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§18.5. New activities

Activities are all about influencing the standard mechanisms which Inform uses, so it might at first seem that there is no need to create new activities: but on further reflection, quite a lot of the writing of interactive fiction involves creating new and systematic ways to do things, and as soon as we have a general rule, we will want to have exceptions. Inform therefore allows us to create our own activities, giving us ways to influence the operation of our own mechanisms.

There are two kinds of activity: those which relate to a specific value (usually an object but not necessarily), and those which do not. Here are some examples of activities being created:

Assaying is an activity.
Analysing something is an activity.
Announcing something is an activity on numbers.

Inform looks for the clue "something" (or "of something") after the activity's name to see if it will work on a value: so analysing and announcing will do, but assaying won't. If we don't specify a kind, Inform assumes the value will be an object, as if we had written:

Analysing something is an activity on objects.

As always in Inform, the names of activities are themselves values.

"assaying activity" has kind activity on nothing
"analysing activity" has kind activity on objects
"announcing activity" has kind activity on numbers

Creating an activity is like creating an action: it automatically makes new rulebooks - "before analysing", "for analysing" and "after analysing" - but they start out empty, so the activity does nothing yet. Just as it does for rulebooks, Inform defines the adjectives "empty" and "non-empty" for activities to test this state:

if the analysing activity is empty, ...

will be true only when all three of its rulebooks are empty.

A newly created activity never happens unless we take steps to make it do so. We can make an activity happen at any time by writing phrases like so:

carry out the (activity) activity

This phrase carries out the given activity, which must be one not applying to any value. Example:

carry out the assaying activity;

carry out the (activity on values) activity with (value)

This phrase carries out the given activity, which must apply to a kind of value matching the one supplied. Example:

carry out the analysing activity with the pitchblende;
carry out the announcing activity with the score;

To make the activity do something useful, we need to put a rule into its "for" rulebook:

Rule for announcing a number (called N): say "Ladies and gentlemen, [N]."

The last for assaying rule:
    say "Professionally, you cast an eye around mineral deposits nearby, noticing [list of rocks in the location]."

"The last" is a technicality about rulebooks (see the next chapter) which, put briefly, guarantees that this rule comes last among all possible "for assaying" rules. This is good form because the whole point of an activity is to make it easy for further rules to interfere - so we deliberately hang back to last place, giving precedence to anybody else who wants it.

The "for" rulebook is one where rules stop the activity, by default, when they take effect - in the same way that the "instead" rules stop actions by default. If this causes problems, we can use:

continue the activity

This phrase should be used only in rules in activity rulebooks. It causes the current rule to end, but without result, so that the activity continues rather than stopping as a result of the rule. This is useful for rulebooks like the "for" rulebook of an activity) where the default is that a rule does stop the activity.

Activities are more useful than they first appear. Every new one provides a context which other activities can observe. We could, for instance, define

Rule for printing the name of a rock while assaying: ...

so that during assays more technical names are used.

arrow-up.png Start of Chapter 18: Activities
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arrow-right.png Onward to §18.6. Activity variables

An Encyclopedia set which treats volumes in the same place as a single object, but can also be split up.

Modifying the rules for examining a device so that all devices have some specific behavior when switched on, which is described at various times.

Adding a "printing the description of something" activity.