On Sat, Dec 27, 2008 at 7:35 AM, Robert Kern <robert.k...@gmail.com> wrote:
>
> I'm spending my holiday tinkering time playing around with Parsing
> Expression Grammars (Google it, if you're curious). I'd like to make a
> PEG implementation that works on arbitrary Python sequences in
> addition to strings and apply it to the task of pattern matching over
> sympy expressions. I have two(!) runtimes implemented now, and they
> seem to work for typical string-parsing tasks (recognition and AST
> construction) but I still need to work on substitution and semantic
> actions.
Pretty cool. I've never heard of PEG before, but after reading some
stuff now, it seems interesting.
> Have you ever felt constrained by the current .rewrite()/.subs()
> functionality?
Yeah. We also have .match().
> Is there some sophisticated pattern-matching available
> in some other symbolic math system that you wish sympy could do? If
I don't know of any, it seems to me Mathematica just has the .match(),
maybe a little bit more polished.
> you can describe what you'd like to be able to do, I'll see what I can
> do about implementing it.
Well, for example I'd like the trigsimp() to be simpler, see
sympy/simplify/simplify.py. It currently uses a mix of .match() and
handwritten parsing. If you know how to make this simpler, for example
using PEG, or anything else, that's be really awesome.
It can simplify some stuff:
In [4]: trigsimp(sin(x)**2+2*cos(x)**2)
Out[4]:
2
2 - sin (x)
But we need to add more rules to it, for example for tan:
In [5]: trigsimp(tan(x)*cos(x))
Out[5]: cos(x)â‹…tan(x)
if you have some ideas how to approach this problem, so that the code
is easy to understand, extensible and maintainable, it'd be cool.
Thanks for any work you do,
Ondrej
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