From: Charles Oliver Nutter <charles.nutter@...>
Date: 2009-02-27T12:26:44+09:00
Subject: Re: Nexus Programming Language

J旦rg W Mittag wrote:
> Actually, I believe it is the other way round: embracing ParseTree
> would preclude alternative implementations like JRuby from being
> implemented in the first place.
 >
> Well, I admit, I was exaggerating. However, until recently, JRuby
> *had* a (somewhat) ParseTree-compatible AST. The reason why they
> decided to drop ParseTree-compatibility was simply that it would be
> impossible to evolve the compiler while keeping the AST compatible.

Not impossible...there's just been practically *zero* demand, limited 
resources, and other things that needed more attention. I'm not opposed 
to supporting PT in JRuby, but it's never been a very high priority. And 
now that ruby_parser can be used for almost all the offline PT-based 
tools, there's even less motivation.

> ParseTree is intimately tied to the actual layout of the AST inside
> MRI and to the actual inner workings of MRI. That would mean that the
> alternative implementations would be intimately tied to the inner
> workings of MRI -- in other words, they wouldn't actually be
> "alternative" implementations any more, just clones.

I've also had an issue with PT basically just *being* MRI's AST. We've 
made numerous changes to the structure of the AST, some of them for very 
good reasons. We've also unified a number of constructs that MRI still 
keeps separate. In short, we've evolved, and while PT did a good job 
exposing the general structure of MRI's AST, it also exposes MRI's 
implementation warts. We don't feel a strong need to emulate those 
warts. But it's doable :) The fact that it hasn't been done says more 
about the demand for it than the challenge of making it happen.

> I wouldn't say that. There are better ways than ParseTree. A
> standardized generic, abstract, high-level parser API for example.

ruby_parser handles some of this, but it's another all-at-once parser; 
you can't stop and start it or handle subsections of code easily. 
Really, it's just a port of MRI's Yacc/Bison parser to Racc.

- Charlie