Contextual handlers

Having programs depend on intentions—effects—rather than specific ways these intentions may take form allows us to swap them out depending on the context without having to change our program’s code. This is important for programs that work in different platforms, with different capabilities, or just want to change aspects of its execution (as often needed for testing).

Core to this is the idea of “contexts”. But how exactly does one choose an effect handler based on these contexts?

Reusable handlers

First, most of the handlers defined in the standard Crochet libraries are reusable. They are defined by the package that defines the effect, and provided through a reusable handler.

For example, in order to use the random number generator package, we can use the scoped-random with-source: _ handler to run a piece of Crochet code with that effect:

let Randomness-source = #random with-seed: 1384172941724;

handle
  let Random = shared-random instance;
  let A = Random between: 1 and: 10;
  let B = Random between: 1 and: 10;
with
  use scoped-random with-source: Randomness-source;
end

The above will choose two integers between 1 and 10 using the source of randomness that we provided in the scoped-random with-source: _ handler—but only in code that’s executed from within the delayed program we provide in the handle block.

Execution goals

Crochet has an idea of “execution goal”. These may correlate with a platform, e.g.: a program may be executed on Windows or on a Web Browser, and each of those would be a different execution goal.

Goals affect which files are included in the program, and which command is used to start it. For example, in a Web Browser program Crochet will start the program by executing the main-html: _ command. Whereas, if we’re running the program in Node.js, Crochet would start the program by executing main: _.

A common use of the Novella package for writing interactive fiction that runs in a Web Browser would define the following entry point command:

command main-html: Root do
  handle
    novella show: "It was a dark, stormy night...";
  with
    use novella with-root: Root;
  end
end

Each of these goals has different ideas of which arguments are relevant for the command. Currently these execution goals are defined in Crochet itself, and are not customisable. But in the future, applications will be able to define different execution goals and what it means to run the program with those goals.