[css-typed-om] Simplifying expressions in CSSMathValue subclasses.

[darrnshn]

In the algorithm for adding CSSNumericValues, we have an explicit step (Step 3) for simplifying the expression if all values are CSSUnitValues with the same unit. However, none of the other subclasses seem to have this step, even though theoretically it should be possible (e.g. min(2px, 3px) == 2px). The inconsistency is a bit confusing and makes addition a special case in terms of implementation. Is there any particular reason for making addition simplify but not the others?

[tabatkins]

.add(), .sub(), and .negate() all have special cases. (.sub() has it by virtue of calling into the .add() algo.)

.mul() could have a special-case here - check if this + every value is a CSSUnitValue with type=number, or at most one has a different type, then return a plain CSSUnitValue accordingly.

.div() could have a special case - if this+values are all CSSUnitValue, and all the values (not this) are number type, we can return a CSSUnitValue.

.invert() could special-case a number type.

I agree that .min() and .max() could have the exact same special-case as .add().

Happy to add these in if others think they're good ideas.

[css-meeting-bot]

The Working Group just discussed Simplifying expressions in CSSMathValue subclasses, and agreed to the following resolutions:

• RESOLVED: mul/div/invert/max/min should have shortcut just like add/negate/sub

The full IRC log of that discussion

<nainar> Topic: Simplifying expressions in CSSMathValue subclasses
<TabAtkins> GitHub: https://github.com//issues/503
<nainar> TabAtkins: Math operations in class - you can say px.add(some other px value)
<nainar> TabAtkins: some operations have shortcuts to return the operation. Have it for add, negate, sub. Dont have it for mul, div, invert, min, max
<nainar> TabAtkins: Have a sketch in the issue of how to do it. Apply these shortcuts to all operations or none?
<nainar> TabAtkins: Add always returns a CSSMathSum or a unit vaoue if easily computable?
<nainar> Rossen: Similiar with what we do to calc? You simplify it down
<TabAtkins> https://drafts.css-houdini.org/css-typed-om/#dom-cssnumericvalue-add
<nainar> TabAtkins: Yes
<nainar> Rossen: would align with what we already expose
<nainar> TabAtkins: People who want to increment length by 1 unit will still get a length unit and not complicated expression
<nainar> dholbert: addition inside of a loop - would be a good idea
<nainar> Rossen: it keeps expanding.
<nainar> RESOLVED: mul/div/invert/max/min should have shortcut just like add/negate/sub