13. Relations

Descriptions of things - "open door", "people in the Drawing Room" - have already had a whole chapter to themselves. But descriptions are only half of the story of Inform's highly flexible language for talking about places, things and circumstances: this chapter is the other half, and is about the "sentence". Of course all text is made up of sentences, but Inform has a more specific meaning than that. Consider the following pieces of source text:

What these three extracts have in common is the sentence "the mouse is in the teapot". Such a sentence can be used in three different ways: to declare the original state of the world, to ask during play if the world currently has that state, or to change things during play so that it does.

Actually, though, only definite sentences about the present can be used in all three ways. A vague instruction like

will fail, because there are so many ways in which Darcy might be able to see the mouse that Inform has no way to know how to arrange matters. And this by contrast is not merely difficult but impossible:

Which cannot be arranged because the past cannot be changed.

Verbs also turn up inside the more complicated descriptions. For instance,

are both descriptions, not sentences, but they contain "to be" and "to be able to see" respectively.

This chapter is about the verbs which can be used in sentences and descriptions. Inform involves many other features which use verbs - the action "taking the mouse" and the phrase "end the story" both use forms of verbs (to take and to end) - but this chapter has nothing to do with them: so for the sake of clarity, we will call verbs that occur in sentences "sentence verbs".

A sentence consists of two nouns with a verb between them. Usually, the two nouns are descriptions, as in:

Here "Mr Collins" and "a lighted room" are descriptions. But there are sentences where one or both of the nouns is a value of some other kind. For instance, in

the sentence "the score is greater than 10" consists of two number values ("the score" and "10") connected by a verb part ("is greater than").

This chapter is about getting the most out of sentences by defining new verbs to express ideas not already built in to Inform. Before we can define a new sentence verb, however, we must first look at the meanings of verbs: which Inform calls "relations".

Relations are what sentences express. They are yes/no questions about pairs of things: for example, to say that the coin is in the purse is to say that a particular relation ("being in") is true about a specific pair of things (the coin, the purse). It is neither a fact about the coin nor about the purse, but about the two together.

Inform comes with a number of relations built in, almost all of which have been used in previous chapters already. The following table names some of the more useful ones, giving examples of sentences to bring them about:

These relation names do not trip off the tongue, but they relatively seldom need to be referred to.

The same meaning can often be expressed by using several different verbs, or using the same verb in several different ways, as in the following examples:

all of which boil down to saying that the coin and purse satisfy the containment relation. Because of that, relations are not the same as verbs. To create a new idea, we will need first to create a new relation, and only then can we set up a verb which allows us to talk about that relation.

Inform has altogether five mutually exclusive ways in which one thing can be physically joined to another one:

This is why we cannot have

simultaneously, and it is a rare exception to the general rule that having one relation does not affect having another.

But there is also a sixth relation used in Inform for these meanings: the possession relation, which is the meaning of the verb "to have". At first sight this looks the same as the carrying relation, but in fact it is a convenient shorthand for "carrying or wearing", provided for conditions rather than assertions:

will be true during play if he is either carrying or wearing the shirt.

Still another relation exists which can be tested, but not declared to be true or false: the concealment relation, which is the meaning of the verb "to conceal". So we can ask:

We can create new relations like so:

Every relation has a name which ends with the word "relation", and in this case the name is "loving relation". While the name is often just two words long, as here, it doesn't have to be:

makes the "adept sensitivity relation". (The limit is 32 words.)

In such a definition, we have to say what kind of thing appears on the left and right of any relation, and also whether "one" or "various" possibilities can exist. In the example

what we are saying is that only people love; that they only love people; and that each person loves only one other person (at any given moment).

The "various" part comes in because, for instance, we might have:

so that various people (Verenka and Liubov, to name but two) love one person (Stankevich). But we are forbidding anyone to love two other people at the same time: Stankevich must decide which of them to love, or pick someone else, or no-one at all. Similarly, we would not allow

It is sometimes convenient to give a name to the other side of a relationship, so to speak. We might imagine:

It would then make sense to talk about "the owner of Loulou", and we could have phrases like "now Flaubert is the owner of Loulou" or "if the owner of Loulou is a woman…" and so forth. This, however, would not be allowed:

because "the pet of Flaubert" would be ambiguous: he might have owned dozens.

The relationships described in this chapter so far are by no means always reciprocated. For instance, if a stone is on a table, then it is never true that the table is also on the stone. And the question may not even be meaningful to ask. If Peter wears a jacket, the jacket does not even have the possibility of wearing Peter.

But sometimes we do want a relation which works both ways equally well. These are simple to set up:

The effect is that various people know various other people, and this is always reciprocated. If Daisy knows Sophie then, automatically, Sophie knows Daisy. This even-handedness is maintained throughout play, so that whatever changes are made it is always true that if A knows B then B knows A.

And similarly for a reciprocal relation between one and another:

In this case, we can again give a name to the partner under a relation:

and now, for instance, we may have that the spouse of John is Yoko and the spouse of Yoko is John.

Since many of these examples have involved people, it might be worth mentioning again that any kind can be involved, not just the "person" kind.

Finally, there is a kind of relation which binds even more strongly.

This is a kind of relation which divides people up: we might wish to have all the Icelandic people related to each other, all the Peruvians to each other, and so on. If there were a Pacific island called Informia with one inhabitant, then that person would be related only to himself. As time goes by, we could imagine people emigrating, and so on, so that these groupings would switch: perhaps everyone would leave Belgium and, for a while, there would be no Belgian nationals at all.

The testing command RELATIONS prints out the current state of all the relations created in the source code. For instance:

That can produce a lot of output. To see only a single relation, or to see it at some intermediate point in a calculation, there's also a testing phrase:

But this is a phrase - not a typed command.

It is all very well to define new relations, but this does nothing if there is no way to assert that they are true, or to ask whether they are true or false. That requires a verb: in fact, a relation is nothing more than what Inform uses as the "meaning" of a verb. The assertion verbs built in to Inform have the following built-in relations as their meanings:

Two of Inform's built-in relations are expressed using prepositions instead:

It would be easy to make verbs for these if we wanted ("to adjoin", say) using the techniques of the next section.

The verb to be is grammatically different from any other, and its meaning is too complicated to be fully expressed by any one relation. A great deal of the Inform program is given over to its "meaning", which we are not allowed to change or imitate. The "equality relation" is simple enough, and is the one implied by conditions like

but to be can have more complicated implications - "if Mr Wickham is hungry" clearly doesn't test whether two quantities are equal. Fortunately the other verbs are much simpler.

There are a few other built-in verbs, as can be seen in the Index, but these are mostly for experts only. For example:

"To mean" can be used to make new verbs, as we'll soon see. Provision is to do with whether something can have a given property: for example, "if R provides the property lighted" tests whether R is able to have this property, not whether it actually has it at the moment.

Here is an example definition of a new verb:

Once this is done, we can write the assertion

which will do the the same thing as

because both verbs have the same relation as their meaning.

Earlier versions of Inform needed to be told how to make other parts of the verb, but that's rarely true now. Just writing:

is enough for Inform to understand "he sports", "they sport", "he sported", "it is sported", "he is sporting", "he had sported" and so on. It works with irregular verbs, too; it has a very comprehensive dictionary. But it's legal to spell out the conjugation if need be:

Occasionally it's convenient to have the relation the other way around. For instance:

With that defined, these two sentences have identical meanings:

Reversed in this sense means that the things related - the subject and object of the verb - are the other way round.

The Phrasebook index contains all the verbs associated with assertions, in the Verbs section. When we add new verbs to our source, those will appear in the Phrasebook as well.

The verbs above ("to grace", "to sport") are short ones, but we're free to make them longer than that. For example:

Here we have "to cover oneself with", four words long; the limit is 29.

The term preposition is used here, a little loosely, to mean anything which we add to the verb to be in order to talk about some relation or other. We have seen many examples already, such as:

These are defined just the way verbs are. Compare the following:

The result of this is that

are exactly equivalent, and so are these two descriptions:

While most prepositions are short ("in", "part of", "suspicious of"), they're free to be longer if need be ("inordinately far away from"): the limit is 30 words, which should be ample.

We can also define verbs as auxiliaries, like so:

Now we can ask if Poirot "can approach" Hotchkiss, and so on.

We have already seen, in the chapter on Descriptions which is a forerunner of this one, that Inform provides not only "adjacent" as a way of seeing if one room is directly connected to another, but also "the best route from A to B", which allows us to see if any sequence of moves connects them.

Something similar - in fact, simpler - is allowed for any relation between objects. Suppose we would like to go sledging: we can go downhill, but not up. Some quite distant places may be reachable, while others close by may not be, even if lower than us, because they would involve climbing again at some point. The following would implement this:

Here we're making use of:

Another example would be the "six degrees of separation" game, where it is claimed that any two people on Earth are connected by a sequence of up to six acquaintances. In an Inform implementation, we might talk about "the next step via the friendship relation from George Bush to Saddam Hussein", for instance, a phrase likely to evaluate to Donald Rumsfeld, and then

would be… but that would be telling.

As with route-finding through the map, finding "the next step via" a relation can be slow. For instance, suppose we have dozens of articles of clothing all partially revealing each other, connected by two relations - overlying and underlying. Then "the next step via" these relations allows us to establish what can be worn on top of what else. If we need to calculate this often, and there are enormous wardrobes of clothes to choose from, speed starts to matter.

Once again there is a choice of algorithms: "fast" and "slow", where "fast" needs much more memory. To make route-finding for a given relation "fast", we have to declare it that way:

Otherwise, the "slow" method will be used.

This "with fast route-finding" note can only be added to various-to-various relations. (Although route-finding through various-to-one and one-to-various relations is fully supported, it exploits the relative simplicity of these problems to use a more efficient algorithm than either "fast" or "slow".)

One last way to create a new relation and, in many ways, the easiest of all. If we write:

then we would be able to talk about a handle being joined to a door, and a door being joined to a handle, and so on. We are not allowed to declare:

because the question of whether they are joined is not for us to decide: that will be for the condition to determine, whenever we test it. Similarly, we cannot meaningfully write

(and Inform will not let us) because this relation is not something we can force either way: we can make it come true by other means, maybe, but we cannot simply make it true by saying so. Lastly, this kind of relation is restricted in that we are not allowed to find paths or calculate numbers of steps through it.

So this way to define relations is, on the face of it, just a sort of verbal trick to write conditions in a more attractive way. The more flexible, changeable relations in previous sections have much greater expressive power. All the same, it is nice to be able to write -

and then to be able to write rules like:

As with other relations, there's no reason why we have to use objects. For example:

which enables us to write:

And here is a mathematical one:

We now find that "2 divides 12", "5 is not a factor of 12" and "12 is divisible by 3" are all true. Again, we are only really gaining a nice form of words, but improving the clarity of the source text is never a bad thing.

Although most of the examples in this chapter have involved objects, relations can connect almost any values together. We can create relations in groups, one to various relations, various to one relations, one to one relations, and various to various relations for any combination of kinds. For example:

How might we make use of this? Clearly it would be impractical to keep trying:

to find out what "caviar" belongs with. It's still harder to find out if it belongs with anything at all -- in theory we would have to try every possibility, which of course is impossible. Instead we have these phrases:

If a partner does exist, then we can find it with:

Of course, there might be many answers to this question, so perhaps these are neater:

Finally, it's sometimes useful to get at the list of all values which can appear on the left or right hand side of a relation. We need tongue-twister like wording to do it, but:

For efficiency reasons, there are no guarantees about what order these lists have - but they can of course always be sorted when found.

As we've seen, most relations have names - "containment relation", for instance. These are themselves values in Inform, though there are a few restrictions on how they are used. (Relations can contain a colossal amount of data, so we don't want to have to copy them casually.)

Consider these two examples:

Here "parity relation" and "joint magnitude relation" are both values of the same kind: "relation of numbers to numbers". In general, every relation is a value of kind "relation of K to L", for the appropriate kinds K and L. So the parity relation doesn't have the same kind as the containment relation, for example. Because it often happens that K and L are the same, we can just say "relation of K" in this case, so we could equally say that the kind of the parity relation is "relation of numbers".

This is useful to know when writing phrases like so:

and now "chart parity relation" will work nicely, but "chart visibility relation" will be rejected (as it should be, because it relates things, not numbers). In general, if R is any relation, we can write

to test, set and unset a relation R between two values. (Inform checks that the values X and Y have the right kind and produces a problem message if not.)

Several useful adjectives can be applied to relations:

So for example it's possible to ask

With some relations, it's possible to clear them out by writing:

and with temporary relations (see the next section), it's even possible to change their valencies (one-to-one vs. one-to-various, etc.) using "now", but only when they are empty. The exceptions where "empty" can't be used are those which can't be changed at all, like the parity relation above, and a few built-in cases such as the support, containment and incorporation relations, where emptying would dissolve the model world in a disastrous way.

So far in this chapter, we've only seen relations which exist permanently during play. The relationships might change - sometimes Red Riding Hood would be in the Woodcutter's Cottage, sometimes not - but the relations themselves were eternal.

In fact, though, we can also create relations to be dynamic data structures, like lists:

Such a relation exists only in the current phrase, and is destroyed when the phrase finishes, like any other "let". Of course there's no verb whose meaning is this relation, but that's no obstacle, because we can manipulate it using "relates":

(At present such a relation cannot be used outside its own phrase.)

It is easy to say what verbs are for: they are to express relations. But what are relations for?

Inform 7's focus on relations between objects is unusual as an approach to interactive fiction; the concept does not exist in most design systems, or rather, it does but is submerged. Traditional design systems do, after all, have the spatial relations of being inside, on top of, and so on. It could well be said that these are the only relationships that inanimate objects ever have. A stone can be on top of a table, and if so then that expresses their entire association.

This is because the stone, and the table, have no opinions, emotions, knowledge or memory. If the stone is taken away and then put back, nothing has changed. People, on the other hand, tend to remember having met each other before; they like being in some places, but not others; their behaviour depends on who, or what, is nearby. Being conscious, they have internal states, unlike the stone. Relations are a simple but powerful way to express and talk about such connections, and although they have numerous uses in physical contexts too, they are at their most powerful when helping to make the characters of interactive fiction come alive.