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69 lines
3.1 KiB
OpenEdge ABL
69 lines
3.1 KiB
OpenEdge ABL
How To Include This Module.
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What to do to make use of the calculus module in a new command-line tool.
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@h Status.
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The calculus module provided as one of the "services" suite of modules, which means
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that it was built with a view to potential incorporation in multiple tools.
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By convention, the modules considered as "services" have no dependencies on
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other modules except for //foundation// and other "services" modules.
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@h Importing the module.
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We'll use the term "parent" to mean the tool which is importing //calculus//,
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that is, which will include its code and be able to use it. As with any
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imported module,
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(*) The contents page of the parent's web must identify and locate the
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module:
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= (text as Inweb)
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Import: somepath/calculus
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=
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(*) The parent must call |CalculusModule::start()| just after it starts up, and
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|CalculusModule::end()| just before it shuts down. (But just after, and just
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before, the corresponding calls to //foundation//.)
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@h Using callbacks.
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Shared modules like this one are tweaked in behaviour by defining "callback
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functions". This means that the parent might provide a function of its own
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which would answer a question put to it by the module, or take some action
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on behalf of the module: it's a callback in the sense that the parent is
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normally calling the module, but then the module calls the parent back to
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ask for data or action.
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The parent must indicate which function to use by defining a constant with
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a specific name as being equal to that function's name. A fictional example
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would be
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= (text as Inweb)
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@d EXPRESS_SURPRISE_CALCULUS_CALLBACK Emotions::zowie
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=
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void Emotions::zowie(text_stream *OUT) {
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WRITE("Zowie!\n");
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}
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=
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The calculus module has only a few callbacks and all are optional. The
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following alphabetical list has references to fuller explanations:
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(*) |BINARY_PREDICATE_CREATED_CALCULUS_CALLBACK|, if provided, is called whenever
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a binary predicate is created. (The Inform compiler uses this opportunity to
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register its textual name, if it has one.) See //BinaryPredicates::make_pair//.
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(*) |DETECT_NOTHING_CALCULUS_CALLBACK|. The Inform run-time has a constant called
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|nothing| which acts as a default value for objects, indicating an absence of
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any object. The existence of this enables some propositions to be simplified.
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If provided, the function should test whether the supplied constant is |nothing|.
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See //Simplifications::nothing_constant// and //Binding::substitute_nothing_in_term//.
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(*) |PROBLEM_CALCULUS_CALLBACK| is called when a proposition is mis-constructed,
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and can prevent the resulting warning from being issued to the terminal as an
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error message: see //TypecheckPropositions::problem//.
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(*) |PRODUCE_NOTHING_VALUE_CALCULUS_CALLBACK| can provide the |nothing| constant;
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see above, and see //Simplifications::not_related_to_something//.
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@ In addition, the following value can optionally be defined:
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(*) |VERB_MEANING_UNIVERSAL_CALCULUS_RELATION| should be the universal relation,
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that is, the binary predicate which defines "relates". This requires specialist
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type-checking: see //TypecheckPropositions::type_check_binary_predicate//.
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