On Feb 25, 2:48 am, Ira Baxter <idbax...@semdesigns.com> wrote: > On Feb 25, 3:05 am, David Kirkby <david.kir...@onetel.net> wrote: > > > On 25 February 2011 05:57, Ira Baxter <idbax...@semdesigns.com> wrote: > > > > Mathematica is otherwise not hard to parse, and you don't need > > > a hand-written parser to do it. > > > > Ira D. Baxter, CTO > > > Semantic Designs, Inc. > > Part of this discussion started because Dave suggested that > writing and maintaining a hand-written parser was harder > than a parser-generator one, and consequently that Wolfram > probably didn't write a parser by hand.
Unless a language changes, it is not often that one is compelled to change a parser. There are lots of reasons not to change a language. Which is harder .. changing a hand-written (typically the only plausible technique is recursive descent), parser vs. changing the syntax table and augments for a (say) LALR or perhaps GLR parser? Why Dave should have an educated opinion on this, who knows. He claims to not be an expert, and I accept that assessment. > > Writing a top-down recursive descent parser isn't terribly hard for > a lot of languages; I've done many this way. > Mathematica in fact is probably singularly > easy, because of its Lisp-like syntax. I suspect you do not mean syntax here, unless you mean FullForm. > Lisp parsers have been > pretty much hand-rolled from day one because of this > rather simply notation with convenient brackets to > guide the parser as to when to push, and when to pop. Um, there are a bunch of things wrong with this simple sentence. 1. Programs that read lisp are called readers, not parsers. One can write a lisp reader for traditional lisp symbolic expressions in a few lines of code. 2. That code would not include any instructions like "push" or "pop". It would be a set of mutually recursive programs to read atoms and to read lists or "dotted pairs". 3. Common Lisp has a highly customizable reader, where programs can be attached to characters. This is fairly hairy. > > The rumors that I heard was the the early versions of MMa > were based in Fortran. My guess is that Wolfram did write > a recursive descent parser by hand, because you can do that > easily even in Fortran. I am unaware of such rumors, but it sounds pretty doubtful. Wolfram and friends wrote SMP at Caltech. I think that Wolfram was able to write programs in C. > > I would further guess there isn't/wasn't any great reason to change > it. > Final remark: the effort to write the parser is tiny compared to > the effort to build the rest of any interesting system attached > to a parser. The same is true for maintenance costs. I agree. For example, I find that merely parsing the language is a lisp program of about 1,300 lines. The pattern matcher is about that size, as is the display program. Doing polynomial and rational arithmetic is again comparable in size to the parser. RJF -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
