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Re: comprehensive list of perl6 rule tokens

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From:
Mark A. Biggar
Date:
May 25, 2005 08:28
Subject:
Re: comprehensive list of perl6 rule tokens
Message ID:
4294998B.2080109@biggar.org
Jeff 'japhy' Pinyan wrote:
> On May 25, Mark A. Biggar said:
> 
>> Jonathan Scott Duff wrote:
>>
>>> On Tue, May 24, 2005 at 11:24:50PM -0400, Jeff 'japhy' Pinyan wrote:
>>>
>>>> I wish <!prop X> was allowed.  I don't see why <!...> has to be 
>>>> confined to zero-width assertions.
>>>
>>>
>>> I don't either actually. One thing that occurred to me while responding
>>> to your original email was that <!foo> might have slightly wrong
>>> huffmanization.  Is zero-width the common case?  If not, we could use
>>> character doubling for emphasis:  <!foo> consumes, while <!!foo> is
>>> zero-width. 
>>
>>
>> Now <prop X> is a character class just like <+digit> and so
>> under the new character class syntax, would probably be written
>> <+prop X> or if the white space is a problem, then maybe <+prop:X>
>> (or <+prop(X)> as Larry gets the colon :-), but that is a pretty
>> adverbial case so ':' maybe okay) with the complemented case being
>> <-prop:X>.  Actually the 'prop' may be unnecessary at all, as we know
>> we're in the character class sub-language because we saw the '<+', '<-'
>> or '<[', so we could just define the various Unicode character property
>> codes (I.e., Lu, Ll, Zs, etc) as pre-defined character class names just
>> like 'digit' or 'letter'.
> 
> 
> Yeah, that was going to be my next step, except that the unknowing 
> person might make a sub-rule of their own called, say, "Zs", and then 
> which would take precedence?  Perhaps <prop:X> is a good way of writing it.

Well we have the same problem with someone redefining 'digit'.  But 
character classes are their own sub-language and we may need to
distinguish between Rule::digit and CharClass::digit in the syntax.  Of 
course we could hack it and say that a rule that consists of nothing but 
a single character class item is usable in other character classes by
its name, but that could lead to subtle bugs where someone modifies that 
special rule to add stuff to it and breaks all usage of it as a 
character class everywhere else.  Now a grammar is just a special kind 
of class that contains special kinds of methods called rules, maybe we 
need another special kind of method in a grammar that just define a 
named character class for later use?  In any case as usual with methods 
a user define character class should override a predefined one of the 
same name.

-- 
mark@biggar.org
mark.a.biggar@comcast.net

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