----------------------------------- btiffin Fri Apr 04, 2008 10:35 pm Tutorial: REBOL parse ----------------------------------- An introduction to REBOL PARSE By Brian Tiffin April 2008 PARSE REBOL, the Relative Expression Based Object Language has many features. One of the key elements of this scripting language is PARSE. PARSE opens the door to Domain Specific Languages or Dialects using REBOL terminology. Unlike Regular Expressions, PARSE works more like the pattern matching made famous by SNOBOL and Icon and can resemble Backus-Naur Form. Code examples use >> to signify the console prompt with Major Modes PARSE has two major modes string parsing block parsing String parsing At its simplest, PARSE is used to pull strings apart, using default delimiters >> parse "Hello World!" none == ["Hello" "World!"] REBOL ate the space and returned a block! with two elements the string! "Hello" and the string! "World!" This is using the default parse rules; specified by none. none is a REBOL word that has a type of none! and a value of none. Other delimiters can be specified, such as comma >> parse "one,two,three" "," == ["one" "two" "three"] or dash >> parse "613-555-1212" "-" == ["613" "555" "1212"] or null >> parse/all c-strings "^(null)" Here the /all refinement was used so that spaces, tabs and newlines are included. Grammar Rules String splitting is all well and good, but parse rules can be far more sophisticated. The rules for PARSE use a dialect of REBOL and as part of the Relative Expression acronym, symbols can have different meanings in different contexts. Alternates The vertical bar allows alternates in a pattern match rebol [] parse "a phone" [ ["a" | "the"] ["phone" | "radio"] ] == true The above would be true for "a radio", "the radio", "a phone" or "the phone". The rules can also be in variables, looking a little more BNF. rebol [Title: "Nerds and Jocks"] article: ["a" | "the"] person: ["nerd" | "jock"] parse "a nerd" [article person] == true REBOL datatype parsing This is where things can get fun. The script rebol [Title: "REBOL values"] parse/all read http://compsci.ca [ thru copy title to ] print titleComputer Science Canada ==false will display the HTML title line from the compsi.ca website. REBOL knows about the tag! datatype. Lexically, anything within angle brackets is treated as a tag! So; the thru keyword asks the parser to scan through the title tag, leaving the parse scanner pointing just past the title open tag. copy title informs the parser to keep a copy of the next match that follow in the REBOL variable title. That pattern match happens to be to . Unlike thru, to asks the parser to match up to the tag, leaving the scanner just before the closing tag. And the REBOL variable title, will end up having the text that is between the and tags of the page that is returned by reading http://compsci.ca, be that index.html, index.php or whatever the webserver delivers. Note, this example will return false from the parse. There is more text beyond the so the parse expression actually fails, but we still get our title. A proper parse expression returning true would be rebol [ Title: "truer title parse" Author: "Brian Tiffin" Date: 04-Apr-2008 ] parse read http://compsci.ca [ thru copy stuff to to end ] print stuffThe to end advancing the parse scanner to the tail of the input; something required for a successful and true parse expression. More to it than what is shown here This is just a sample of what can be done with PARSE. There are more keywords, more patterns, more features. But being REBOL; not hundreds, just a dozen or so. With that toolkit of features, a lot of power exists. Last example To end this little tutorial; evaluated parsing. After each successful match, the parse dialect allows expression evaluation. This is where the power of PARSE becomes more apparent, (though perhaps not in this trivial example). This example includes parse looping, REBOL value parsing and expression evaluation. Note that the input is not a string!, but a block! of loadable REBOL values. The some keyword causes a one or more repeat to occur inside the parse, and skip advances the parse scanner (used here to skip over non integers). rebol [] parse [a 1 and a 2 and a 3 and a 1024] [ some [ set val integer! (print [val "squared is" val * val]) | skip ] ] 1 squared is 1 2 squared is 4 3 squared is 9 1024 squared is 1048576 == true Part of the reason for this example is also to raise curiosity so that you will want to visit the online manual at rebol.com to Please note; I froth at the mouth a little bit when I evangelize REBOL, out on the lunatic fringe. Cooler heads do prevail in the majority of the REBOL community and others can do a better job of explaining things than I. Think of me as the nut case sports fan all dressed in body paint with a flame spewing top hat, far too excited to actually make any sense, but meaning well, and hopefully adding to the fun of the event. And I just realized, I didn't even show a keyword Dialect or DSL. Maybe next time if anyone is curious for more. Cheers ----------------------------------- A.J Sat Apr 05, 2008 1:04 pm Re: Tutorial: REBOL parse ----------------------------------- Nice! You should ask Tony if you could make a REBOL Forum :D ----------------------------------- btiffin Mon Apr 07, 2008 9:35 am Re: Tutorial: REBOL parse ----------------------------------- Thanks A.J. I'd do my best to populate a REBOL forum. :) Next Tutorial will either be PARSE and DSL or something on the ease of Client / Server when using the message passing features of REBOL. Cheers, Brian ----------------------------------- Mackie Mon Apr 07, 2008 9:44 am RE:Tutorial: REBOL parse ----------------------------------- If you want to start a Rebol forum, you should make some more, introductory tutorials. If people start leanring, and posting the creation, a forum would probably follow, if you want REBOL forum that is. ----------------------------------- btiffin Mon Apr 07, 2008 12:23 pm Re: Tutorial: REBOL parse ----------------------------------- Mackie; I was thinking the same thing. Let things build up until it seems reasonable to have a full forum. One thing that happened on RebolTalk ... too many forum divisions. There is definitely a balance; one that is easily broken, but hard to define. I've been plopping recent REBOL related posts in this Functional Programming group. A small misnomer, as REBOL isn't clearly defined as Functional ... nor is it OO, nor declarative, nor ... but it has features of many of these paradigms. I won't be pestering Tony anytime soon. ;) But I do hope that the august members of this board give REBOL at least a glance. Cheers ----------------------------------- Mackie Mon Apr 07, 2008 6:42 pm RE:Tutorial: REBOL parse ----------------------------------- I'll be glad to check out REBOL, once I have more experience. For now I'm sticking with Ruby. :D but it does look cool. ----------------------------------- reboltutorial Sun Jun 28, 2009 11:44 pm RE:Tutorial: REBOL parse ----------------------------------- I read chapter 15: http://www.rebol.com/docs/core23/rebolcore-15.html#section-8" Code: spacer: charset reduce [tab newline #" "] spaces: [some spacer] rule: ["a" spaces "b" spaces "c"] parse/all "a b c" rule is OK but if I change rule to just Code: rule: ["a" spaces copy varb to spaces "c"] parse/all "a b c" rule Rebol Console outputs error: Code: ** Script Error: Invalid argument: some spacer ** Where: halt-view ** Near: parse/all "a b c" rule >> Why ? ----------------------------------- btiffin Tue Jul 21, 2009 7:13 pm Re: Tutorial: REBOL parse ----------------------------------- TO sadly only accepts datatype! or value, not a pattern or dialect command. http://www.rebol.com/docs/core23/rebolcore-15.html#section-10.3 If you'd like to work out alternative parse mechanics, we can talk more. Something liketo end will allow the parse to succeed. Without the to end, parse will return false (having not exhausted the input), but will still set varb to "b ". Cheers ----------------------------------- tbrown01 Thu Jan 14, 2010 12:57 am RE:Tutorial: REBOL parse ----------------------------------- Great! You've done your assignment well! Keep it up.... *removed links* ----------------------------------- chiropractic marketing Sat Sep 25, 2010 2:14 am Re: Tutorial: REBOL parse ----------------------------------- Is there anything anymore that's considered a general programming language? Kind of like C used to be, or is it still? *removed link* ----------------------------------- DtY Sat Sep 25, 2010 6:54 pm Re: Tutorial: REBOL parse ----------------------------------- Is there anything anymore that's considered a general programming language? Kind of like C used to be, or is it still? *removed link*I love how this message is *almost* passable as something a human would say.