#11776: Holding an expression unevaluated: Something like hold_all() would be
nice.
---------------------------+------------------------------------------------
Reporter: Pap | Owner: jason
Type: enhancement | Status: new
Priority: minor | Milestone: sage-4.7.2
Component: misc | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
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Comment(by nbruin):
Replying to [comment:2 Pap]:
> I know, that's why I suggested a different approach in the end of the
Sage worksheet I posted (http://test.sagenb.org/home/pub/6). Something
like
{{{
unevaluated_expr=hold_all( integrate(x^2+1,x) + diff(tan(x),x) )
}}}
That is exactly what can't work. This is equivalent to the python code
{{{
unevaluated_expr=hold_all(integrate(x^2+1,x).__add__(diff(tan(x),x)))
}}}
The meaning of this in python is that what is inside the parentheses gets
executed and the result of that gets passed to {{{hold_all}}}. By the time
{{{hold_all}}} executes, it is already too late. You need to inform
{{{integrate}}}, {{{__add__}}} and {{{diff}}} that they need to behave
differently from what they would normally do. The "hold" parameter does
that, but as you observe, it is rather burdensome that it potentially has
to be supplied to all routines involved (the fact that not all relevant
routines accept "hold" yet is just a matter of a bug to fix).
Basically what is needed is a flag on SR that sets "hold=True" to be the
default rather than "hold=False". I don't know how easy and how thread-
safe such a flag would be. It would definitely violate the stipulation
that parents be immutable.
A "clean" solution would be to allow a second instance of SR that does
have "hold=True" as default? This would allow something that you'll
probably find painful too:
{{{
sage: SRheld = SymbolicRing( hold_by_default = True )
sage: SRheld(1) + SRheld(3) #note the need to turn 1,3 into symbolic
objects before adding
1 + 3
sage: x = SRheld.var('x')
sage: integrate(x^2+1,x)
integral(x^2+1,x)
sage: xt = SR(x) # the normal SR still behaves as before
sage: integrate(xt^2+1,xt)
1/3*x^3+x
}}}
I'm afraid the proposed namespace magic I proposed earlier will never
fully work because {{{SR(1).__add__(3)}}} can't be reached that way. So we
need to find a convenient place to store the "hold default value". SR
itself would be a reasonable place except that changing the value
definitely changes how SR behaves and parents are supposed to be
immutable.
Perhaps if we make a context manager that sets and resets a
"hold_by_default" flag, we localize the potential trouble a bit.
{{{localvars}}} has set a precedent for such:
{{{
with held_function_context(SR):
expr=sin(pi)^2+cos(pi)^2
}}}
If in addition it would hold a lock on SR we would be thread safe as well
(just not very thread friendly).
I think there is merit in having "hold" facilities more readily available
but to implement it requires some serious architectural considerations and
likely some relatively comprehensive modifications.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11776#comment:3>
Sage <http://www.sagemath.org>
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