From: Kaspar Schiess <eule@...>
Date: 2005-01-27T17:10:35+09:00
Subject: Re: When little languages grow...

(In response to news:Pine.GSO.4.60.0501261622400.24999
@brains.eng.cse.dmu.ac.uk by Hugh Sasse Staff Elec Eng)

> I seem to have run into my parsing problem again.  Whatever I'm
> doing I usually end up having to parse non-simplistic input, and I'm
> still not happy about the apparently available solutions to this.
> So I'm wondering what other people do.

Just a few random thoughts: 

- All of this depends on how complex your mini-language is. Should it be 
Turing complete or just really something like fstab ? See 
http://www.faqs.org/docs/artu/ch08s01.html (a chapter from the Art of 
Unix Programming, including a taxonomy of languages). 

- If your language needs loopiness and control constructs like Ruby has 
(or should look like Ruby), then you could help me think about a cut-down 
version of Ruby for configuration files: This version of Ruby should be 
accessible from within Ruby and be totally safe (=cut off of the external 
world). Such a thing would have to be execution timeouted (endless loops 
and stack overflows) too... 

- Your problem was essentially the topic of one of my semester projects. 
I tried out a lot of different parser generators, for different 
languages. None really could score in the category of simplicity. 
Probably just a complex subject. But the problem of having to create a 
minilanguage pops up all the time, as you say. 

- Parse error reporting: A lot of programming languages have bad parse 
error reporting, which essentially comes from the fact that the parser 
generator does not help you in this. ANTLR is a bit better there, but IMO 
still sucks. 

I think there are two really good ideas in this thread: 
a) Create and Integrate Grammars as first class objects in Ruby. 
(Clifford Heath)
b) Use a cut-down version of Ruby that is designed with security in mind 
(various people, this has been done).

I can't help feeling that we're missing a really simple way to do all of 
this ... 

kaspar

hand manufactured code - www.tua.ch/ruby