A large part of playing interactive fiction involves referring to the objects and locations that make up the game's world. The information in this chapter will assist you in coding your game in such a way that the JACL parser will select the object the player intended as often as possible. All references to objects made by the player are parsed for the following possible structure:
objects [from other objects] [except objects [from other objects]]
The JACL parser will build two lists of objects for any reference made by the player. The first will contain all the objects for inclusion while the second will contain all the objects referred to after the word except for exclusion. Once the two lists have been built, the second list will be subtracted from the first list. Both lists can make multiple use of the word from to refer to objects, however, the word except can not appear more than once.
When the player refers to an object during the course of the game they may use as many or as few of the object's names as are required to uniquely identify it. For example, consider the following three objects:
object ball_1: ball object ball_2: small red ball object ball_3: big red ball
Presuming all these objects were in the current location, if the player referred to ball, then ball_1 would be selected. This is because the JACL interpreter divides the number of names an object has by the number of names supplied to come up with a best match. Although all three objects have the name ball, ball_1 matches 100% while ball_2 and ball_3 only match 33%. If the player referred to red ball, then ball_1 would automatically be excluded, as it does not have the name red. On the other hand, both ball_2 and ball_3 have the names red ball, and both would match 66%. In this case, a message would be displayed stating that the reference was ambiguous. If the player referred to big, then ball_3 would automatically be selected as neither of the other two objects have the name big at all.
It is important to be aware of the number of names that you give each object, and how they relate to each other. It is generally best to have two similar objects have the same number of names, although in some cases you may wish to nominate one to be the default by giving it fewer names.
For example, in The Unholy Grail there are two objects: beige agar and brown agar. To avoid problems when looking up agar in the encyclopedia, a third object named simply agar was added. This object stays permanently in limbo and therefore does not affect any physical manipulation of the two real agar objects. It does, however, get selected when using commands such as ask_about that accept objects that are *anywhere.
It also possible to define plural names for an object. When the player uses a plural name all the objects that have that plural name will be selected. For example, the above objects could all be given the plural name balls using the following code:
object ball_1: ball plural balls object ball_2: small red ball plural balls object ball_3: big red ball plural balls
If during the game the player was to type the command take balls, all of the above objects would be selected and three separate take commands would be issued. It is the use of the plural name that tells the parser that a reference to multiple objects was intended and that the reference is not ambiguous. It is also possible to further qualify a reference to a plural name by using one or more regular name. For example, the command take red balls would result in only the small red ball and the large red ball being taken.
When the parser can't narrow an ambiguous reference to a single object on its own, the best places to nudge it toward the right answer are the object's name list and the grammar lines that introduce the verb. Make the intended object the one with the most matching names (or the only one with a particular name), or scope the grammar line with *held, *present, *here or *inside so that out-of-context objects are excluded before the disambiguation step is reached. See the section on Grammar Statements for the full list of scope keywords.
If two equally-named objects remain after scope filtering, the interpreter prints an "ambiguous reference" message and asks the player to be more specific. There is no per-object callback for overriding this decision in the current build; the parser source contains the skeleton of a planned disambiguate hook but it is not active.