9. Time

With the material from previous chapters, we are now able to set up a simulated world, to respond to the player's actions within it, and to make it change in perhaps unexpected ways.

The resulting experience can be as lively as we want to make it, but so far we have no very good ways to give it any structure: a beginning and an end, for instance, or a sense of plot. This next chapter is all about the passage of time, and it begins at the beginning.

We have already seen an example of how to write a rule which applies just once, at the start of the story:

The "when play begins" rules are checked only at the start of a story, not when a saved session is restored from disc. What happens is that these rules are followed, then the story's banner is printed, then the initial room description is printed up, and then the player is asked for a first command.

Traditionally-written stories award points throughout play, as an indication of progress. If we want to be traditional, we must first write:

Without this, the SCORE, NOTIFY ON and NOTIFY OFF commands do not work; the final score is not shown at the end of a story; and the status line above the player's text area shows only the turn count, not (as is more usual) both the score and the turn count. Changing the "score" (see below) has no visible effect, though it is not actually illegal.

With "Use scoring" in place, we can award points as follows:

substituting whatever number we feel is appropriate. We should be careful not to give out the same points over and over, that is, not to reward the same basic achievement many times over if the player simply repeats the same action. This, for instance, is open to abuse:

The player may simply take the trophy, drop it again, take it again, … and win five points every time around. We can prevent this by phrasing the rule more carefully:

("Was handled", not "is handled", because this rule happens after the trophy has been taken - so by the time this rule has been considered, the trophy is always handled.) Rather than being an open-ended scoring system, IF normally has a maximum possible score, which can be specified with a sentence like so:

The score and maximum score are just numbers that vary, so we can freely change them:

Another tradition of interactive fiction is that the player has a current 'rank' according to how far his or her score has got. We can (but need not) choose to provide such rankings, and should do so by specifying a table like this:

Typographically, tables in Inform look as much as possible like those found in non-fiction books: they can have many columns, so this is only a simple example (drawn from the actual rankings used by Infocom's Zork I, 1979). Each line in the source represents one row in the table, and the entries on a line must be separated by at least one tab character. (An entry might of course have several words with spaces in between, so a space is not enough to separate entries: this is the only context when Inform distinguishes between spaces and tabs.) The table must occupy a single whole paragraph, with no skipped lines or missing entries. We are free to use extra tabs to indent it if we like.

Ordinarily we must explicitly ask to use the information presented in a table, but the table of rankings is a special case: Inform uses it automatically, if it is provided at all. To be recognised it must have the name "Rankings" and must have two columns named and arranged as shown. The scores should be given in ascending order. Customarily, the score in the final row should be the maximum possible achieved in the story - so that only a player with maximum possible score can be awarded the final ranking - and the value of "maximum score" is automatically set to this bottom-row value if it has not been explicitly set anywhere else in the source text.

Short of something like a power cut, the story can only end when one of the two participants chooses to end it: either the player, by deciding that enough is enough, or us.

In story-telling, there are many kinds of ending: happy, sad, decisive, bittersweet, surprise. Inform doesn't try to interfere in that kind of artistic choice, but it does need to know one thing about the ending: is it final, or not? Many authors like to make additional menu items available if the player has completed the story right through, but not if she has reached an early or wrong ending. (See the activity "amusing a victorious player", for example.)

The closing message is traditionally set out in asterisks:

This style is traditional in IF, and goes back to 1980 if not earlier.

We can test the current state like so:

The rulebook "when play ends" is the matching bookend to "when play begins". It is followed when the story decides to end (not when the player simply gives up and quits), and before any epitaph like

appears. For example:

Surprisingly, the end is not always the end:

The passage of time in interactive fiction is broken up into a succession of turns, in each of which the player types a request and is given a response. Usually each such request triggers one action, but sometimes a whole sequence are fired off, as when the player types "get all" in a cluttered room.

As we've seen, the variable "turn count" holds the number of turns of play so far. By convention turn number 0 is the time when Inform prints up the banner and any initial text; it becomes turn number 1 when the player's first command is typed.

One of the last things to happen in each turn is that Inform will apply any rules which have been set to occur "every turn", like so:

This is equivalent to writing:

Note that the text about blossom, which will quickly become tiresome, is said at the end of every turn, not at the beginning, and in particular not before the player's first opportunity to type a command.

As usual when defining rules, we can add stipulations: any condition can be attached using "when".

Inform keeps track of the time of day automatically: play ordinarily begins at 9 AM and each turn takes one minute. In some works of interactive fiction, time of day is crucial: in others, it is irrelevant or even, by a sort of tacit convention, non-existent. So Inform does nothing to act upon the time, or to reveal it to the player, without instruction. Nevertheless it is there.

A sentence like the following allows the initial time to be set up as something other than 9 AM:

Here, "3:13 PM" is a constant value of a kind not seen before: it's a kind of value called "time", and the value "time of day" is a time that varies. After one turn it will be 3:14 PM, then 3:15 PM and so on.

Note that the sentence above is an assertion (a statement about the initial state of affairs), not an instruction which can be part of a rule. It would be equivalent to write:

We more often change "time of day" to take care of drastic events:

Now that we have the time of day, we can of course use this value in room descriptions and the like:

It seems odd, though, to read a precise numerical description of the time here: after all, it isn't a digital clock. A friendlier version would use:

To reiterate an example which came up earlier, we could even work the time of day into the command prompt, which would lend the proper sense of urgency to a story played out against the clock:

Clocks and watches vary considerably in how much detail they show, and we tend not to report the time over-precisely: half-past ten is an elastic concept. The following room description for the Clock Chamber comes across much more naturally:

The phrase "… to the nearest …" rounds off the given time, just as it sounds; as we'll see later, it can actually round off any arithmetic values, not just times. For instance, "9:58 PM to the nearest ten minutes" is 10:00 PM.

In talking about lengths of time, rather than times of day, it's useful to have these:

Carrying out easy calculations with times is straightforward:

Here we are using two phrases:

Similarly, we have conditions:

We will occasionally need to perform more complex calculations with time, and in order to do that, we have a way to convert the time of day to numbers. Thus the phrase "the minutes part of …" takes a time and produces a number from 0 to 59; similarly "the hours part of …" extracts a number from 0 to 23, using the twenty-four hour clock.

To go the other way, we can convert any number to a duration by writing "minutes" or "hours" after it. For instance:

Note that "clock error" is a number, but "clock error minutes" is a time.

We often want to arrange for something to happen at some point in the future. Here is yet another timepiece:

The event here is called "the egg-timer clucks". It only happens if we instruct so, using one of the following phrases:

If we know in advance what time we want something to happen, we can more simply write:

(Note that in either case such rules begin with the word "at": they are the only rules allowed to begin with the word "at".)

A small warning: timed events like these only have a chance to occur during the turn sequence, that is, once every turn. In most stories, one turn takes one minute, so there will in due course be a turn happening at exactly (say) 11:35 AM. But if the clock is being advanced faster than this, it's possible that there are turns at (say) 11:32 AM and then not until 11:37 AM. But an event set for 11:35 AM will nevertheless happen -- it will run at the first available turn after that time, which will be 11:37 AM. Events can thus happen up to half an hour late, though Inform cancels them if the elapsed time is greater than that.

The Scenes panel of the Index can be a useful way to see what events have been set.

There are two ways that descriptions of actions can be used as conditions. First, we can simply describe an action, and then the condition will be true if that is what the player is trying to do, and not otherwise:

This is actually an abbreviation for the longer, some would say preferable form:

Secondly, we can talk about the past as well as the present, which is very useful since interactive fiction often contains situations which are changed by earlier events.

More on the past tense later follows in the next section: note that "we are taking" has become "we have taken". For the rule to apply, it is not enough that the action "taking the lantern" has been tried: it must have succeeded. Note also that it's enough for any actor in the story to have successfully taken the lantern: it doesn't have to be the player.

The remaining sections of this chapter go into more technical ways to think about the progress of the story through time, and can be skipped at a first reading.

Conditions are clauses which require Inform to make a decision: is such-and-such true, or not true? We have already seen conditions attached to rules using "when":

and, as we shall later see, we will often want to write instructions like:

The condition is "the Sorting Hat is in the Hall", and during play this will sometimes be true and sometimes false.

A condition in the form "X is Y" is of course written in the present tense, and refers to the current state of affairs. Three other tenses are allowed. First, the present perfect:

is true if it has ever been the case that "X is Y" at the start of any turn (or any action). So, for instance,

will be valid if and only if the gate has ever been made open by any action (even if it is closed now), or if it started out by being open when play began.

Next is the past tense:

holds if and only if "X is Y" was true at the start of the most recent action. This is convenient when trying to describe what has changed in the course of the action, but sometimes also when making the action itself happen. For instance:

Completing the set is the past perfect:

which records whether "X has been Y" was true at the start of the most recent action. All these verbs can of course be negated (though "wasn't" and "hadn't" are disallowed as poor style: we use "was not" and "had not" instead). So for example,

is true if the player hadn't visited the Ballroom at the start of the most recent action.

Something we must watch out for is that variables might not have the same values in the past that they have now. As a result, writing conditions such as "if the noun has been open" is a bad idea, because in the past "the noun" very likely referred to something different. It is really only safe to talk in the past tense about definite, fixed things: "if the Great Gates of Kiev have been open" would be fine.

There are two further ways to examine the historical record. Given any condition, we can say

(all of which are synonymous: the words once, twice, thrice, one, two, three, four, five, six, seven, eight, nine, ten, first, second, third, fourth, fifth, sixth, seventh, eighth, ninth and tenth all mean what they obviously should). The result is true if the condition holds now and has held for only one previous spell in the past. A condition holding for, say, fifteen consecutive turns without a break counts as only one "time" - so what we mean by "twice" here is that it is true now, was previously false for a while, and was previously true for a while before that, but no more. In effect, then,

is true if this is the third visit to the Ballroom. We can also say

or similarly "less than", "at least", "at most". It would be more natural, though, to say

The adjective "only" (or equivalently "exactly") can be added to obtain

To recap, this means there have been exactly three visits to the Ballroom in history, whereas

means there have been exactly three visits, the third of which is still going on - an important distinction.

So much for "times" - spells in which a condition is true. We can also test the length of time, in turns of play, that something has been true. Thus:

means that the condition holds now, and held at the start of this turn, at the start of last turn, and at the start of the turn before that. In particular:

will be false if the hat is not currently worn (even if it has been often in the past) and, on the other hand, will be true if the hat has been worn for twenty turns. Here again we can be more specific. These are synonymous:

all requiring that the hat wasn't worn four turns ago. As before, "more than", "less than", "at least" and "at most" so forth can also be used - say, "for at least 21 turns".

A warning: we must be careful when writing something like

since this tests whether it has ever been true that the noun of the then action was open: not whether the current noun-object has ever been open.

Lastly, note that the beginning of play - when (usually) initial text and a banner is printed, followed by a room description - counts towards these counts. In effect, this is a turn: one in which the player compulsorily performs the looking action, rather than being asked for a command. (By convention it is numbered as turn number 0, and doesn't contribute towards the turn count.)