compile-time regex redux

[janderson]

kenny wrote:
> BCS wrote:
>> Walter Bright wrote:
>>> kenny wrote:
>>>
>>>> I know I'm asking for a lot, but the way templates handle string are 
>>>> still kinda weird to me. Would string parsing in this sort of way be 
>>>> absolutely impossible with templates? I have not had good luck with it. 
>>>
>>>
>>> I just haven't thought about this enough. Certainly, however, solving 
>>> the problem in a more general, D-ish way than regex would be a much 
>>> bigger win. Regex works only for a subset of problems (can't do 
>>> recursive descent parsing with it).
>>
>> As I see it the biggest problem with compile time parsing in D is that 
>> building non linear structure is a pain. Tuples implicitly cated when 
>> passed together an this make some things really hard. Allowing a tuple 
>> to be a member of another tuple would put D template in the same class 
>> as LISP.
>>
>> Another things that might make things easier is some way to mark a 
>> template as "evaluate to value and abandon". This would cause the 
>> template to be processed but none of the symbols generated by it would 
>> be kept, only the value. Of course, suitable restrictions would apply.
> 
> so it would be like writing normal D code inside of a template? Could we 
> use phobos or do more metastring functions like find, strip, etc. need 
> to be reinvented in D. Or can I take those functions and just put a 
> wrapper around it? I think this is getting into the security issues 
> again, but something like this:
> 
> auto my_text = meta trim_whitespace(import("myxml.xml")); // obviously a 
> better keyword should be used, and can only be used in global scope too.
> 
> where trim_whitespace is an actual function that is actually defined up 
> in the file somewhere, and will be compiled, used on the import file, 
> then discarded and the result stored into auto my_text? A day or so ago, 
> someone mentioned a .rc compiler. I personally would use it to parse 
> stuff for interface elements .. to generate them dynamically. We already 
> have libraries to parse XML (XHTML), CSS, and other config type things. 

I don't see any security issues if the compile-time language is 
restrictive enough.  You could still load in files using the new import 
command.

 > It would be SUPER AWESOME to be able to just re-use them as a template
 > without any other extra work!!! Oh man, that has me really excited, if
 > that's possible :)
 >
 > woah!

I agree, this is only a sub-set of what could be done.  You could even 
try out / invent new language features for D before suggesting them on 
the newsgroup and post the code.  Of course that may be a possibility 
with regex as well.

-Joel