§21.6. 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).
![]() | Start of Chapter 21: Lists |
![]() | Back to §21.5. Building lists |
![]() | Onward to §21.7. Lists of values matching a description |
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.
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". |
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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.
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". 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.
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". |
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