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Pekr, I'm done. Thanks. Initially I could not figure out how to escape 
the space character.

the funny thing (that confused Pekr) about it is, that you did not 
realize, that there was no need to escape the space character :-)

for Graham : delimeters = delimiters

I should've read the entire core manual before posting. Chapter 15 
addresses this clearly.

Processing text and rules seems so natural to rebol. I think I'm 
going to enjoy this.

Once you get used to PARSE, it's very hard to use other languages. 
:-)

I've uploaded my old Whitespace interpreter that I implemented in 
PARSE to GitHub: http://github.com/hostilefork/whitespacers/raw/master/rebol/whitespace.r

What's interesting about it is the comparison that can be made to 
implementations in other languages.  I've collected implementations 
of whitespace interpreters in several other languages (by other people) 
in that project: http://github.com/hostilefork/whitespacers/

Ok, continuing the discussion from "Performance" group, I'd like 
to ask for some help with parsing rebol format files.

Basically, I'd like to be able to extract a block near the beginning 
or end of a file, while minimizing disk access.

The files to be parsed could be large, so I don't want to load the 
entire contents, but chunks at a time.

So my parse rule should be able to detect when the input has been 
exhausted and ask for another chunk.

(When extracting a block near the end of a file, I'll have to parse 
in reverse, but I'll try to implement that later.)

And why you don't want to use the load/next which was advised?

Using LOAD/NEXT, I still have to use a O(n^2) algorithm. I'd now 
like to do my own parse, which can be O(n).

As far as I know, there is no way to instruct LOAD/NEXT to only read 
chunks from the file as necessary to load the next value.

Which is why, in that algorithm, I had to iteratively: load a chunk, 
append it and try LOAD/NEXT until it succeeded.
Which gives the algorithm O(n^2) performance.

My question for this O(n) parse algorithm is:
What rebol syntax do I need identify?
I suppose all I need is:
 - Comments (lines beginning with semi-colon ; )

 - Strings (single-line "" and multi-line {} ) watching out for escape 
 sequences eg. {^}}
 - Blocks

yes

Have I missed anything ?

just that comment may not be on line beginning so far

and you want to get just the first block? Skipping any content before 
it?

Ok, yes, it may be preceded by whitespace. That seems easy to deal 
with.

Well, I'd like the algorithm to be general enough to get any number 
of block in the file. So far I need just the first, second and last 
blocks.

And you need to parse the Altme's *.set files or something else as 
well?

The *.set files are what I need it for, yes.

I imagine it could be useful in other similar situations, so I'd 
like it to be pretty general.

I suppose a bonus functionality is to be able to get nested blocks.

(And a super bonus will be to get any datatype at any level, but 
I won't bother doing that until I need it.)

Um, actually, I remember years ago I wanted to load the rebol header 
of many files, avoiding loading the whole of each file.

Also char! must be supported:  #"["

So this algorithm could be used for that, too.

Must it ?

I think if I can parse single-line strings correctly, then a bracket 
inside won't cause a problem.

This means I'll be basically ignoring datatypes which allow strings 
in their syntax, and just jumping to the string part.

you are right

I don't think there's any way to make any type with a literal bracket 
in it (except blocks, of course). (But I am worrying about that a 
bit.)

I tried to make some words with a single unmatched literal bracket, 
or literal string delimiter, but I failed so far. They don't load, 
so they won't be in well-formed rebol format files.

And then I tried issues, files, and I can't do it there, either.

One caveat:

Misidentifying as a block, types like (what are they called?) "inline 
types"?
eg.  #[none]

If I don't recognise it as none! (or maybe issue!) , then I might 
accidentally take it as a block.

Or how about this:
#[block! [hello]]

I would probably want to extract  [hello], but if I don't recognise 
this form, then I'd get  [block! [hello]]  instead.

But that's ok, I can probably tweak the parse rule to support those 
later.

Does anyone have any advice on how I should structure this algorithm?

I don't feel confident as I haven't studied parsing theory deeply.
http://en.wikipedia.org/wiki/Parsing

Should I do lexical analysis and syntactic analysis separately ?

I think I can do it all with just one parse, but it might not be 
a good idea.

I'll make a start.

try this: http://box.lebeda.ws/~hmm/rebol/load-first-block.r

but it's not well tested. You can make the chunk size better. It's 
just 200 bytes just to not find the block easily in the first one.

(bigger)

It will not be bug free yet.. the case like #[block! [hello]] will 
fail now as I'm reviewing it.

Having a look. Thanks for posting that.

Also the chunk inside comment line will fail now.. so it's far to 
be complete yet.

I have to say you pulled that out pretty quickly.

the load-last-block function would be more complicated.

I just found something interesting.

I remember Gabriele saying he thought PARSE would convert chars it 
encountered in its rule with strings before using, so these are equivalent:
	parse "a" [#"a"]
	parse "a" ["a"]

(Of course, the first one is a char and not a string, so consumes 
less memory.)

But I was just thinking it might be clearer to use strings instead 
of chars in the parse rule.
Then I discovered you can use issues:
	parse "a" [#a]

and the escape characters is interesting as you only need to type 
one of them in the issue:
	parse "^^" [#^]

Anyway, that's a side-issue.

I would not use it.. I think you chould use char! where you expect 
char!. btw... the script is updated, but would require some test-cases 
(for chunk bordes inside strings/blocks/comments) to make it safe 
for use.

Thankyou very much for this contribution.

It's time for me to sleep. Good night.

Anton, the cost of disk reads dwarfs the cost of LOAD/next. And PARSE 
is much slower at loading REBOL data than LOAD. You might consider 
finding out the max size of the value you are loading, rounded up 
to multiples of 4096 (disk blocks), and just READ/part a bit more 
than that from the disk for each file. Then LOAD/next from the resulting 
string. There is no reason to do speculative reads once you have 
an upper bound on the size you will need to read. In a language like 
REBOL, minimizing disk reads sometimes means minimizing the number 
of calls to READ, not just the amount read.