[css-values] Clarification on what the second `<easing-function>` in first clause in calc-interpolate do?

[weinig]

The calc-interpolate() grammar is currently:

<calc-interpolate()> = calc-interpolate(
  [
    <progress-source> && [ by <easing-function> ]?
    && <easing-function>?
  ] ,
  <input-position>{1,2} : <calc-sum> ,
  [ <easing-function>? , <input-position>{1,2} : <calc-sum> ]#? )

In that first clause:

  [
    <progress-source> && [ by <easing-function> ]?
    && <easing-function>?
  ] ,

There are two <easing-function> productions.

The first one, with a by prefix, is defined in 7.1.1.1. Easing Interpolation Progress: the by <easing-function> argument , and "It applies the specified easing function to the “timeline” of progress as a whole by modifying the interpolation progress before it’s applied to".

The second one, however, I am not 100% clear on.

I think it is the <easing-function> that https://drafts.csswg.org/css-values-5/#valdef-calc-interpolate-easing-function describes as "appearing in the first argument" and therefore "specifies the “default” easing function to be used between each stop.", but since it that is the "7.1.2. Defining the Interpolation Map" I am not completely confident.

This could maybe be made a bit clearer if the various arguments were aliased. Perhaps something like:

<timeline-easing-function> = <easing-function>
<default-easing-function> = <easing-function>
<surrounding-stops-easing-function> = <easing-function>

<calc-interpolate()> = calc-interpolate(
  [
    <progress-source> && [ by <timeline-easing-function> ]?
    && <default-easing-function>?
  ] ,
  <input-position>{1,2} : <calc-sum> ,
  [ <surrounding-stops-easing-function>? , <input-position>{1,2} : <calc-sum> ]#? )

[cdoublev]

In my opinion, interleaving the default segment <easing-function> (or any other segment option) between <progress-source> and by <easing-function> should not be allowed. For example, calc-interpolate(by ease ease 0.5, ...) should be invalid.

calc-interpolate([<progress> && <easing-function>?], <interpolation-map>) with <progress> = <progress-source> [by <easing-function>]? seems more appropriate and clearer. And for the generic notation, interpolate-function([<progress> && [<easing-function> || <segment-options>]?], <interpolation-map>).