#715: Parents probably not reclaimed due to too much caching
-------------------------------------------------+--------------------------
Reporter: robertwb | Owner: somebody
Type: defect | Status: needs_work
Priority: major | Milestone: sage-4.8
Component: coercion | Keywords: weak cache
coercion
Work_issues: Comparison of the third key items | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies: #9138,
#11900
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Comment(by SimonKing):
It really is not an easy question whether or not we should have "is" or
"==".
On the one hand, we have the lines
{{{
!python
if y_mor is not None:
all.append("Coercion on right operand via")
all.append(y_mor)
if res is not None and res is not y_mor.codomain():
raise RuntimeError, ("BUG in coercion model: codomains
not equal!", x_mor, y_mor)
}}}
in sage/structure/coerce.pyx seem to imply that comparison via "is" is the
right thing to do.
But in the same file, the coercion model copes with the fact that some
parents are not unique:
{{{
!python
# Make sure the domains are correct
if R_map.domain() is not R:
if fix:
connecting = R_map.domain().coerce_map_from(R)
if connecting is not None:
R_map = R_map * connecting
if R_map.domain() is not R:
raise RuntimeError, ("BUG in coercion model, left domain
must be original parent", R, R_map)
if S_map is not None and S_map.domain() is not S:
if fix:
connecting = S_map.domain().coerce_map_from(S)
if connecting is not None:
S_map = S_map * connecting
if S_map.domain() is not S:
raise RuntimeError, ("BUG in coercion model, right domain
must be original parent", S, S_map)
}}}
That would suggest that comparison by "==" (the old behaviour or
`TripleDict`) is fine.
Perhaps we should actually have to variants of `TripleDict`, one using
"is" and one using "==".
Note another detail of sage/structure/coerce.pyx: We have
{{{
cpdef verify_action(self, action, R, S, op, bint fix=True):
}}}
but
{{{
cpdef verify_coercion_maps(self, R, S, homs, bint fix=False):
}}}
Note the different default value for "fix". If "fix" is True then the
coercion model tries to cope with non-unique parents by prepending a
conversion between the two equal copies of a parent.
Since the default is to fix non-unique parents for actions, but not for
coercion maps, I suggest that a "=="-`TripleDict` should be used for
actions and an "is"-`TripleDict` for coercions.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/715#comment:35>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
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