Chapter 20: Lists
20.6. Lists of objects

While there are lists for any kind of value, the form that is useful most often is "list of objects". There is nothing really different about this:

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

makes a list of two objects as usual. However, a much faster and more systematic way to build lists of objects is to use descriptions. If D is any description then "the list of D" produces a list of all the objects matching D. For instance:

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 Inform 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 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:

let L be the multiple object list;
alter the multiple object list to L;

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. The multiple object list can be read at any time, but should be altered only up to the point when the "generate action rule" in the turn sequence rules takes effect.


402
* Example  What Makes You Tick
Building a fishing pole from several component parts that the player might put together in any order.

RB

Suppose we want to let the player build a fishing pole out of three parts: a hook, a string, and a stick.

There are several things we must account for here. One is that our combination verb should be insensitive to ordering: it shouldn't matter whether the player types COMBINE STICK WITH STRING or COMBINE STRING WITH STICK.

Second, we need to make sure that our implementation handles intervening stages of assembly gracefully. The player should be able to combine string and hook first, or string and stick first, and be able to complete the assembly in either case.

Our implementation here uses a table of lists to determine which combinations of inputs should produce which result object. Because we sort our lists before comparing them, we guarantee that the player's ordering doesn't matter: COMBINE STICK WITH STRING will have the same effect as COMBINE STRING WITH STICK.

What's more, our implementation could be expanded to account for many other assemblages, if we wanted object-building to be a running theme of puzzles in our game.

"What Makes You Tick"

Understand "combine [something] with [something]" as combining it with. Combining it with is an action applying to two things. Understand the command "connect" as "combine".

Understand the command "attach" as something new. Understand "attach [something] to [something]" as combining it with.

The combining it with action has an object called the item built.

Setting action variables for combining something with something:
    let X be a list of objects;
    add the noun to X;
    add the second noun to X;
    sort X;
    repeat through the Table of Outcome Objects:
        let Y be the component list entry;
        sort Y;
        if X is Y:
            now the item built is the result entry.

Check combining it with:
    if the item built is nothing or the item built is not in limbo,
        say "You can't combine [the noun] and [the second noun] into anything useful." instead.

Carry out combining it with:
    move the item built to the holder of the noun;
    remove the noun from play;
    remove the second noun from play.

Report combining it with:
    say "You now have [an item built]."

Limbo is a container. Limbo contains a hookless fishing pole, a hooked line, and a complete fishing pole.

Streamside is a room. The player carries a stick, a wire hook, and a string.

Table of Outcome Objects
component list   result   
{stick, string}   hookless fishing pole   
{wire hook, string}   hooked line   
{hooked line, stick}   complete fishing pole   
{hookless fishing pole, wire hook}   complete fishing pole   

Test me with "combine stick with string / i / combine pole with hook / i".

This kind of implementation makes sense if we don't intend the player to take the fishing pole apart again, or to refer to any of its component parts once it is built. For an alternate approach that does allow assembled objects to be taken apart again, see "Some Assembly Required".

403
** Example  Formicidae
Manipulating the order in which items are handled after TAKE ALL.

RB


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