What is the best way to implement code ligatures? With dlig or calt? Same or separate font?

[thundernixon]

Code ligatures are not always readable to people who are unfamiliar with them. They also add a fair number of glyphs and coordinates to a font, and therefore add a significant amount of data to final font files.

We should be sure that:

• Code ligatures are not on by default
• In web fonts served by Google Fonts, code ligatures are split into an extended Unicode range, rather than served with the initial, basic Latin charset (Could "code ligatures" be split into a different unicode-range, for font-serving CSS? google/fonts#2103) are not served if users do not want to use that feature
• If there are specific Recursive Sans font files, code ligatures are subset out.

[arrowtype]

Code ligatures are not subset out of the most simple downloads from Google Fonts, and probably can't be any time soon (without a change also happening in CSS and browsers for the unicode-range property). This means that:

1. Code ligature glyphs download every time

In Fira Code, code ligatures use standard ligatures plus contextual alternates (rather than discretionary ligatures, as would seem to be the more obvious feature to use). In Cascadia Code, only contextual alternates are used.

In VS Code, it appears that these exact two features (liga and calt) are turned on with the option "editor.fontLigatures": true.

https://github.com/microsoft/vscode/blob/master/src/vs/editor/browser/widget/media/editor.css#L27-L30

On the web, this means that:

2. Code ligatures are on by default

From https://codepen.io/thundernixon/pen/yLBWzMd

These two things would obviously not be the appropriate behavior for the default version of Recursive.

So, I think I will follow the example of Fira Code and Cascadia Code, and make "Recursive Code" – a separate build of Recursive that has code ligatures. (I think Noemi Stauffer had this idea first. As it turns out, it's an even better idea than I initially realized!)

Update: Dave has expressed that he doesn't want duplicate families on Google Fonts, so I will investigate a little deeper on how I might create a single variable font that better navigates this issue.

As a start, I have posted to tonsky/FiraCode#854 to ask why Fira Code uses calt for ligatures. Ideally, I think dlig would be used for ligatures, and that VS Code would update its settings slightly to activate code ligatures with dlig.

[arrowtype]

For reference, from the OpenType spec:

standard ligatures "cover the ligatures which the designer/manufacturer judges should be used in normal conditions. Example: The glyph for ffl replaces the sequence of glyphs f f l."

contextual ligatures "Unlike other ligature features, 'clig' specifies the context in which the ligature is recommended. This capability is important in some script designs and for swash ligatures. Example: The glyph for ft replaces the sequence f t in Bickham Script, except when preceded by an ascending letter."

discretionary ligatures "Replaces a sequence of glyphs with a single glyph which is preferred for typographic purposes. This feature covers those ligatures which may be used for special effect, at the user’s preference. Example: The glyph for ct replaces the sequence of glyphs c t"

contextual alternates "In specified situations, replaces default glyphs with alternate forms which provide better joining behavior. Used in script typefaces which are designed to have some or all of their glyphs join. Example: In Caflisch Script, o is replaced by o.alt2 when followed by an ascending letterform."

[arrowtype]

UPDATE: I'm moving these planning notes to docs/code-ligatures--implementation

(Here's a permalink to the current commit.)

[arrowtype]

I may loop back on this and offer a version that uses clig, but I believe dlig to be the option that best follows the OT spec, so I will make that the main strategy.