Re: Naming RFC: Properties, property sets, presets

[Flaming Hakama by Elaine]

---------- Forwarded message ----------
From: Urs Liska <lists@openlilylib.org>
To: "lilypond-user@gnu.org" <lilypond-user@gnu.org>
Date: Sun, 12 Jul 2020 15:38:03 +0200
Subject: Naming RFC: Properties, property sets, presets
Hi all,

I'm writing some documentation for the new openLilyLib feature set of
properties, and I think this is the (last) moment to clarify some of
the naming.

I have implemented the concept of a set of properties, which is a place
to store typed variables. I'm pretty confident that the terms
"property" and "property set" are appropriate. To demonstrate:

\definePropertySet my-function.appearance
#`((thickness ,number? 1)
   (color ,color? ,red)
   (label ,markup? "")
   (extra-width ,number-pair? (0 . 0))
   (Y-position ,integer? 0))

This defines the set of properties applicable for my-function, along
with type predicates and default values.

Property values can (if necessary) be retrieved with
  \getProperty my-function.appearance.label
and changed with
  \setProperty my-function.appearance.color #green
  \setProperty my-function.appearance.color "blue" % fails type check

The actual use of properties is from within functions:

myFunction =
#(with-property-set define-music-function (mus)(ly:music?)
  '(my-function appearance)
  #{
    \once \override NoteHead.color = #(property 'color)
    #mus
  #})

Within a function created with the with-property-set macro a function
(property <key>) is available to produce the current value of the
property (which can be the currently set global value or a value passed
in the function

{
  \myFunction c' % => (property 'color) => red
  \setProperty my-function.appearance.color #blue
  \myFunction c' % => (property 'color) => blue
  \myFunction \with { color = #green } c' % => (property 'color) =>
#green
}

###

So far I'm pretty sure property and property set is the right naming.
However, there's one more step, and here I have been suggested to
reconsider the naming.

Properties can not only be changed globally or per instance but also
through something I so far call "presets". Alternative suggestions for
that concept were "contexts" or "environment", but I'm really not
convinced about them. So I'm looking for either common support for
either name or a better suggestion.

A "preset" is a subset of a property set with individual property
values. When invoking the function a preset can be requested while
properties not included in the preset are left unchanged. Presets can
inherit to create cascading structures.

\definePreset \with {
  thickness = 3
  Y-position = 2
} my-function.appearance default

\definePreset \with {
  parent = default
  color = #green
} my-function.appearance style-one

\definePreset \with {
  parent = default
  color = #magenta
} my-function.appearance style-two

Using it the properties included in the preset are used while others
keep the current global value. Additionally arbitrary properties can be
overridden locally:

{
  \myFunction \with {
    preset = style-two
    label = "Foo"
    thickness = 2 % properties from presets can be overridden too
  } c'
}

###

So, to cut a long story short: What do you think this "is", i.e this
should be named: presets, contexts, environments, something else? If
you should think about styles, this has been discussed before, but a
property set isn't necessarily limited to matters of appearance, it
could configure arbitrary things, e.g. export target, lyrics language,
composition algorithm parameters, anything.

Actually I'd prefer one of two answers: A confirmation that I'm good to
go with "preset", or a better suggestion that is so striking that I can
immediately go with it.

Thanks
Urs

To be clear, to make sure I understand it correctly, when you are calling 

    \myFunction \with { 

        preset = style-two 

    } c'

that is a synonym for, assuming your original definition of style-two:

    \myFunction \with { 

        parent = default

        color = #magenta 

    } c'

Is that correct?

If so, to me, the elements of "preset" are in the vicinity of what I would call arguments, as they are values you supply to a function.  But since these arguments are all properties, it's probably better to call them properties.  Since they can be overridden, I'd think of them as property defaults.

I would find it clearer as 

\defineDefaultProperties \with {
     parent = default
     color = #magenta
} my-function.appearance style-two

{

    \myFunction \with { 

        defaultProperties = style-two 

    } c'

}

If you wanted a more generic name, I would go with:

\defineArgumentDefaults \with {
     parent = default
     color = #magenta
} my-function.appearance style-two

{

    \myFunction \with { 

        argumentDefaults = style-two 

    } c'

}

HTH, 

Elaine Alt

415 . 341 .4954                                           "Confusion is highly underrated"

elaine@flaminghakama.com

Producer ~ Composer ~ Instrumentalist ~ Educator

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