[Cuis-dev] No regex in parser

[Casey Ransberger]

Hi Mariano,

I’m familiar with PetitParser, OMeta/Ohm as well. 

What I’m after is more like, “has anyone studied ad-hoc parsers in depth?”

I have a sneaking suspicion that when the grammar is sufficiently trivial, an ad-hoc parser ends up being the smaller, simpler code base versus a Chomskyan setup with a parser generator. But, how do I confirm that this gut-feeling is correct?

I can point at anecdotal evidence. Smalltalks, Lisps, and Forths don’t seem to need the — heh — Chomskyan overhead. Again, the grammars are relatively simple, and that’s probably the “why.” But I’m looking for a formal inquiry into the “how.”

Maybe there isn’t one and I have to do it myself. Cuis would be a great subject, but I’m lazy and not at a college, so I look for research papers. What I’m hoping I can find is research into ad-hoc parsers (with a focus on the scanners they employ.)

Anyway I think I’ve burned enough bandwidth on something only tangentially relevant to the list, so I’ll bow out now. 

Thanks for indulging me, people of Cuis :)

—Casey

> On Sep 24, 2019, at 4:54 AM, Mariano Montone via Cuis-dev <cuis-dev at lists.cuis.st> wrote:
> 
> Hello,
> 
>> El 24/9/19 a las 00:16, Casey Ransberger via Cuis-dev escribió:
>> First, I should say I’m aware that there are external packages that do regular expressions. I’m interested in how we get by without them. 
>> 
>> Found myself marveling again tonight that the Smalltalk parsers of the world generally haven’t used them. That really astounded me at first, as I had been raised on lex/flex for lexical analysis. Later I learned about parsing expression grammars and saw those used for both scanning and parsing.
>> 
>> I still don’t know how to write a scanner without either of those tools, I’m afraid. I’m also honestly not terribly familiar with how the various Smalltalks go about this.
>> 
>> For context: I’ve run across a case where I want to build a scanner in a language that lacks any regular expression support at all, and thought I might look to Cuis for a good example of how to go about it. Sorry that this is only tangentially on-topic.
>> 
>> Questions: 
>> 
>> * What classes / methods should I look at to see the magic?
> 
> Smalltalk uses all sorts of magic in different areas, but I don't think
> there's anything special about how it handles parsing. The Scanner and
> Parser classes are implemented manually.
>> 
>> * Is there a formal name for the kind of pattern matching that Cuis does when tokenizing input? Asking because I’d like to track down research papers as a matter of habit.
> 
> If you are willing to reasearch, then I would look at Parsing Expression
> Grammars. Smalltalk implemention of PEGs is PetitParser from Lukas
> Renggli. PetitParser combines PEGs with a combinators api; it is
> somewhat special, not easily found in other programming languages. The
> PetitParser package for Cuis works very nicely. And this is cool
> Smalltalk stuff using them: http://scg.unibe.ch/research/helvetia
> 
> Cheers,
> 
> Mariano
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