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.
>
>>
>> I think a few things are generally universally accepted (correct me
>> if I'm wrong though):
>> 1. sympy/core/assumptions.py should be removed, and everything patched
>> up to use sympy/assumptions/*
>> 2. sympy/core/logic.py should be removed and replaced with
>> sympy/logic/*
>> 3. Assumptions should probably have the right references to be
>> properly deleted from a context when their symbol is deleted -- this
>> goes for any AssumptionContext (not just the global one)
>>
> I'm not sure about 1: sympy/core/assumptions.py and sympy/assumptions/*
> should be merged, but I don't know how exactly the merge should happen.
>
>>
>> Ronan: Can you elaborate on this? (my memory is failing me at the
>> moment)
>>> * Remove the cumbersome and confusing Assume() syntax
>
> The idea is to write '~Q.real(x)' instead of 'Assume(x, Q.real, False)'.
> It's shorter, easier to understand and doesn't suggest wrongly that
> creating the object does anything beyond that.
>
+1. And I agree that Assume() can be confusing (especially considering that
Maple's assume() actually does create global assumptions, for example).
Aaron Meurer
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