§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.
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Suppose we want to add rules so that any time we examine a charred object (and most of our objects can be charred), a line about the charring is appended to the end of the object description. We could use "after examining...", but perhaps we would prefer for the sentence about the charring not to appear in its own paragraph. This is an ideal occasion for a new activity. We look at the action index for "examining" to identify the rule that causes the old behavior (in this case, the "standard examining rule"); replace this with a new rule that calls our activity; and write our "printing the description" activity in such a way that it uses an object's description without forcing a paragraph return afterward. Then we will use "after printing the description" to add our line about charring, and make sure that the paragraph return does occur before the prompt. So:
This instruction replaces a normal piece of the examine action, the standard examining rule, with another one of our own devising. (The replacement of the standard examining rule will be explained in more detail in the chapter on rulebooks.)
All we have done here is enclose what is usually just a rule inside an activity. This means that we can now write before and after rules for the activity, and also add special instructions like "Rule for printing the name of something while printing the description of something" -- this may not be likely to arise often, but Inform now has the concept of "printing the description of something" as a separate context of action. Next we add the modification that lets us append to the description without a new line:
"run paragraph on" here will mean that we do not get a paragraph break following the description, even if it ends with a period. We also insert a space, so that our follow-on comments will be properly punctuated.
The instead at the end of this line stops Inform for going on with any other "after printing the description of..." rules. The standard library also has rules for printing additional text about containers and supporters with visible contents, and devices that are switched on; with this current system, we could add those as "after printing the description" rules as well, building up a complete paragraph if we wanted. But for simplicity we won't exemplify all of that here. The effects would be much the same as with the "charred" line. Now, because we want to make sure that we always do get a paragraph break after our description, we add this rule last after all the other rules. "Last" and "first" rules are covered in more detail in the chapter on rulebooks.
The player's description is handled in an unusual way, and this will produce a space paragraph break there where it should not. Instead, therefore, we will add an instead for examining the player (probably a good idea anyway):
The "printing a description" activity may be useful for other games, and can be imported just by lifting section 1. |
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Suppose we want to add rules so that any time we examine a charred object (and most of our objects can be charred), a line about the charring is appended to the end of the object description. We could use "after examining...", but perhaps we would prefer for the sentence about the charring not to appear in its own paragraph. This is an ideal occasion for a new activity. We look at the action index for "examining" to identify the rule that causes the old behavior (in this case, the "standard examining rule"); replace this with a new rule that calls our activity; and write our "printing the description" activity in such a way that it uses an object's description without forcing a paragraph return afterward. Then we will use "after printing the description" to add our line about charring, and make sure that the paragraph return does occur before the prompt. So:
This instruction replaces a normal piece of the examine action, the standard examining rule, with another one of our own devising. (The replacement of the standard examining rule will be explained in more detail in the chapter on rulebooks.)
All we have done here is enclose what is usually just a rule inside an activity. This means that we can now write before and after rules for the activity, and also add special instructions like "Rule for printing the name of something while printing the description of something" -- this may not be likely to arise often, but Inform now has the concept of "printing the description of something" as a separate context of action. Next we add the modification that lets us append to the description without a new line:
"run paragraph on" here will mean that we do not get a paragraph break following the description, even if it ends with a period. We also insert a space, so that our follow-on comments will be properly punctuated.
The instead at the end of this line stops Inform for going on with any other "after printing the description of..." rules. The standard library also has rules for printing additional text about containers and supporters with visible contents, and devices that are switched on; with this current system, we could add those as "after printing the description" rules as well, building up a complete paragraph if we wanted. But for simplicity we won't exemplify all of that here. The effects would be much the same as with the "charred" line. Now, because we want to make sure that we always do get a paragraph break after our description, we add this rule last after all the other rules. "Last" and "first" rules are covered in more detail in the chapter on rulebooks.
The player's description is handled in an unusual way, and this will produce a space paragraph break there where it should not. Instead, therefore, we will add an instead for examining the player (probably a good idea anyway):
The "printing a description" activity may be useful for other games, and can be imported just by lifting section 1. Suppose we want to add rules so that any time we examine a charred object (and most of our objects can be charred), a line about the charring is appended to the end of the object description. We could use "after examining...", but perhaps we would prefer for the sentence about the charring not to appear in its own paragraph. This is an ideal occasion for a new activity. We look at the action index for "examining" to identify the rule that causes the old behavior (in this case, the "standard examining rule"); replace this with a new rule that calls our activity; and write our "printing the description" activity in such a way that it uses an object's description without forcing a paragraph return afterward. Then we will use "after printing the description" to add our line about charring, and make sure that the paragraph return does occur before the prompt. So:
This instruction replaces a normal piece of the examine action, the standard examining rule, with another one of our own devising. (The replacement of the standard examining rule will be explained in more detail in the chapter on rulebooks.)
All we have done here is enclose what is usually just a rule inside an activity. This means that we can now write before and after rules for the activity, and also add special instructions like "Rule for printing the name of something while printing the description of something" -- this may not be likely to arise often, but Inform now has the concept of "printing the description of something" as a separate context of action. Next we add the modification that lets us append to the description without a new line:
"run paragraph on" here will mean that we do not get a paragraph break following the description, even if it ends with a period. We also insert a space, so that our follow-on comments will be properly punctuated.
The instead at the end of this line stops Inform for going on with any other "after printing the description of..." rules. The standard library also has rules for printing additional text about containers and supporters with visible contents, and devices that are switched on; with this current system, we could add those as "after printing the description" rules as well, building up a complete paragraph if we wanted. But for simplicity we won't exemplify all of that here. The effects would be much the same as with the "charred" line. Now, because we want to make sure that we always do get a paragraph break after our description, we add this rule last after all the other rules. "Last" and "first" rules are covered in more detail in the chapter on rulebooks.
The player's description is handled in an unusual way, and this will produce a space paragraph break there where it should not. Instead, therefore, we will add an instead for examining the player (probably a good idea anyway):
The "printing a description" activity may be useful for other games, and can be imported just by lifting section 1. |
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