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§2.2. Making rules

The other kind of sentence tells Inform what should happen in certain circumstances, and reads like an instruction issued to someone:

Instead of taking the crate, say "It's far too heavy to lift."

This is a "rule", and it changes the crate's behaviour. The player who tries typing "take crate", "pick up the crate" or similar will be met only with the unhelpful reply "It's far too heavy to lift." The many different kinds of thing which the player can do are called "actions", and are always written as participles: "taking ...", for instance, or "putting ... on ...".

Inform is built on a mass of several hundred rules, some quite complex, and it could even be said that Inform is that mass of rules. We never see the complexity behind the scenes because the whole aim is to provide a basic, penny-plain, vanilla flavoured sort of realism. It would be surprising if one could put the crate inside itself, so a rule exists to forbid this. It would be surprising if one could drop something which was already on the ground, and so on. These basic rules of realism are the ones which every new Inform project starts with.

A rule always starts with a situation which it applies to, and then follows with one or more things to do. Here's an example where the situation is "Before taking the crate" - the player is just starting to try to pick the box up - and there's a three-step process to follow, but steps 2 and 3 happen only if step 1 comes out in a particular way:

Before taking the crate:
    if the player is wearing the hat:
        now the hat is in the crate;
        say "As you stoop down, your hat falls into the crate."

The steps to follow here are called "phrases". Inform knows about 400 built-in phrases, but most of them are needed only occasionally. These three are used over and over again:

if tells Inform to do something only if some "condition" holds, here "the player is wearing the hat";
now tells Inform to change the situation, here so that the hat moves to the crate; and
say tells Inform to say something, that is, to write some text for the player to read.

Every one of the built-in phrases has a definition somewhere in this book. The full definition of "say" will come later, but in the simple form above it writes out the given text for the player to read. (Normally this text is simply shown on screen, not spoken aloud, unless software adapted for partially sighted people is being used.) Phrase definitions are all linked to in the Phrases page of a project's Index.

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