Custom functions in Stencila

Oliver Buchtala, 17 May 2018

Nokome Bentley and Oliver Buchtala have investigated options to write custom functions in Stencila and found a very simple and generic solution.

Cells can return a special value of type function. This does not differ very much from other return values, such as a number. Returned functions are however special, they are passed around just as the information necessary to ask the Stencila execution context to actually run the function. This approach introduces only a rather tiny concept, but has a huge impact: for example a cell executed in R can register a function, which can be executed in every other context, e.g. in Javascript. Reactive evaluation, i.e. automatic updates, are coming for free, as functions are just treated as regular values.


A function could be defined in Javascript:

function hello(you) {
  return "Hello " + you + "!""

When executed in Stencila's JavascriptContext, this cell returns a function that is known to the Stencila Engine by the name hello. Any other cell can then call it, for example a cell written in Mini:

me = 'Anna'

The engine knows that me = "Anna" and that hello is a function registered the by the JavascriptContext. When it comes to evaluation of the second cell, it will ask the context to evaluate hello with the value Anna for it's you argument.

This is how it looks in action: