12.10. Action variables

| Chapter 12: Advanced Actions |

| 12.10. Action variables

For some complex situations, it can be useful to keep track of a few values throughout the processing of the action. This is not an everyday occurrence: in the Standard Rules, for instance, only two or three out of 90 actions need to do this. But suppose we want to write a more deluxe version of our "photographing" action. This time, rather than having a single thing called the "camera", we will provide a whole range of possible cameras, varying in quality:

> Photographing is an action applying to one visible thing and requiring light. Understand "photograph [something]" as photographing.

> The Studio is a room. Sally is a woman in the Studio. A foam-lined tote bag is in the Studio.

> A camera is a kind of thing. A camera has a number called picture quality. The digital SLR camera is a camera in the tote bag. The player carries a camera called the instant one-shot camera. The picture quality of the SLR camera is 10. The picture quality of the one-shot is 2. Definition: a camera is sharp if its picture quality is 5 or more.

And we will want the photographing action to have the player use the best-quality camera which comes to hand. We will give the action a variable called the 'camera photographed with', thus:

> The photographing action has an object called the camera photographed with.

Every action's variables must be named differently from those of all other actions, because there are some "before" rules (for instance) which take effect for many different actions, and which might need access to any of their variables. So action variables should be named in a way marking out to which action they belong. The best way to do this is to include the past participle of the action name - just as "camera photographed with" contains the past participle "photographed" of the action "photographing".

This value is created when the action begins, and disappears when the action ends. (If the action should happen a second time before the first time was completed, a second copy of the value is created, leaving the original undisturbed.) When the action begins, the value starts out as something neutral - so if it is a number, it starts out as 0, if a text, it starts out as the blank text "", and so on. Here it is an object, so it starts out as nothing - the value meaning no object at all. But of course we want to give it a value ourselves. We can do that using the "setting action variables" rulebook. For instance:

> Setting action variables for photographing:
> now the camera photographed with is the sharpest camera which is carried by the actor.

The "setting action variables" rulebook is run through before even the before rules, and it has no power to stop or change the action. Its rules should say nothing and do nothing other than to set rulebook variables like this one. Note that it is intended to work for any actor, not only the player: so rather than referring to the player as the performer of the action, we need to write "the actor", as in the example above. (See subsequent sections for more on actors.)

We can now write rules such as:

> A check photographing rule:
> if the camera photographed with is nothing:
> say "You can hardly photograph without a camera, now can you?" instead.

Only rules to do with the photographing action - before, instead, after, check, carry out, or report rules, and so on - are allowed to see the 'camera photographed with' value: it's the private property of the action.

A further elaboration allows us to make rules about photographing neater to write. If we create our variable like so:

> The photographing action has an object called the camera photographed with (matched as "using").

...then we are now allowed to add an optional 'using ...' clause onto a description of the action. The clause has to be introduced with a single word: here, it's 'using'. For instance, we could write rules such as

> Instead of photographing something using the one-shot camera:
> say "But you promised to give this to Sally's nephew."

> Check photographing something using the noun:
> say "That would require some sort of contraption with mirrors." instead.

> Report photographing something using a sharp camera:
> say "You feel cool and important as the shutter clicks."

(This is the method used by the Standard Rules to attach optional clauses such as 'to', 'with' and 'through' to the going action.)

198
| Example Further Reasons Why All Poets Are Liars The young William Wordsworth, pushing a box about in his room, must struggle to achieve a Romantic point of view. |
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199
| Example The Second Oldest Problem Adapting the going action so that something special can happen when going from a dark room to another dark room. |
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Text in this example is drawn from Will Crowther's original 1976 FORTRAN implementation of ADVENTURE, the founding work of the genre, whose source code was rediscovered by Dennis G. Jerz in 2007. Note the capitals: the program ran on an early computer without lower case lettering. They look a little mimsy now, but picture them glowing green on an old-style cathode ray tube monitor in a darkened room late at night. The problem alluded to is that the player is forbidden to walk between two dark rooms, so that he must always have light to see by from at least one end of any movement. Writing source text to achieve this is tricky to get right in every case, because the determination of light is hard to do. Here we interleave the necessary rules into the existing "going" action, using a new action variable to record the number of ends which are dark as experienced by the player, which might be 0, 1 or 2: "THE SECOND OLDEST PROBLEM" The going action has a number called the dark terminus count. Setting action variables for going: now the dark terminus count is 0; if in darkness, increment the dark terminus count. The last carry out going rule: if in darkness, increment the dark terminus count; if the dark terminus count is 2, end the story saying "YOU FELL INTO A PIT AND BROKE EVERY BONE IN YOUR BODY!" instead. And now three early rooms to try this out. COBBLE CRAWL is a room. "YOU ARE CRAWLING OVER COBBLES IN A LOW PASSAGE. THERE IS A DIM LIGHT AT THE EAST END OF THE PASSAGE." DEBRIS ROOM is west of COBBLE CRAWL. "YOU ARE IN A DEBRIS ROOM, FILLED WITH STUFF WASHED IN FROM THE SURFACE. A LOW WIDE PASSAGE WITH COBBLES BECOMES PLUGGED WITH MUD AND DEBRIS HERE,BUT AN AWKWARD CANYON LEADS UPWARD AND WEST." AWKWARD CANYON is west of DEBRIS ROOM. "YOU ARE IN AN AWKWARD SLOPING EAST/WEST CANYON." DEBRIS ROOM and AWKWARD CANYON are dark. Rule for printing the name of a dark room: say "DARKNESS" instead. Rule for printing the description of a dark room: say "IT IS NOW PITCH BLACK. IF YOU PROCEED YOU WILL LIKELY FALL INTO A PIT." instead. Test me with "w / e / w / w". This is only the second oldest problem in the IF literature: the earliest puzzle is unlocking the steel grate which bars entrance to the cave.

200
| Example Removal TAKE expanded to give responses such as "You take the book from the shelf." or "You pick up the toy from the ground." |
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201
| Example Puff of Orange Smoke A system in which every character has a body, which is left behind when the person dies; attempts to do something to the body are redirected to the person while the person is alive. |
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202
| Example Croft Adding special reporting and handling for objects dropped when the player is on a supporter, and special entering rules for moving from one supporter to another. |
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