World: r3wp

Nice work Francois. Good conversation here too, with important details 
brought out by Ladislav.

Yes... Ladislav reminds me some basic math! God, I felt so stupid 
about this associativity bug! 

The reason why I developped parse-expression.r is because I need 
it to build an companion app for one of the best math book: Calculus 
3d edition from Smith & Minton! For now, I have developped a rebol 
library to transform any vid face into a function plotter, and parse-expression.r 
allows me to use human readable expression in the gui instead of 
guru rebol code :)

Ladislav:

How would you interpret x**-1/2 :
(1) [divide power x -1 2]
or 
(2) [power x divide -1 2]


A previous version of parse-expression.r returned (2)... but i considered 
this as a bug and changed it for (1) (see history 0.9.2 in the header 
of the script). But now, with our discussion on associativity, i 
am not sure anymore... 

Thanks for your help!

Precedence of ** is higher than /
The right form should be (1)

Thanks, this confirms my feeling

the associativity concern operators with same precedence.
** is right associative, the other ones are left associative

So, 2 ** 3 ** 4
is evaluated from rigth to left

like
2 ** (3 ** 4)

>> expr: parse-expression "2**3**4"
== [power 2.0 power 3.0 4.0]

It works :)

hehehe... It has been a long time since I had to implement such a 
challenging problem. It is so fun!

Must have something like yours somewhere, I used the stack approach 
though

like the HP48... I would like to see your stack approach. Should 
be interesting.

Lot of sources everywhere.
But I will look for it tomorrow.

I used this way
http://www.programmersheaven.com/2/Art_Expressions_p1

Thanks for the link. Seems like I did some kind of tree way...

>> std [1 ** - 2 / 3] translate
== [divide power 1 negate 2 3]

, i.e. you could as well "consult" the %evaluate.r script

A new version (0.9.5) of parse-expression.r is available on rebol.org 
(http://www.rebol.org/view-script.r?script=parse-expression.r). This 
fixes two bugs I found when writing the documentation, which is also 
available on rebol.org

Francois, don't forget to post update notices to the Announce group 
as well.

in R2 is there a single word which terminates all depths of parse 
rules?  I can't remember

hehe... I've been trying things for 15 minutes and just as I write 
this... I finally get it... hehehe   'END.

That triggers a backtrack to an alternate if there is one.

yeah... I just did a few tests, and it doesn't work in my case, which 
generates and endless rule   :-(

anything else?

I'm parsing code and I want to stop at any syntax error... right 
now I have no clue what to do!

END means end-of-input, not end-of-rule. If you really want to break 
out, try putting the parse call in a LOOP 1 and then BREAK in a paren, 
or in a function and RETURN or EXIT.

ah, loop 1 [] ... good idea   


I guess I can also force the end of the string... by using this old 
trick...
[here: (here: tail here) :here] 

though using break in a loop is much easier in my case.

thx for the tip... its odd  that I've never had to solve this case 
before , after all these years of parsing   :-)

If you want to fail to an alternate, you can assign the block [end 
skip] to a variable that would normally be set to none, and then 
make references to that variable whenever you want to trigger a failure 
condition.

yeah.. I've already done that one, that's what I usually need... 
but in this case, there is only one success and many failures... 
so its much more work to include the exit rule everywhere.

opt-fail: none

parse "abc" [some [["a" | "b" | (opt-fail: [end skip])] opt-fail]]

If there is only one success and many failures, you can make failure 
the default and set it to something non-failing in the success case. 
If you need to distinguish between failures, set a local variable 
with some distinguishing data. Remember, a predictable failure is 
just an unwanted success.

yeah but I still need to put opt-fail in all rules so its a burden 
I don't need, in fact its even going to slow down the parsing, so 
i'd rather just use a loop   :-)

loop 1 [    parse "abc"   [some [["a" | "b" |  (err-msg: "failed!" 
break)] ]]    ]

BREAK/return is your friend for distinguishing failures :)

Watch out when porting to R3 though, as PARSE itself is considered 
a loop there for BREAK in parens.

ah yes... forgot about that, even better.

in R3, does ( BREAK ) return the whole parse?

(BREAK) breaks out of the whole parse, as does RETURN; BREAK breaks 
out of an interation, which all enclosing rules are, so it will break 
out of any block, not just ANY, SOME and WHILE blocks.

what's the best practice for recognizing words with certain syntax? 
(besides using the various word variants)

i was thinking about using the datatype notation or similar pre-/postfix 
characters

Can you give us examples of these words, you want to get recognized 
by parse? Then it may be easier to give opinions.

If you mean things like "word!", then this is a way:

alfa: charset [#"a" - #"z" #"A" - #"Z"]
numeric: charset [#"0" - #"9"]
alfa-numeric: union alfa numeric

parse "word!" [copy word [some alfa any alfa-numeric "!"] (probe 
word)]

Will also parse "word2!, but not "1word!".

i was not talking about matching string input. that's obvious

parse [some word! other* stuff']

I'm not sure, I understand, but maybe you mean something like:


>> parse [a few words of type word! "string" 1] [some word! string! 
integer!]
== true

Else it would be easier with an actual example.

this was an actual example.

i can match it with [some word!] but i would like to differentiate 
between the words based on what is their last character

i know it's suboptimal, just wondering if still possible somehow 
to reject a matching rule later somehow

parse [xxx*] [set w word! (unless #"*" = last to-string w [doesnt-match])]

Ah ok, then you need to convert the word to a series, for example 
a string, and check on last letter:


parse [some word! other* stuff'] [some [set word word! (print last 
form word)]]

It's tricky to reject a rule later, and PARSE has changed over time, 
so I'm not sure.