# Action Results

One of the main programming tasks in writing an adventure game is handling special-case commands from the player - commands that do something special, rather than using the basic library handling. The TADS 3 library uses "object oriented programming" techniques that let you write special-case command handlers for individual objects or for entire classes of related objects, but this article assumes you already know all about that sort of thing: dobjFor(), verify(), action(), and so on. So we're not going to talk about that. Instead, this article is about how you return information from your special action handlers, to tell the library what happened and what to do next.

The handling for a specific action is divided into a series of phases: verification, preconditions, checking, and action. Since each phase serves a distinct purpose, each phase has its own kinds of results. So, you shouldn't expect to signal the same kinds of results from the "verify" phase that you would from the "check" phase. Since each phase is different, we have to consider them one at a time.

Verify
Pre-conditions
Check
Action
Library Messages

## Verification

The primary purpose of the verification phase - really the only purpose - is to figure out which object or objects are the most logical for a given action. This lets the parser pick the best interpretation automatically when the player's phrasing is ambiguous, so that we don't have to constantly bother the player asking for clarification.

A side benefit is that we can often use this same information to rule out some actions entirely, since our "logicalness" determination can frequently tell that an action is completely illogical. This saves us the trouble of writing any code in the other phases for handling this particular action on this particular object, since we can just rule out the object in the verify phase based on the logicalness rating we have to come up with anyway. But always keep in mind that this is just a side benefit; it's easy to slip into thinking that the point of the verify phase is to rule things out, but it's not, because we'll still have plenty more chances to rule things out later, in other phases. Any ruling-out that we can do in the verify phase is just a free bonus that saves us a little work.

### What does "logical" mean, anyway?

Before we go on, we should define what "logical" means in the context of verifiers. This is really important to understand, because it's at the heart of what the verification stage is for. Here's the key: "logical" means what the player thinks it means.

So, what does "what the player thinks it means" really mean?

English, like any natural language, is full of ambiguity. It's capable of very fine precision, as you can plainly see if you read any ANSI specification document or any legal contract. But there's a high price for that kind of precision, paid in the form of verbosity and linguistic convolution - which, again, you can plainly see in any ANSI spec or any contract. There's a reason precision comes at such a great cost, and it's not that lawyers and engineers are bad writers. The real reason is that precision of the kind needed in engineering and legal documents just isn't nearly as important in everyday speech as speed and expressiveness, so natural languages evolved to express information concisely, at the cost of allowing lots of ambiguity. In everyday speech, there's so much shared context that we can communicate volumes with a few words, knowing that our shared context will fill in all of the things we leave poorly specified in our actual words.

Think about two friends talking, and one of them saying to the other, "Joe is retiring to Florida." So, who's Joe? First off, any English speaker would know it's a man's name, so we've narrowed it down to "male human named Joe"; that's still pretty darn ambiguous, but these two people talking are friends, so they have a shared idea of "Joe." They might each know multiple people named Joe, and might even have several acquaintances named Joe in common, but there will be one person that they both understand to be the one they can talk about as just "Joe." If they didn't have such an understanding, or if the speaker wanted to talk about one of the other Joes they both know, the speaker would have qualified it somehow: "Joe Smith," or "Bob's friend Joe," or "that guy from school who was always going on about his hernia problems" - the speaker would qualify it according to the shared context with the listener.

That's what we mean by "what the player thinks it means": we have to imagine the player sitting at the keyboard, having just read the moving prose you've written to describe the room in which the player character finds herself. What's in the player's mind now? What does the player mean by "Joe" here, in this location in the game, at this time?

### What "logical" doesn't mean

There are a couple of important distinctions we need to draw here.

First, note that we're not interested in what the player character thinks; we're interested in what the player thinks. Whatever theory anyone has about the relationship between player and player character, there's no question that it's the player typing those commands, so our job parsing the input is to understand the meaning the player meant to convey.

Second, we're especially not interested in what the author thinks, or what the "narrator" thinks, or what the parser thinks, or what an omiscient observer who can look at all of the object properties thinks. This is a key point to understand and to keep in mind while writing your game, because it's sometimes easy to forget about the player and think instead in terms of the objective reality (so to speak) of the game world.

For example, if a particular key opens a particular door, it's easy to fall into the trap of thinking that that particular key ought to be a more logical match for UNLOCK DOOR WITH KEY than any other key. That's the "objective reality" trap: the key is in fact the correct one for the action, but that doesn't matter - the only thing that matters is whether or not the player knows this. If the player doesn't know it, then there's no reason to assume that the player means that key over any other.

(The library has a Key class that operates on exactly this principle. Initially, one key is pretty much like any other. Once the player has successfully used a particular key to open a particular door, though, the library remembers that successful combination, on the assumption that the player will also duly note and remember the combination. On future attempts, the key that the player used successfully will be considered the most likely one in cases of ambiguity.)

Another example, roughly the opposite of the previous one: there's a door. It looks perfectly ordinary, but unbeknownst to the player, it's actually a fake door, cleverly constructed to look exactly like a real door, but not openable. Should OPEN DOOR be ruled out as illogical? No, because the player has no way of knowing that the door's a fake. If the fake is so convincing that even attempting to open it would not reveal its falsity (maybe it just looks like a real door that's stuck, for example), then it would still not be illogical to try opening it again and again. Only when the player finds out that the door isn't actually a door - maybe by inspecting it more closely, or by being told by a character in the game - will OPEN DOOR stop making sense to the player.

### Verifiers and game state changes: a deadly combination

A general note on writing verify handlers: as in TADS 2, it's important that a verify handler never modifies the state of the game. For example, a verify handler should never move an object from one location to another, and it should never change an object from open to closed. The reason is simple: the parser can call the same verifier several times in the course of an action, and it can call a verifier "tentatively," before deciding if the object is really involved in the action. If a verifier does anything to change game state, then it will make its change too many times, or simply at the wrong times.

### How multiple results are handled

A single verification call can have several separate results, so you don't actually return results as function return values. Instead, you add the results to the current result set. Fortunately, this isn't as complicated as it sounds, because the library provides a set of macros that make it easy to add a result. Typically, a verify routine checks some conditions, adds one or two results, and might inherit the default handling from a superclass.

To determine if a command will be allowed to proceed, or to compare the logicalness of different possible object matches for a noun phrase, the library looks at the worst result of the verification. The library pays attention to the worst result because verification is essentially a negative sort of process: we're looking for problems, reasons the object is a bad choice, so naturally we go with the biggest problem we find. Note that this means that if there are no verify results at all, the parser takes this to mean that there are no problems - objects are assumed logical by default, unless a verifier says otherwise.

Because the worst result prevails in cases of multiple results, you don't have to worry that superclass code that you inherit could overrule any objections that your code might raise. If you raise an objection by adding an "illogical" result of some kind, your objection will be obeyed; the worst that can happen is that a superclass can find a worse objection, in which case that more serious problem will upstage your objection. On the other hand, this means that you can't overrule a superclass or subclass to make the object more logical; if some other code objects, there's no way to retract that objection in your code. The only way to prevent superclass code from declaring something illogical when you want to make it logical is to avoid inheriting the superclass code in the first place - which is easy, since it just means you don't put an "inherited()" call in the method.

### Examples

Here are some examples of how to use verifiers.

For something that just plain makes no sense at all, we use "illogical." For example, it makes no sense to open a coffee mug:

       dobjFor(Open)
       {
         verify() { illogical('{You} can\'t open a coffee mug! '); }
       }

For things that might make sense some of the time, but don't make sense right now because of the current state of an object, we use either "illogicalNow" or "illogicalAlready," the latter being for cases where the action is redundant because it tries to put the object into a state it's already in. For example, a door can be opened and closed, but if it's already open, then opening it again doesn't usually make any sense. We use "illogicalAlready" for this kind of redundant command.

       dobjFor(Open)
       {
         verify()
         {
           if (isOpen)
             illogicalAlready('{The dobj} {is} already open. ');
         }
       }

We'd use illogicalNow instead of illogicalAlready if the object's state makes the command illogical, but for some reason other than redundancy. For example, we can't board an inflatable rubber raft when it's deflated.

Note that it's not necessary to do anything at all if we don't want to raise an objection, so there's no "else" in the "if" statement above. If we don't raise an objection, the parser assumes that the object is perfectly logical.

Sometimes, a particular action is possible on essentially any object, but it's more sensical on certain objects than others. For example, you could try to read almost anything, but things like books and magazines are much more likely than others to be the object of a READ action. You can handle this sort of thing using "logicalRank," which lets you indicate that an action is logical, and give different objects different relative rankings. For reading, you could give ordinary objects a low ranking - the library uses a value of 50 on its arbitrary ranking scale for this. (The default ranking for an object that doesn't provide a specific ranking is 100.) So, for ordinary objects, you could do this:

       dobjFor(Read) { verify() { logicalRank(50, 'not readable'); } }

The 'not readable' string, by the way, isn't something that will ever be shown to the player - it's not an error message. It's just a "key" value that the parser uses to identify the rankings. If every matching object has the same rank with the same key, the parser will know that it can't tell those objects apart on the basis of that ranking, so it'll ignore that ranking and look for some other ranking to use instead.

### Verifier result types in detail

Here are the macros to add verify results. These are listed in descending order of logicalness, so you can read two things into the ordering. First, an item later on the list will upstage an item earlier on the list when both items are added to the same object's verify results, because the worst result is always the prevailing one. For example, if an object's verify uses both "logicalRank" and "illogical," the overall result will be "illogical." Second, during disambiguation, once the parser has found the overall result for each possible match to a noun phrase, it will pick the object or objects with the best results, so an object with an overall result from earlier in the list will be chosen over objects with overall results later in the list.

logical. You don't usually have to bother with this one, but it's provided anyway, to make the set complete. Remember, you can't overrule an objection by making an object more logical, and everything starts out logical by default, so this result really has no effect. About the only reason to use this is if you want to add emphasis to the source code, for the benefit of a human reader looking at the code - it won't affect anything in the parser, but a human reader would be able to see that you specifically intend for the action to be logical.

logicalRank(rank, key). This indicates that the object is logical, and qualifies the logicalness with a ranking.

The key argument is any arbitrary string value. This is a name - any name you want to make up - for whatever quality it is you're ranking. The purpose of the key is to permit multiple logicalRank results for a single verify, keeping them all separate. When it comes time to choose the results, the parser will try to compare apples to apples by comparing each logicalRank result to others with the same key. If two objects have the same rank value with the same key, the parser will know that both objects are exactly the same for that particular quality, and that it should therefore ignore that quality and look instead at any other logicalRank results that could distinguish the two objects.

The rank argument is any number; the higher the number, the more logical the ranking. The default ranking for a regular "logical" object is 100. Other than this, the meaning of the ranking is up to you; you can use this to fine-tune the parser's automatic object chooser for particular situations where you want it to pick one object over another.

The library uses logical rankings in a few places to do this kind of fine-tuning, and when it does, it has its own conventional rank values. You don't have to use the same values, but it might be convenient to do so. The library's conventional rank values are:

- 150: an especially good fit; this object is an especially likely match for the action. For example, a book might use this for a READ command.
- 140: similar to 150, but slightly less ideal. The library uses this for cases where an object is entirely appropriate in some general way (a key being used to unlock something, for example), but isn't the most likely unique individual object (it's not the particular key known to open this particular lock, for example).
- 100: the default ranking used by the "logical" macro. This object is a good candidate, with nothing that would make it seem unlikely, but nothing that would make it seem more likely than anything else.
- 80: slightly less than perfect. This object is a good match, but with some temporary and readily correctable attributes that make it less than perfect. The library uses this for objects with unmet preconditions attached, so that it chooses objects that can already meet any preconditions over those that need work first.
- 70: slightly less than perfect, with some attributes that aren't necessarily problems, but make it seem likely that the player would probably be referring to something else. For example, we might want to favor items already being held when we read them.
- 60: same as above, for slightly more unfavorable attributes.
- 50: logical, but not especially likely. This object can be used as a match for the action, but probably isn't the best choice. This is used for cases where the object would not normally be expected to be a good choice for the action.

logicalRankOrd(rank, key, ord). This is the same as a regular logicalRank result, but further qualifies the result with a "list ordering," which is a number given by the ord argument. The list ordering determines how important the result is relative to other logicalRankOrd results. The lower the list order, the earlier the result goes in the result list for the object. The earlier in the list, the higher the priority. So, the lowest ord value is the most important.

The basic logicalRank result type uses a list order of 100. Other than this, the order number is arbitrary. The library uses list order 150 for precondition results, but doesn't assign any ordering values other than this. The reason the library uses 150 for precondition results is that it wants precondition results to be considered less important than other factors in determining logicalness; because of the ordering, precondition results will only matter when there aren't any ordinary logicalRank results at all.

My advice is not to worry about this one - just use the plain logicalRank result instead. logicalRankOrd is included mostly for the library's use; games shouldn't usually need such microscopic control over result orderings.

dangerous. This result indicates that the action is not one that a player would perform lightly, because of apparent danger in the situation. This makes the object only slightly less logical than the default "logical" result, but its main purpose is to prevent the action from being undertaken as an implied action, or as a default action.

As always, what's important is what the player perceives. If something is in fact dangerous, but the danger isn't apparent to the player, you shouldn't add a "dangerous" result.

For example, going north through the airlock door would imply OPEN AIRLOCK DOOR. It's probably pretty obvious to a player that you don't open an airlock door unless you have your spacesuit on, so you'd probably want to add a "dangerous" result to this action. This would prevent OPEN AIRLOCK DOOR from being performed as an implied action for NORTH, and it would prevent the airlock door from being chosen as the default object if the player just types OPEN. Note that "dangerous" doesn't rule out the object. If the action is explicitly and unambiguously performed on the object, then we'll allow the action to proceed despite the danger.

illogicalAlready(msg, params...). The action is illogical, because whatever it's trying to do is already done. For example, OPEN DOOR is illogical when the door is open - but it's not always illogical, since it's perfectly logical when the door is closed.

The parameters provide a message to display to the player, to explain why the action is illogical. Within game code, you'll almost always use a simple single-quoted string here - something like this:

       illogicalAlready('{The dobj} is already open. ');

Note that you can use "{xxx}" substitutions here. For an overview of how those work, see the article on message substitution parameters. Note also that when you give the message as a string, there are no additional parameters (so the params list is left empty).

Within the library itself, you'll never see single-quoted strings in these messages. Instead, the library uses the alternative way of specifying the message, which is to provide a property pointer for a library message, along with optional parameters to the library message property. The library uses message properties rather than strings for two reasons. First, it makes it easier to translate the library to other languages, since the messages are all separated from the main library and gathered in one place. Second, it makes it easy to use different messages for different actors. We'll see more about using library message properties later in the article.

illogicalNow(msg, params...). The action is illogical on this object, due to the object's current state, but might be logical at other times. For example, BOARD RAFT is illogical when the inflatable rubber raft is deflated, but isn't always illogical.

As with illogicalAlready(), the arguments give the message to display and its parameters.

illogical(msg, params...). The action is illogical on this object at all times. This differs from illogicalAlready and illogicalNow in that the action is always going to be illogical on this object by the very nature of the object. For example, TAKE BUILDING simply makes no sense, since a building is obviously not an object you can pick up and carry around; this isn't a momentary condition of the building, but an intrinsic aspect of its nature.

As with illogicalNow, the msg argument can be a simple single-quoted string giving the message text, or it can be a library message property.

illogicalSelf(msg, params...). The action is illogical because it's attempting to do something to an object using the object itself. For example, PUT BOX IN BOX.

The reason the library provides this special separate macro, rather than simply using illogical() for these cases, is that it's relatively easy for a player to inadvertantly attempt an action like this by using a plural phrase in a command. By flagging these errors specially, we make it possible for the parser to filter them out when they occur in plurals, avoiding unnecessary messages.

nonObvious. This indicates that the object is not obvious for the action. This makes the object less likely, in terms of logicalness, than any of the "illogical" results, so it'll be chosen for disambiguation purposes only after exhausting all of the other possibilities. It also prevents the action from being used implicitly, and it prevents the object from being chosen as a default.

This can be used for situations where an object is actually a good match for a command, but this isn't meant to be obvious to the player, because the object serves a hidden purpose. For example, suppose you can pick a lock with a hairpin, but this is a puzzle. You obviously can't make the hairpin illogical for UNLOCK DOOR WITH something, but if you didn't do anything at all, and there were no other key-like objects present, then a simple UNLOCK DOOR would pick the hairpin by default. nonObvious handles this: it allows the object to be used in a command when the command is explicit and unambiguous, but it ensures that the object will never be supplied as a default for the command, and that the command will never be attempted implicitly.

inaccessible(msg, params...). The action is impossible because the object is inaccessible. This indicates that the object is in scope, but it's inaccessible to a sense that's needed for the action, such as touch or hearing. You usually won't have to use this result type in a game, since accessibility to a sense is almost always handled with a precondition. If you write your own sense-checking precondition, look at the existing ones in the library (touchObj, objVisible, objAudible, and so on) to get an idea of how to use this.

## Pre-conditions

One of the things that comes up over and over again when writing command handlers is checking for certain basic requirements before performing a command. For example, before you can use a key to unlock a door, you have to be holding the key. After writing a few command handlers, it becomes clear that the same requirements tend to come up a lot for different actions. "You have to be holding something" comes up not only for UNLOCK DOOR WITH KEY, but for all sorts of other actions as well: TURN SCREW WITH SCREWDRIVER, CHOP WOOD WITH AXE, PUT BOOK ON SHELF, SHAKE CAN, THROW DART AT TARGET.

It's tempting to think that these requirements should be tested in the action itself, but in practice, that's a bad place to make these checks. The problem with checking these requirements on the action is that it doesn't allow for cases where the requirements vary. For example, you might think it's a reasonable rule to say that EAT X requires "X must be held," and so encode that rule in the EAT action. But while "X must be held" might apply to a hot dog or a sandwich, it probably wouldn't apply to a steak or a bowl of soup; for those, being able to touch the object is sufficient. It's much better to test these requirements for each object, since that makes it easy to vary the requirements accordingly. Specifying the individual requirements for every action for every object might sound incredibly tedious at first glance, but remember that TADS 3 is object-oriented, so all we'll really end up doing is writing the requirements for a base class or two, and then overriding this inherited set of requirements for special cases.

The library's approach to testing these common requirements is something called "pre-conditions." A pre-condition is a requirement that you indicate must be met before the command can proceed. If the requirement isn't met, the command isn't allowed. This means that once you specify a pre-condition for a given action, you can stop worrying about that condition; your action handlers will never have to test for it, since they'll never even be invoked if the pre-condition fails.

The pre-condition mechanism has two really powerful features. First, it lets you take a common condition that applies to all sorts of different actions, such as "object must be held," and write the code to test that condition just once, by creating a PreCondition object. Every time you need to apply that condition, you merely list the PreCondition object in the pre-condition list for the object action where you want to apply it; there's no additional code to write for the object, since the check is entirely contained in the PreCondition object. For example, if you're creating a SANDWICH object, and you want to require that the player be holding the object before eating it, you'd simply list the objHeld precondition for EAT SANDWICH:

       // on the sandwich object
       dobjFor(Eat) { preCond = [objHeld] }

Second, a pre-condition can do more than just test that a requirement is met: a pre-condition can actually try to bring the requirement into effect automatically, using an "implied command." An implied command is simply a command that is obviously implied by the requirement; for example, "object is held" pretty obviously implies TAKE OBJECT to bring the requirement into effect. This lets your game overcome the common adventure-game annoyance of parser errors telling you what you have to do rather than just doing it; it's annoying to be told "You have to take the sandwich first," because if the parser knows this, why doesn't it just do it already?

(Note that the automatic implied command of a pre-condition is sometimes not desirable, because it would give away a puzzle, or because it would do something that the player would in all likelihood avoid because of danger. In these cases, you can avoid the implied command in one of two ways. The easiest way is not to use the pre-condition in the first place, but check for the condition explicitly somewhere else, such as in the check() routine. The other way, which is often better but a little trickier, is to use a "dangerous" or "nonObvious" verify result for the implied action.)

You attach pre-conditions to a particular object for a particular action, by using the preCond property in a dobjFor() or iobjFor() group. The preCond property simply returns a list of PreCondition objects. For example, we might add this to a screwdriver object:

       iobjFor(UnscrewWith) { preCond = [touchObj] }

In many cases, you'll want to simply add a pre-condition or two to the default set of conditions inherited from a base class. To do this, simply use inherited() to pick up the base class conditions, and add in your additional conditions:

       dobjFor(Read) { preCond = (inherited() + touchObj) }

In some cases, you'll need to remove a pre-condition applied in a base class. For example, the base Thing class in the library applies an objHeld pre-condition to EAT, but some objects don't need to be held to be eaten. For these kinds of objects, you can remove the objHeld condition by inheriting the default and then subtracting out the condition you don't want:

       dobjFor(Eat) { preCond = (inherited() - objHeld) }

Here's a list of the pre-conditions defined in the library.

objVisible - ensures that the actor performing the action can see the object. This doesn't imply any action if the condition isn't met; it simply disallows the action if the object isn't visible.

objAudible - ensures that the actor performing the action can hear the object. There is no implied action. Note that this doesn't require that the object is actually making any noise, or that the actor can perceive the kind of noise it's making; it merely requires that the object is within "hearing range" of the actor, using the normal sense connection system.

objSmellable - ensures that the actor performing the action can smell the object. There is no implied action. Note that this doesn't require that the object actually have an odor, or that it's a kind of odor that the actor can detect; it merely requires that the object is within "smelling range" of the actor.

actorStanding - ensures that the actor performing the command is in a standing posture. This implies the action STAND.

actorTravelReady - ensures that the actor performing the command is "ready for travel," meaning that the actor is in a suitable state for travel to a new location. The precondition defers to the actor's immediate container to determine the exact condition required, via the container's isActorTravelReady() and tryMakingTravelReady() methods. The default, defined in BasicLocation, simply ensures that the actor is standing. You could use this precondition to enforce novel travel rules, such as preventing travel while the actor is handcuffed to the wall. This precondition is applied by default to all travel commands by TravelConnector objects (which are the objects that link rooms together for normal room-to-room travel).

TravelerDirectlyInRoom - ensures that the traveler (which is usually an actor attempting to do something like "go east," but could be another kind of object, such as a vehicle) is located directly in the object. This implies an action that depends on the traveler and on the object, but which attempts to move the traveler into the object. (Note that this precondition is a class - it has to be instantiated when needed with some extra parameters, namely the actor doing the travel, the connector being traversed, and the room the traveler is required to be in.)

actorDirectlyInRoom - ensures that the actor is directly in the object. This is similar to TravelerDirectlyInRoom, but applies directly to the actor performing the command, even if the actor isn't the same as the traveler (the traveler differs from the actor when the actor is in a vehicle, for example).

actorReadyToEnterNestedRoom - ensures that the actor performing the action is standing, and is ready to enter the object as a nested room. The exact meaning of this is provided by the actor and by the object.

canTalkToObj - ensures that the actor performing the action can talk to the object, as defined by the actor's canTalkTo() method. There is no implied action.

objHeld - ensures that the object is being directly held by the actor performing the action. Implies TAKE.

touchObj - ensures that the actor performing the action can touch the object, which means that there are no intermediate containers that would block the actor's reach. This condition will attempt to perform implied commands to remove any obstructions, starting with the obstruction nearest the actor (in terms of containment). The exact means of removing obstructions vary according to the obstructions; for example, the obstruction created by a closed container is usually removed by opening the container.

roomToHold - ensures that the actor has room to directly hold the object. This will implicitly try to create room in the actor's hands by rearranging the actor's inventory. The exact means of rearrangement are defined by the actor via its tryMakingRoomToHold method; the implementation in the library attempts to move directly-held items into any "bag of holding" in the actor's possession, if possible.

objNotWorn - ensures that the actor is not wearing the object. Implies a DOFF command.

objOpen - ensures that the object is open. Implies an OPEN command.

doorOpen - ensures that the door is open in preparation for travel. This is essentially the same as objOpen, but has the special feature that it will only report that the door needs to be opened if the actor can see the door; if the actor can't see the door, the report merely indicates that travel is not possible. Doors are slightly special in this respect in that a travel action can depend upon a door which the actor can't even see because of darkness.

objClosed - ensures that the object is closed. Implies a CLOSE command.

objUnlocked - ensures that the object is unlocked. Implies an UNLOCK command (with no indirect object).

dropDestinationIsOuterRoom - ensures that the destination for a DROP command is an outermost room (that is, it's not nested within a containing room). If this isn't the case, then we'll perform the action that the TravelerDirectlyInRoom would perform when used with the outermost room of the actor's drop destination. (This is a pretty specialized condition that probably won't be needed in game code.)

objBurning - ensures that the object is lit (i.e., its isLit property is true). Implies a BURN command (with no indirect object).

objEmpty - ensures that the object has no contents. Implies TAKE child FROM object, where child is the first item within the object. We only try to remove one object because we'll be called repeatedly until the condition is fully met - the pre-condition mechanism always checks all of the pre-conditions again after any pre-condition performs an implied action.

You can extend the list above by defining your own PreCondition objects. If you want to enforce some set of conditions that's beyond the capabilities of the library PreCondition objects listed above, you can usually do so by creating a custom PreCondition. For details on how, see the separate article on Custom Preconditions.

## Check

As we discussed back in the verify section, the verify routine is for determining how logical an action seems from the player's perspective, irrespective of whether or not the action is actually possible or meaningful within the game world. When we want to disallow an action for reasons that wouldn't be obvious to the player, we wait until the "check" method to halt the command.

The "check" routine can do one of two things: it can let the command proceed, or it can display an error message and use "exit" to stop the action.

If the "check" routine wants to stop the action, it can simply display a message directly, using a double-quoted string; doing this is exactly the same as using mainReport. The routine can also use reportFailure, but this isn't necessary: the parser automatically considers the action to be a failure if the "check" routine uses "exit" to halt the action.

Here's a sample "check" routine. This implements a piece of clothing that we don't want to allow the player to remove - it's not important to the game to let the player character undress, so we want to simply disallow it. However, it's perfectly logical from the player's perspective to try the action, so we don't want to make it illogical; this is why we must handle it in "check" rather than in "verify."

      dobjFor(Doff)
      {
        check()
        {
          "It would be highly unladylike to get undressed away from the
          privacy of one's own bed chamber. ";
          exit;
        }
      }

## Action

The "action" routine is where the body of the action is carried out. If we've made it this far, we know the action is logical, that it's passed all of its pre-conditions, and that we've made it past any "check" conditions.

In addition to actually carrying out the action, this routine must generate a message to describe what happened. The easiest way to do this is to display a message directly, using a double-quoted string:

      dobjFor(Open)
      {
        action()
        {
          makeOpen(true);
          "Okay, {the dobj} {is} now open. ";
        }
      }

In addition, the library defines several macros that you can use instead of showing a message directly.

mainReport(msg, params...). This specifies the "main" result message for the action. There can be any number of main reports for a single action, so this isn't an exclusive sort of "main."

If msg is a single-quoted string, this acts exactly like showing a message directly with a double-quoted string. There are no additional parameters with a single-quoted string message.

The msg argument can also be a property pointer, in which case it's a library message property, and params are any parameters required for that library message method. The library will get the actual message text from the library message object; we'll see more about this later in the article.

Game code can usually use the simple single-quoted string form of this call. The library, on the other hand, always uses the property pointer form, since this keeps all of the messages in the library together in one place, to facilitate translating the library.

reportAfter(msg, params...) This adds a report that works pretty much like a main report, except that it'll be displayed after all main reports. Even if another main report is added after the reportAfter is added, the reportAfter will still be moved so it appears in the final transcript after all of the main reports.

reportBefore(msg, params...) This adds a report that appears before any main reports. Even if other main reports have already been generated, a reportBefore will be moved so that it appears in the final transcript before the first main report.

reportFailure(msg, params...). This shows a failure message. If the action routine checks a condition that it wasn't possible to test eariler (in "verify" or "check"), and finds that the command cannot proceed, the action routine can use this to show a message and mark the action as a failure.

It's not strictly necessary to indicate that an action failed, but it's useful information that's made available to the code that initiated the action. When a command is performed as a nested action, the outer action that called the nested action can test to see if the nested action succeeded or failed, and proceed accordingly. Whenever you're writing an action handler, and you consider the outcome of the action to be a failure, you should use reportFailure to indicate this to callers.

defaultReport(msg, params...). This generates a "default" report. This is a special kind of report used to indicate a very simple, minimal acknowledgment of a successful action. The important thing about default reports is that they merely acknowledge that we have done exactly what the player asked. For example, if the player enters the command OPEN BOX, and the response is "Opened," this would count as a default report: it tells the player only that we have done exactly what they asked, and nothing else. If opening the box reveals something inside, that wouldn't be part of the default report, because the player's command didn't say anything about revealing the object. (Even if the player expected the object to be revealed, that part still wouldn't be a default report - the revelation wasn't part of the explicit request, no matter what the player actually expected.)

The important thing about default reports is that they'll only be displayed if there's not also another report of some kind, and the command was explicit. We don't show default reports for implied commands, because, by definition, a default report merely acknowledges a command, and a command that the player didn't actually enter requires no acknowledgment. We leave out default reports if there are any other reports as a convenience to custom overrides; a custom "action" routine can freely inherit base class code from the library without having to worry about any default message that the library code generates, since the default message won't be shown if the custom code shows a main report instead.

Default reports will probably not be used very often in game code. They're mostly for low-level library classes that implement the basic commands.

defaultDescReport(msg, params...). This is similar to defaultReport, but is used for descriptions. Like defaultReport, a defaultDescReport is not shown if there are any other reports for the action; unlike defaultReport, though, a defaultDescReport is displayed for an implied action.

The purpose of defaultDescReport is to provide an easy way of describing something as ordinary, without contradicting any special features that are added by other parts of a generated description. For example, suppose you have a table that has no special description:

       >x table
       You see nothing special about it.

Now, suppose there's something on the table. The library's normal Surface code would add a mention of the items on the table; if we left the "nothing special" message, we'd have a somewhat self-contradictory description:

       >x table
       You see nothing special about it.  On the table are a newspaper
       and a pair of sunglasses.

Well, we clearly do see something special about it: we see that it contains the newspaper and the sunglasses. defaultDescReport helps with this. In the first case, where the table is empty, there will be no other reports for the action, so we'll keep the default description. In the second example, however, we have some additional reports, so the defaultDescReport will be suppressed, and all we'll see is this:

       >x table
       On the table are a newspaper and a pair of sunglasses.

cosmeticSpacingReport(msg, params...). This adds a message for internal spacing only; this is usually used to add a blank line or a paragraph break. The important feature of this type of report is that it doesn't count against any default reports: if there are default reports for the action, and no other reports, a cosmeticSpacingReport won't suppress the default reports.

extraReport(msg, params...) This adds an additional report, without affecting any default reports. If there are default reports and no other reports, this won't suppress the default reports.

## Library Messages

This section is mostly for people interested in the internal technical details of the library, for people translating the library, and for authors of library extensions. Game authors will not usually need to know how the library messages mechanism works.

In most cases, game authors will want to write the text of their messages directly within the game code. For the library, though, interspersing messages directly within code would be a problem, because it would require library translators to edit virtually every library module, and to search laboriously through the entire library looking for messages to translate. It would also create a huge merging problem as the library evolves: for every new version of the library, a translator would have to repeat the process from scratch on every module in the library that was updated. For its own messages, then, the library needs to separate the message text from the main library code.

The library separates message text from program code by putting all of the messages in a set of object properties. The objects that define these message properties are gathered into a single library module. A translator can translate all of the messages in the library simply by replacing this single module. (There are other parts of the library apart from the messages that require translation, so translating the library requires more than just replacing the messages and more than just replacing one module, but that's a separate story.)

The library groups the messages into three main places.

### libMessages

The libMessages object is the default message repository. This object contains many default description messages and little text fragments that are used to construct larger messages. There's nothing special about this object or about getting its messages; when the library wants to display a message from libMessages, it simply uses code like this:

       say(libMessages.roomDarkName);

### playerMessages and npcMessages

For messages generated during parsing - disambiguation questions ("which box do you mean..."), missing object questions ("what do you want to open?"), unknown word errors, and a few others - the parser doesn't look to the default libMessages repository. Instead, it uses an object provided by the command's target actor - that is, the actor to whom the command is directed: Bob in "bob, go north," for example. (When the target actor can't be determined, or isn't yet known at the time the error message is displayed, the parser uses the player character by default.)

To get this parser-specific message repository, the parser calls the method getParserMessageObj() on the target actor. The default version of this method returns the object playerMessages for the player character, and the object npcMessages for all other actors.

The playerActionMessages and npcActionMessages objects are both based on the basic libMessages object. This means that any message that these objects don't override is inherited from libMessages. Many parser error messages are defined in libMessages, since the library doesn't have a need to vary these by actor in most cases.

If you want to customize any of the parser-related messages for a particular actor, you can override getParserMessageObj() for that actor to return your own custom object. Then, you must define this object so that it provides all of the methods defined in (and inherited by) npcMessages. A simple way to override just a few messages is to create a new object that inherits from npcMessages, and then just override the methods for the messages you want to change.

### playerActionMessages and npcActionMessages

Much as the parser has its own set of special message repository objects, the action execution system has a separate set of objects. The action system, like the parser, gets the special message object from the command's target actor.

The action system calls the method getActionMessageObj() to get its special message object. By default, Actor.getActionMessageObj() returns the object playerActionMessages if the actor is the player character, and returns the object npcActionMessages for any other actor.

The library automatically retrieves the appropriate object, using getActionMessageObj(), whenever you use any of the "verify" or "action" result macros with a property pointer. The library then calls the property pointer, using the additional parameters defined in the macro, on the library message object. So, for example, if you have a mainReport like this:

       mainReport(&noResponseFrom, self);

...then the library turns this into a call like this:

       gActor.getActionMessageObj().noResponseFrom(self);

This handling applies to illogicalNow(), illogicalAlready(), illogicalSelf(), illogical(), inaccessible(), mainReport(), reportAfter(), reportBefore(), reportFailure(), defaultReport(), defaultDescReport(), cosmeticSpacingReport(), and extraReport().

### Varying messages by actor

Because the library obtains the parser and action message object individually for every target actor, it's easy to customize the messages according to which actor is performing a command. The library uses this extensively by separating player character and NPC messages into separate objects: playerMessages and playerActionMessages for the player character, and npcMessages and npcActionMessages for non-player characters.

For the player character, messages tend to be acknowledgments more than descriptions:

       >take the box
       Taken.

       >open it
       Opened.

       >put the coin in the box
       Done.

For non-player characters, however, the library defines messages that describe the action more fully. The library makes this distinction because the simple acknowledgments that are fully appropriate for the player character don't sound right when another character is the one performing actions.

       >bob, take the box
       Bob takes the box.

       >bob, open it
       Bob opens the box.

       >bob, put the coin in the box
       Bob puts the coin in the box.

You can take advantage of the per-actor message objects to customize the responses that a particular actor produces for particular actions. If you want to customize a few responses for a particular character, you merely have to do the following:

- Create a custom message object for your actor. Make it inherit from npcMessages (if it's a parser message repository) or npcActionMessages (if it's an action message repository), so that you only have to define the methods that you want to customize - the rest will automatically be inherited from the base object.
- Define the messages you want to customize by defining methods on the object that override the standard library messages. For example, here's a message object that customizes TAKE and DROP:

         bobActionMessages: npcActionMessages
           okayTake = '{You/he} grudgingly pick{s} up {the dobj/him}. '
           okayDrop = '{You/he} disdainfully put{s} down {the dobj/him}. '
         ;

- In the Actor object for your character, override getParserMessageObj() and/or getActionMessageObj() so that they return your custom message objects.

### npcMessagesDirect

The library provides a variation on the standard npcMessages object, called npcMessagesDirect. This variation overrides many of the standard parser error messages to make them sound as though they were being stated aloud, within the game, by the NPC:

        >bob, get the box
        Bob looks around.  "I don't see any box."

Some people prefer this style of parser errors, because it creates the impression that the player is talking directly to the NPC. Most authors these days prefer the default style, which is based on the idea that the player isn't talking directly to the NPC's, but rather to an unseen parser/narrator that exists on the same plane as the player, outside of the story world; when there are parsing errors, they're in this external "meta-game" context, not in the context of the story world - it's the parser that failed to understand the command, not the NPC.

If you want to use this style of message for one of your actors, you can simply override getParserMessageObj() for that actor to return npcMessagesDirect. If you want to use this style for all of your actors, you can modify Actor to return npcMessagesDirect from getParserMessageObj() when isPlayerChar() returns nil.

TADS 3 Technical Manual
Table of Contents | TADS 3 In Depth > Action Results
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