On 30 Apr., 23:41, "Aaron S. Meurer" <asmeu...@gmail.com> wrote:
> On Apr 30, 2011, at 1:24 PM, Ronan Lamy wrote:
>
>
>
> > Le samedi 30 avril 2011 à 13:36 -0400, Haz a écrit :
> >> So a few things to respond to...
>
> >> Tom:
> >>> Can you describe what went wrong with these branches? In particular,
> >>> why can the following naive strategy not work:
>
> >>> 1. Make the new system at least as good as the old one. That is
> >>> whenever I can write foo.is_bar I can write ask(foo, Q.bar) and I
> >> will
> >>> get at least as good an answer. In particular Symbol('x',
> >>> positive=True) should register automatically in the global
> >> assumptions
> >>> that x > 0 (if I understand correctly how the new system works...).
> >>> 2. Replace all queries foo.is_bar by ask(foo, Q.bar).
> >>> 3. Remove all implementations of the is_bar properties.
> >>> 4. Remove all remaining remnants of the old system.
>
> >> This is what was largely tried with a branch last summer:
> >> -https://github.com/haz/sympy/tree/disconnect-assumptions-2
>
> >> There was push-back from the community since removing the
> >> Symbol('x', positive=True) syntax was largely frowned upon.
>
> > As I recall, the problems were that it was slow and broke a lot of
> > things. Symbol(x, positive=True) is easy to hack back in.
>
> >> Vinzent:
> >>> I think the only problem could be that ask() is much slower for
> >>> trivial queries, needs to be verified. This should be not too hard to
> >>> fix though.
>
> >> I don't think this is the case. Trivial queries (those that already
> >> have obvious implications) are compiled into quick reference
> >> dictionaries. In particular:
> >> -https://github.com/sympy/sympy/blob/master/sympy/assumptions/ask.py#L114
>
> > I think that the problem is with non-trivial queries, for which that
> > code does nothing.
>
> >> In the end, everything in the new assumption system is done with a
> >> particular assumption context -- design decisions must be made on
> >> which context is used in the ambiguous cases. There is also an issue
> >> (feature request) that makes things more natural that will be worked
> >> on after the assumptions shift has taken place -- things like the
> >> following would be possible:
>
> >> with <some assumptions on variables>:
> >> <do stuff>
>
> >> This gives the assumptions an explicit local scope. The branch from
> >> last summer was mostly filled with fixing test cases so they used the
> >> new assumption system (with the global assumptions), and then cleaned
> >> up after themselves.
>
> >> It should also be emphasized that it isn't just a simple notion of
> >> cleaning up assumptions after yourself -- the cache interacts in quite
> >> an intertwined way. Under the right assumptions, expressions will be
> >> simplified, and variables will be assigned certain values. If that's
> >> stored in the cache, changing any related assumption will invalidate
> >> the result. For this reason, I had to manually clear the cache on a
> >> regular basis. I think that the cache is only really needed for
> >> lengthy solve algorithms (so you don't recompute an expression over
> >> and over), in which case we can probably wipe it after any sympy
> >> command is invoked (i.e., only use the cache while sympy is computing
> >> things, and then flush it just before returning the result). However,
> >> the branches from last summer never made it this far.
>
> > The cache is the source of a lot of problems. We should try to use it as
> > little as possible, even if it's not really possible to remove it at
> > this time.
>
> If a specific algorithm is slow without the cache, I think it should be
> rewritten to not do duplicate calculations.
>
Why? For example in gruntz, the performance depends quite a lot of
caching mrv computations, limit computations, and order computations.
Since these hardly arise in a controlled manner, the only way I can
see to not recompute is to roll a separate cache - what would be the
point of this?
IMHO we should definitely keep the cache. When something breaks it, it
is ill-designed.
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