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There are certain necessary conditions that tend to recur commonly in IF actions. In order to read the book or examine the chest, the objects in question have to be visible. In order to hit the troll with the sword, or move the nest with the stick, or unlock the chest with the gold key, the sword, stick or key first have to be held. These common conditions are encapsulated in TADS 3 as PreCondition objects, since they represent the common preconditions of various types of a command.

preCond property in dobjFor or iobjFor propertyset contains (or, if it is a routine, returns) a list of the preconditions needed for the object in question to be used in the action in question. For example, in the case of reading the book and examining the chest you might define:
preCond = [objVisible]  
Whereas, in the case of hitting the troll or whatever, in the appropriate iobjFor() section (for AttackWith, MoveWith, UnlockWith) you might define:
preCond = [objHeld]  
The library already defines sensible default preconditions for most actions, so you often don't need to worry about them. However, as your own games become more sophisticated, you may want to adjust the preconditions that apply to a particular action on a particular object. For example, you may decide that a particular book needs to be held as well as being visible in order to be read, so you might define:
redBook: Readable 'little red book*books'   'red book'

   preCond = [objVisible, objHeld]  
Like check() and verify() routines, preconditions can veto an action if certain conditions aren't met. Unlike check() and verify(), however, they can be used to change the game state to meet the precondition. For example, the objHeld precondition will first check to see if the object is already held; if it isn't, it will try to make the actor take the object (via an implicit action, i.e. one that is reported as '(first taking the book)' or whatever). If the implicit action succeeds, the action is allowed to proceed (provided there's nothing else preventing it). If not, the action is disallowed, with a message like '(first trying to take the book)' that explains the reason for the failure (e.g. "The book is out of reach."). Some preconditions, however simply test whether the condition is met, and disallow the action if it isn't. For example, an objVisible precondition makes no attempt to make an object visible if it isn't (in the general case it's impossible to know what implicit action, if any, could bring this about), it simply vetoes the action if the object can't be seen by the actor.

The library defines several preconditions, including
objOpen, objClosed, objUnlocked, touchObj, actorStanding, objAudible. For a complete list, see the pre-conditions section of the "TADS 3 Action Results" section of the Technical Manual. It is also perfectly possible (and often useful) to define your own, although that is beyond the scope of this guide. The best way to get a full understanding of preconditions is to study the library file precond.t.

Getting Started in TADS 3
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