Absolute value function for scalar fields

[gh-mjungmath]

Currently, the abs operator cannot be applied to scalar fields:

sage: M = Manifold(2, 'M', structure='topological')
sage: X.<x,y> = M.chart()
sage: f = M.scalar_field({X: x*y}, name='f', latex_name=r"\Phi")
sage: abs(f)
Traceback (most recent call last)
...
TypeError: bad operand type for abs(): 'ScalarFieldAlgebra_with_category.element_class'

In this ticket, we add this feature.

[gh-mjungmath]

New commits:

​fcd64c9

Trac #32396: add __abs__ to chart_func and scalarfield

[tscrim]

LGTM.

[gh-mjungmath]

Thank you for the review!

[egourgoulhon]

Thanks for implementing this. Any reason for the choice of Abs rather than abs for the function name?

[gh-mjungmath]

No particular reason. Do you prefer abs or maybe |.|?

[egourgoulhon]

Replying to gh-mjungmath:

No particular reason. Do you prefer abs or maybe |.|?

I don't have any strong preference. abs sounds more inline with other function names in Sage, while |.| is closer to mathematical notation, so I would slightly prefer the latter. Travis, what do you think?

[tscrim]

I am slightly worried about possible ambiguity with |.|. I might go with abs over Abs just because of \lvert compared to \lVert and to match Sages's abs. However, I didn't (and still don't) really care for which one as it is clear documentation-wise what it is doing (and hidden from the causal user). So I am not the person to ask for a real opinion on this.

[git]

Branch pushed to git repo; I updated commit sha1 and set ticket back to needs_review. New commits:

​2095de3

Trac #32396: adjust abs function name to general Sage convention

[gh-mjungmath]

Alright, I changed it to abs with small letters. I find it a strong argument to say that this follows general Sage convention.

[tscrim]

Thanks. I am setting back to a positive review.