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§17.17. Context: understanding when

We have now seen several different forms of "Understand" sentence: for instance,

Understand the colour property as describing a building block.
Understand "mix [colour] paint" as mixing paint.
Understand "rouge" as red.
Understand "curious girl" as Alice.

Any of these may optionally have a condition tacked on: for instance,

Understand "mix [colour] paint" as mixing paint when the location is the Workshop.
Understand "rouge" as red when the make-up set is visible.

In principle, "when ..." can take in any condition at all. In practice a little care should be exercised not to do anything too slow, or which might have side-effects. (For instance, referring the decision to a phrase which then printed text up would be a bad idea.) Moreover, we must remember that the "noun" and "second noun" are not known yet, nor do we know what the action will be. So we cannot safely say "when the noun is the fir cone", for instance, or refer to things like "the number understood". (We aren't done understanding yet.) If we want more sophisticated handling of such cases, we need to write checking rules and so on in the usual way.

Contexts can be useful to make sense of things having different names depending on who is being spoken to, as here:

Understand "your" as a thing when the item described is held by the person asked.

With this rule in place FRODO, GIVE ME YOUR RING means that Frodo will know which ring is meant, even if there are a couple of dozen other rings present.

If the name of something has to change completely, perhaps because the player's understanding of events has changed completely, then Inform's standard way of handling names can be a nuisance. When an item or room is created, Inform automatically makes its name understood as referring to it (in fact, it makes each individual word in that name understood). For instance,

The Wabe is a room. The blue peacock and the sundial are in the Wabe.

means that the player can type EXAMINE BLUE PEACOCK or PUSH SUNDIAL or SHOWME WABE or TAKE BLUE, and so on. This is almost always a good thing, and here there's no problem, because peacocks and sundials are not usually disguised. But here is a case where a disguise is needed:

The secret document is a privately-named thing in the drawer.
The printed name of the secret document is "[if the secret document is handled]secret document[otherwise]dusty paper".
Understand "dusty" and "paper" as the secret document.
Understand "secret" and "document" as the secret document when the secret document is handled.
After taking the secret document for the first time: say "Heavens! It is the secret document!"

As this demonstrates, the either/or property "privately-named" makes Inform create a thing or room which starts out with no automatic understandings at all. The name it happens to have in the source text is ignored. If we simply write:

The ungraspable concept is a privately-named thing in the Dining Room.

then nothing the player can type will ever refer to it; though he will see it, and even be able to pick it up by typing TAKE ALL.

The reverse property is "publicly-named", which all things and rooms are by default.

Inform has four built-in kinds of object (room, thing, direction and region), and all of those have this either/or property. When we create new kinds, they're normally kinds of those four fundamental ones, so they pick up the same behaviour. But if we create a new kind of object outside of these four, that won't be true unless we make it so:

A concept is a kind of object. A concept can be privately-named or publicly-named. A concept is usually publicly-named.

(Privately-named is a property which only affects how Inform creates the object, and it can't usefully be given or taken away during play. "Understand ... when ..." is the way to change names during play.)

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*ExampleQuiz Show
In this example by Mike Tarbert, the player can occasionally be quizzed on random data from a table; the potential answers will only be understood if a question has just been asked.

A bookshelf with a number of books, where the player's command to examine something will be interpreted as an attempt to look up titles if the bookshelf is present, but otherwise given the usual response.