[REBOL] Re: Parse and and recursion local variables?
From: moliad:gma:il at: 17-Mar-2007 19:09
hi Pekr,
I have not completely followed the code part, as its complex but the
recursion issue is simply by the very nature of parse... parse rules are not
a stacked function calls. they are branches of execution with automatic
series pointer rollback on error. so as you are traversing a series, you
really only jump and come back ... no stack push... as you have no
variables
to push on the parse. if rebol did an explicit copy of the
parse rules (thus localizing each rule at each instances), I can tell you
that memory consumption and speed drop would not only be dramatic, but would
render the parser unusable in any large dataset handling.
the current parser is a stream analyser.. where you decide what to do
next... the fact that the stream has a graph, tree, or recursive data
organisation is not parse's fault.
in the current implementation, we are able to parse 700MB files using a
15000 lines of parse rules (cause I know someone who has such a setup) and
it screams!... if we added any kind of copy... any real usage would just
crawl crash rebol and need GBs RAM. as we speak I have a 1300 line parse
rule which handles several kb of string in 0.02 seconds (or less).
So, this being said I know parse is a bitch to use at first... hell I gave
up trying each year for the last 7 years...but for some reason, I gave it
another try (again) in the last months... and well, I finally "GOT" it. its
a strange process, but it suddenly becomes so clear all becomes obvious.
The only thing I can say (from my own experience) ... don't give up...
really do go to the end of your implementation and eventually you might
GET
it too. ;-)
The only thing I can say about parse, is that its usually MUCH easier (and
faster too) to parse the original string data and construct your data set AS
a loadable string. in this way, you just brease through the data linearily
(VERY FAST, no stack issues) and append all nesting in the loadable string
as you go.
simple generic html loading example:
hope this helps
-MAx
;---------------------------------------------------
rebol []
;-
;- RULES
html: context [
output: ""
;protect the global space
data: none
attr: none
val: none
attrstr: none
alphabet: "abcdefghijklmnopqrstuvwxyz"
not-quotes: complement charset {"}
alpha: union charset alphabet charset
1234567890abcdefghijklmnopqrstuvwxyz_ABCDEFGHIJKLMNOPQRSTUVWXYZ
nalpha: complement alpha
path: not-quotes ;union alpha charset "%-+:&=./\"
space: charset [#" " #"^/" #"^-"]
spaces: [any space]
attribute: [
spaces
copy attr some alpha {=}
spaces [ copy val some alpha | {"} copy val any not-quotes {"}]
(append output rejoin [attr " {" val "}"])
]
in-tag: [
"<"
copy data some alpha
( append output join data "[^/" )
spaces any attribute spaces
">"
]
out-tag: [ ["</" thru ">"] ( append output "^/]") ]
content: [copy data to "<" (if all [data not empty? trim copy data]
[append output rejoin [ "{" data "}"]]) ]
;href: [ spaces {href="} copy attrs any path {"}]
;link-tag: ["<A" spaces some [href (append parsed-links attrs ) |
attribute ] ">"] ;[[copy ref-url href (print ref-url)] | attribute ]]
rule: [some [content [ out-tag | in-tag ] ] ]
]
parse/all {<html> <body> <h3 >tada</h3><p><FONT color="#000000" > there you
go :-)</FONT></p> </body> </html>} html/rule
probe load html/output
html-blk: load html/output
; XPATH anyone ;-)
probe html-blk/html/body/p/font/color
ask "..."
On 3/17/07, Petr Krenzelok <petr.krenzelok-trz.cz> wrote:
