I write the code which could cause infinite 
loop, then be it. For me it causes the opposite reaction - some/any 
are not safe to use, let us use while instead ....


something like: parse str [some [to "abc"]] is so obvious and self 
explanatory, that actually not looping forever almost feels like 
parse error. But - even if I don't like it, maybe most such infinite 
loop hits are more difficult to notice, so that actually the prevention 
might be ok, I don't know. As for me though, I would probably prefer 
some internal capability to detect such case, and some debug option 
to show last rule/position, where it happens ...


I am not fluent enough with parse theory, but maybe it also relates 
to your loop vs matching note above ...

BenBran:
Not sure where to put this so asking here:


I downloaded a web script and it has a  snippet I don't understand:
buffer: make string! 1024         ;; contains the browser request
file: "index.html"
parse buffer ["get" ["http"  |   "/ "  |  copy file to " " ]]

what does:

copy file to " "

mean or do?
tia

The copy and to are parse operations. COPY copies the data covered 
by the next operation, the TO. TO covers the data from the current 
parse position until the first instance it can find of its argument.

So, copy file to " " is the equivalent of this regular REBOL code:
file: if find data " " [copy/part data find data " "]

Sort of. The actual code is a little more complex, more like this:

either tmp: find data " " [file: if 0 < offset? data tmp [copy/part 
data tmp]] [break]

The break being a parse match fail, and file being set to none for 
a zero-length match.

I get whats happening now.  If i compare buffer and file I see the 
clipped text:

>> probe file
== "index.html"

>> probe buffer
{GET /a.html HTTP/1.1
Host: localhost

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/531.21.8 
(KHTML, like Gecko) Version/4.0.4 Safar
i/531.21.10

Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-US
Accept-Encoding: gzip, deflate
Connection: keep-alive
Address: 127.0.0.1}

>>probe parse buffer ["get" ["http" | "/ " | copy file to " "]]
== false

>> probe file
== "/a.html"
 
Should I have been able to see the results instead of  == false?

false is the value returned by the parse function

if you want the value you have to change the parse rule

umm.. parse returns either true or false ...

true if the rule completes to the end, false otherwise

ok I see.  Thanks.

Was going to reply but Graham types faster :)

parse buffer [ "get" [ "http" | "/" | copy file to #" "  ( print 
file) ] to end ]
will return true

PARSE returns true if the rule matches and covers the entire input, 
or false otherwise. Your rule matched but there was input left over. 
PARSE's return value doesn't matter in this case, just whether file 
is set or not. If you are using R3 you can do this too:
parse buffer [ "get" [ "http" | "/" | return to " "]]

That would return the file instead of setting a variable and not 
return false because of leftover input.

>> parse "GET /a.html HTTP/1.1" ["get " return to " "]
== "/a.html"

Note that /all is the default in R3 so you need to specify space 
after GET.

for completeness in R3 - I tried the lines above:

>> parse "GET /a.html HTTP/1.1" ["get " return to " "]
** Script Error: Invalid argument: ?native?
** Where: halt-view
** Near: parse "GET /a.html HTTP/1.1" ["get " return to " "]

I must be missing something simple

What version of REBOL are you using? system/version ...

>> help system
SYSTEM is an object of value:
   version         tuple!    2.7.7.3.1
   build           date!     1-Jan-2010/12:15:27-8:00
   product         word!     View
   core            tuple!    2.7.7
   components      block!    length: 60

That is R2, not R3.

doh!

You were right, it was something simple :)

lol :-)

yes it works perfect in R3. Thanks again.

>> parse [ <tag> ] [ copy t tag! ]
== true
>> t
== [<tag>]

never noticed it made a block! before

>> parse [ <tag> ] [ set t tag! ]
== true
>> t
== <tag>

There's a difference between COPY and SET in block parsing mode.

From the docs:

SET - set the next value to a variable
COPY - copy the next match sequence to a variable

Good the remember when dealing with "sequences":

>> parse [ <tag> </tag> ] [ copy t [ tag! tag!] ]
== true
>> t
== [<tag> </tag>]
>> parse [ <tag> </tag> ] [ set t [ tag! tag!] ]
== true
>> t
== <tag>

the = to.

I've always used 'set ... not sure why I used 'copy this time!

<?xml version="1.0"?>

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"><SOAP-ENV:Body><SelectResponse 
xmlns="http://sdb.amazonaws.com/doc/2009-04-15/"><SelectResult><Item><Name>2010-01-29T09:54:48.000ZI3s3NjIxRjZERDI1MUY0QzQyMDk4M0JDMzkwMERGOEQxQTVDRDY5MzEwfQ==</Name><Attribute><Name>Subject</Name><Value>hello?</Value></Attribute><Attribute><Name>Userid</Name><Value>Guest</Value></Attribute><Attribute><Name>UTCDate</Name><Value>2010-01-29T09:54:48.000Z</Value></Attribute></Item><Item><Name>2010-01-29T09:58:36.000ZI3swMTZBODg3QjAxNDQ2NEU5OENCNTA3OTc5OTg0Mjc1MTJGQzkxQTc0fQ==</Name><Attribute><Name>Subject</Name><Value>First 
Message</Value></Attribute><Attribute><Name>Userid</Name><Value>Graham</Value></Attribute><Attribute><Name>UTCDate</Name><Value>2010-01-29T09:58:36.000Z</Value></Attribute></Item><Item><Name>2010-01-29T11:06:18.000ZI3tFREFCRUYwNTY4OTdBMzcwODM2NzJGQUE5MzAwRUE3NjYwMTMwMTY5fQ==</Name><Attribute><Name>Subject</Name><Value>Index 
working</Value></Attribute><Attribute><Name>Userid</Name><Value>Graham</Value></Attribute><Attribute><Name>UTCDate</Name><Value>2010-01-29T11:06:18.000Z</Value></Attribute></Item></SelectResult><ResponseMetadata><RequestId>14873461-626a-44bf-2d7d-c1b23694b2e0</RequestId><BoxUsage>0.0000411449</BoxUsage></ResponseMetadata></SelectResponse></SOAP-ENV:Body></SOAP-ENV:Envelope>

results: copy []
	parse result [
		thru <SelectResult> 
		some [
			thru <Item> copy item to </Item> (
				?? item

    if parse item [ thru <Name> copy itemid to </Name> thru {<Name>Subject</Name>} 
    thru <Value> copy subject to </Value> thru {<Name>Userid</Name>} 
    thru <Value> copy userid to </Value> thru {<Name>UTCDate</Name>} 
    thru <Value> copy utcdate to </Value> to end ][
					repend results [ utcdate itemid userid subject ]
				]
			)
		]
	]

This parse works fine in R2, but doesn't work in R3 ... I coudn't 
see why last night ... still can't ...

Is that result a block or string ?