Chapter 20: Lists
20.6. Lists of objects

Lists can be made of values of any kind (including other lists), but lists of objects are especially useful. We could always make these "by hand":

let L be {the pot plant, the foxglove};

But it is usually easier and clearer to use descriptions.

list of (description of values) ... value

This phrase produces the list of all values matching the given description. Inform will issue a problem message if the result would be an infinite list, or one which is impractical to test: for instance "list of even numbers" is not feasible.

While that works nicely for many kinds of value ("list of recurring scenes", say), it's particularly useful for objects:

let L be the list of open containers;
add the list of open doors to L;

means that L now contains the open containers (if any) followed by the open doors (if any). Or, for example:

let L be the list of things;
remove the list of backdrops from L;

makes a list of all non-backdrops.

As mentioned above, lists of objects can be said in two additional ways:

"[L with definite articles]"
"[L with indefinite articles]"

And as mentioned below, they can be sorted in property value order:

sort L in P order;
sort L in reverse P order;

where P is any value property. In all other respects, lists of objects are no different to other lists.

One special list of objects inside the command parser is worth mentioning. This is the "multiple object list", and is used in commands like this:

>GET ALL
foxglove: Taken.
snake's head fritillary: Taken.

After the command parser has decided what constitutes "ALL" (a process which can be influenced using the "deciding whether all includes" activity), it forms up a list and then runs through it, starting an action for each in turn. Here the list has two entries and Inform generates the actions "taking the foxglove" and then "taking the snake's head fritillary".

For two technical reasons this isn't stored as a "list of objects that varies" - first because it needs to exist even in low-memory situations where we can't afford full list-processing, and second because there are times when changing it might be hazardous. Instead, two phrases are provided to read the list and to write it back:

multiple object list ... list of objects

This phrase produces the current multiple object list as a value. The list will be the collection of objects found to match a plural noun like ALL in the most recent command typed by the player. If there is no multiple object, say if the command was TAKE PEAR, the list will be empty: it won't be a list of size 1.

alter the multiple object list to (list of objects)

This phrase sets the multiple object list to the given value. The list is ordinarily the collection of objects found to match a plural noun like ALL in the most recent command typed by the player, but using this phrase at the right moment (before the "generate action rule" in the turn sequence rules takes effect).


416
* Example  What Makes You Tick
Building a fishing pole from several component parts that the player might put together in any order.

RB
417
** Example  Formicidae
Manipulating the order in which items are handled after TAKE ALL.

RB


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