4.3.3.2 Custom Formatters

In order to associate a directive character with a custom formatter function, use the :function keyword instead of the :standard one (see Quick Start for an example). It is expected that you provide a function designator as a value, that is, a function name, object, or even a lambda expression.

The function in question must comply with the regular formatter protocol, as such custom directives are eventually translated into standard ~/ ones.