What to do to make use of the calculus module in a new command-line tool.

• §1. Status
• §2. Importing the module
• §3. Using callbacks

§1. Status. The calculus module provided as one of the "services" suite of modules, which means that it was built with a view to potential incorporation in multiple tools.

By convention, the modules considered as "services" have no dependencies on other modules except for foundation and other "services" modules.

§2. Importing the module. We'll use the term "parent" to mean the tool which is importing calculus, that is, which will include its code and be able to use it. As with any imported module,

• ● The contents page of the parent's web must identify and locate the module:

Import: somepath/calculus

• ● The parent must call CalculusModule::start() just after it starts up, and CalculusModule::end() just before it shuts down. (But just after, and just before, the corresponding calls to foundation.)

§3. Using callbacks. Shared modules like this one are tweaked in behaviour by defining "callback functions". This means that the parent might provide a function of its own which would answer a question put to it by the module, or take some action on behalf of the module: it's a callback in the sense that the parent is normally calling the module, but then the module calls the parent back to ask for data or action.

The parent must indicate which function to use by defining a constant with a specific name as being equal to that function's name. A fictional example would be

    @d EXPRESS_SURPRISE_CALCULUS_CALLBACK Emotions::zowie

    =
    void Emotions::zowie(text_stream *OUT) {
        WRITE("Zowie!\n");
    }

The calculus module has only a few callbacks and all are optional. The following alphabetical list has references to fuller explanations:

• ● BINARY_PREDICATE_CREATED_CALCULUS_CALLBACK, if provided, is called whenever a binary predicate is created. (The Inform compiler uses this opportunity to register its textual name, if it has one.) See BinaryPredicates::make_pair.
• ● DETECT_NOTHING_CALCULUS_CALLBACK. The Inform run-time has a constant called nothing which acts as a default value for objects, indicating an absence of any object. The existence of this enables some propositions to be simplified. If provided, the function should test whether the supplied constant is nothing. See Simplifications::nothing_constant and Binding::substitute_nothing_in_term.
• ● PROBLEM_CALCULUS_CALLBACK is called when a proposition is mis-constructed, and can prevent the resulting warning from being issued to the terminal as an error message: see TypecheckPropositions::problem.
• ● PRODUCE_NOTHING_VALUE_CALCULUS_CALLBACK can provide the nothing constant; see above, and see Simplifications::not_related_to_something.

§4. In addition, the following value can optionally be defined:

• ● VERB_MEANING_UNIVERSAL_CALCULUS_RELATION should be the universal relation, that is, the binary predicate which defines "relates". This requires specialist type-checking: see TypecheckPropositions::type_check_binary_predicate.