| Chapter 16: Understanding |

| 16.1. Understand

During play, the computer and the player alternate in writing messages to each other: in the player's case, these are short instructions, usually saying what to do next. A wide range of such "commands" are automatically understood, but these only apply to the standard built-in actions. (This wide range is conveniently tabulated at the foot of the Actions index.) If we want the player to be able to command new actions, then we need to specify what is to be understood as what. For this, we supply special sentences starting with the word "Understand".

Suppose we return to the earlier example of a newly created action:

> Photographing is an action applying to one visible thing and requiring light.

We then supply lines of grammar (as they are called) for Inform to recognise, like so:

> Understand "photograph [someone]" as photographing.

> Understand "photograph [an open door]" as photographing.

As usual, the square brackets indicate something which stands for text, rather than text to be taken verbatim. "[someone]" needs to be the name of anything of the kind "person", for instance (though as usual that person will need to be in sight of the player for the name to be accepted). The first word - in these examples "photograph" - must be something definite, not a substitution like this.

For obvious reasons, this pattern of words needs to match the expectations of the action. Photographing applies to "one visible thing" - the "visible" just means it does not need to be touched, only seen - so neither of these would be allowable:

> Understand "photograph" as photographing.
> Understand "photograph [someone] standing next to [something]" as photographing.

The first is bad because it supplies no things at all, the second because it supplies two: what we want, of course, is just the one.

221
| Example XYZZY Basics of adding a new command reviewed, for the case of the simple magic word XYZZY.

We have seen before how to define a new action from scratch, but we may want to review here, using a simple command that requires no objects. "XYZZY" Understand "xyzzy" or "say xyzzy" or "cast xyzzy" as casting xyzzy. Casting xyzzy is an action applying to nothing. Check casting xyzzy: if the player does not wear the amulet of elocution, say "You are unable to articulate the second 'z' separately from the first, and the spell fails in a disdainful puff. Must be Parisian magic." instead; if the player has the plate, say "The plate of cheeses twitches uncomfortably, aware that it should be doing something, but not sure what." instead. Carry out casting xyzzy: move the plate to the player. Report casting xyzzy: say "Under the influence of the Amulet of Elocution, you pronounce this as Xhi-zee. And lo, from nowhere, a [plate] appears!" The amulet of elocution is a wearable thing. It is carried by the player. The description is "A heavy gold ring on a chain. If heated in an ordinary house fire, it glows with the words, 'Moses Supposes His Toeses Are Roses.'" The plate is a portable supporter. On the plate is a very ripe ooze. Instead of smelling the ooze, say "It smells like socks. This is going to be wonderful." The ooze is edible. The printed name of the plate is "plate[if the plate supports the ooze] of cheese[end if]". The description of the ooze is "Definitely genuinely cheese." Understand "cheese" as the ooze. Instead of eating the ooze: remove the ooze from play; say "You are transported..."; move the player to Paradise. The Cheez Factory is a room. "All around you are squares of pressed orange polymer, or possibly cheez. Your only hope is the magic word your uncle taught you: XYZZY." The squares of pressed orange polymer are scenery in the Factory. The description is "You see nothing special about the squares of pressed orange polymer. Nothing special at all." Understand "square" or "cheez" as the squares. Paradise is a room. The description is "Well, it might just be one of the posh upper rings of purgatory, if you're entirely honest with yourself." Test me with "x squares / x amulet / x cheese / xyzzy / wear amulet / xyzzy / x ooze / smell ooze / eat ooze". XYZZY is a magic word from the original Adventure, and many other games respond to it with some sort of amusing message.

222
| Example Indirection Renaming the directions of the compass so that "white" corresponds to north, "red" to east, "yellow" to south, and "black" to west.

223
| Example Xylan Creating a new command that does require an object to be named; and some comments about the choice of vocabulary, in general.

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