Opened 4 years ago
Last modified 3 years ago
#22707 needs_work defect
Remove CombinatorialFreeModule.__contains__
Reported by: | SimonKing | Owned by: | |
---|---|---|---|
Priority: | major | Milestone: | sage-8.2 |
Component: | algebra | Keywords: | |
Cc: | sage-combinat, nthiery | Merged in: | |
Authors: | Simon King | Reviewers: | |
Report Upstream: | N/A | Work issues: | |
Branch: | u/SimonKing/Remove_custom_contains (Commits, GitHub, GitLab) | Commit: | ba5f963354c69109a6c5abaaaf5dfd1ecbcbafa8 |
Dependencies: | Stopgaps: |
Description
Parent.__contains__
is implemented so that x in R
is basically the same as R(x)==x
.
CombinatorialFreeModule overrides it, so that it becomes x.parent()==R
. That notion of containment is totally against Sage's coercion framework and should finally (after 9 years) be removed.
Change History (5)
comment:1 Changed 4 years ago by
- Branch set to u/SimonKing/Remove_custom_contains
comment:2 Changed 4 years ago by
- Commit set to ba5f963354c69109a6c5abaaaf5dfd1ecbcbafa8
- Status changed from new to needs_review
comment:3 Changed 4 years ago by
- Cc sage-combinat nthiery added
This broadens the result of __contains__
by allowing things that convert into the CFM, so it is a behavior change. I'm not against this change, and I doubt anyone is actually using this distinction, but it is something to be aware of.
sage: F.<x,y> = FreeAlgebra(QQ) sage: R.<x,y> = FreeAlgebra(ZZ) sage: elt = 2*x + y sage: elt in F False sage: elt in R True sage: F(elt) == elt True sage: F(elt) in R False
Also, in the cases of returning False
, it is likely a slower check.
comment:4 Changed 4 years ago by
Hi Simon, Travis,
Hmm, the behaviour implemented in Parent
is tricky and questionable.
Generally speaking, anything involving mathematical equality is
intrinsically a potential can of worm; see e.g.
https://wiki.sagemath.org/EqualityCoercion.
Let's take an example:
sage: S = SymmetricFunctions(QQ) sage: S.inject_shorthands() sage: f = s[3]; f s[3] sage: f.parent() Symmetric Functions over Rational Field in the Schur basis sage: e Symmetric Functions over Rational Field in the elementary basis sage: f in e False sage: e(s[3]) e[1, 1, 1] - 2*e[2, 1] + e[3]
As a user, I might not expect s
to be in e
: it's certainly a
symmetric function, but not one expressed in the e basis. There is a
canonical coercion between the two, but it is nowhere trivial.
Well, maybe I just have to accept that there is no perfect answer and consistency should just be the priority. I don't remember: did we specify explicitly somewhere what the semantic of contains should be, and how it should behave w.r.t. coercion?
In summary, like for equality, I am not comfortable with having coercion involved, but maybe that's ok. On the other hand conversion certainly should not be involved, as there is nothing canonical with them. Maybe that's more or less taken care of by the equality test of R(x)=x.
comment:5 Changed 3 years ago by
- Milestone changed from sage-8.0 to sage-8.2
- Status changed from needs_review to needs_work
does not apply
I didn't run the tests yet...
New commits:
Remove custom containment test in CombinatorialFreeModule