[REBOL] Re: [parse] Special characters in block parsing
From: volker:nitsch:gma:il at: 3-Apr-2005 19:43
Actually both methods do the same. Only Antons method inlines the "/"
in the main rule:
!>> compose [set word (to-lit-word first [/])]
== [set word '/]
while mine use a sub-rule and references it by name. so i have only to
create that sub-rule in a special way. you could write
divide-word: compose [(to-lit-word first [/])]
to make that rule, thats not the main point ;)
sub-rules are a good trick with parse and compose, because parens have
special meanings in both. in parse it means "process", in compose
expand
. and if you feed compose a parse-rule with parens, it gives
chaos.
;this works:
divide-word: to-lit-word "/"
parse[/] [set word divide-word (?? word)]
;this not
parse[/] compose[set word (to-lit-word "/") (?? word)]
;because compose replaces the (?? word) too.
about: strange '/ is an invalid lit-word, but to-lit-word "/" not:
the rebol parser has a complicated job. here it has to decide what "/"
shall be. it could be the start of /refinement too. so it prefers to
say "hey, invalid!". actually it is invalid by parsers rules, not the
implementation. the parser can not sort it out. but to-lit-word does
not go through the parser, it just takes the string and makes it a
word. and that string is clearly defined: the stuff between "". so
that works. only if you mold it, mold simply join ' and the word-text
and outputs "'/", which is not loadable again. so dont save "handmade"
words!
On Apr 3, 2005 5:12 PM, Arie van Wingerden <[apwing--zonnet--nl]> wrote:
> Volker Nitsch wrote:
>
> >Composing was my first thought too.
> >But this looks cleaner (with complex rules):
> >
> >divide-word: to-lit-word "/"
> >probe parse [/][ set word divide-word ]
> >probe word
> >
> >On Apr 3, 2005 4:39 PM, Anton Rolls <[antonr--lexicon--net]> wrote:
> >
> >
> >>Hi Arie,
> >>
> >>
> >>
> >>>>parse [/] compose [set word (to-lit-word first [/])]
> >>>>
> >>>>
> >>== true
> >>
> >>
> >>>>word
> >>>>
> >>>>
> >>== /
> >>
> >>Anton.
> >>
> >>
> >>
> >>>Hi all,
> >>>
> >>>while experimenting with a small dialect for calculations, I stumbled
> >>>into a problem.
> >>>
> >>>I've done simple things like:
> >>>
> >>> >> parse [6 * 3] [set num1 number! '* set num2 number! (print num1 *
> >>>num2) ]
> >>>18
> >>>== true
> >>>
> >>>However, when I try:
> >>>
> >>> >> parse [6 / 3] [set num1 number! '/ set num2 number! (print num1 /
> >>>num2) ]
> >>>** Syntax Error: Invalid word-lit -- '
> >>>** Near: (line 1) parse [6 / 3] [set num1 number! '/ set num2 number!
> >>>(print num1 / num2
> >>>) ]
> >>>
> >>>REBOL complains, because (of course) '/ is not a proper lit-word.
> >>>
> >>>How can I still parse the / ? I already tried a charset, but I suppose
> >>>that's only possible with string-parsing, or not?
> >>>
> >>>TIA
> >>> Arie
> >>>
> >>>
> >>--
> >>To unsubscribe from the list, just send an email to
> >>lists at rebol.com with unsubscribe as the subject.
> >>
> >>
> >>
> >>
> >
> >
> >
> >
> Hi Anton / Volker,
>
> many thanks for these useful solutions. In my case Volker's solution is
> somewhat simpler to use, but I can imagine situations where Anton's
> solution might be more appropriate!
> I tried both and indeed both do work fine!
>
> What I find strange however is that specifying '/ is an invalid lit-word
> but to-lit-word "/" is not.
> This looks like a contradiction to me...
> Any ideas why this works that way (and I am glad it does ;-) ?
>
> Thanks again!
> Arie
>
> --
> To unsubscribe from the list, just send an email to
> lists at rebol.com with unsubscribe as the subject.
>
--
-Volker
Any problem in computer science can be solved with another layer of
indirection. But that usually will create another problem.
David
Wheeler
