Burro

One of the features I've previously mentioned as planned before 0.1 is the ability to disable ligatures. Ligatures are, generally speaking, a good thing! However, there might be times where the author wishes to disable them.

As of tonight, this is now possible. Burro has a new command, .ligatures, which accepts a "relaxed boolean" argument. The idea of a relaxed boolean is also new for Burro. This means that any of on, true, or yes parse to a truthy value, while off, false, and no are falsy.

I added these new booleans because the other words can sometimes flow more nicely than true or false. In this case, I like .ligatures[on] more than .ligatures[true] (this matches mom's terminology as well).

To learn how to implement the new feature, I found myself reading the OTF specification—not all 640 pages of it, but just the section on font features! Fonts support several features related to ligatures:

• liga: default ligatures
• clig: contextual ligatures
• dlig: discretionay ligatures
• rlig: required ligatures

Currently, Burro only supports turning liga, clig, and dlig all off. This is done by disabling that set of features from the font file before handing off to the font shaping step. A future improvement could be to allow for finer grained control here.

Even with .ligatures[off], Burro leaves the required ligatures enabled. This is done because those ligatures are required for a reason. Some scripts might require certain characters to be combined. The specification also recommends that rlig never be disabled.