5.1 A Turn of Events

The player in a way controls the execution of an Alan adventure. Each of his inputs are taken care of and acted upon by the run-time system. The execution of an Alan adventure starts by executing the start section. Then the player is prompted for a command.

The player input is analysed according to the explicit and implicit syntax rules and converted to an execution of verb bodies (global and in possible parameters) or exits (in case of directional commands).

After the players command has been taken care of all rules are evaluated and possibly executed. Then each of the other actors execute one step (if active) and for each actor the rules are evaluated again. Finally any events that are scheduled are fired before prompting the player again.

So to summarise:

 
get and execute a player command
evaluate all rules
for each actor
	execute one step (if active)
	evaluate all rules as above
end
check for and execute any pending events

Then the user is prompted for another command and everything is repeated.

A player command may be either a verb or a direction. A verb is executed by checking the syntax of the input, performing any preconditions (checks) and then executing the verb bodies (as described in Verbs ). A directional command is executed by finding any exit in that direction, evaluating the checks and the body (if any) of that exit and locating the hero at the new location.

5.2 Player Input

The syntax defined in the Alan source is the basis for what the player is allowed to input. Commands with these formats form the basic statements available to the player. In addition the following combinations and variations are possible:

• concatenating of statements using AND or THEN , like
  > open the door then enter
• the use of IT to refer to the last object mentioned in the previous command, e.g.
  > take the book and read it
• references to multiple objects using AND , this allows
  > take the blue vase and the pillow
• reference to multiple objects using ALL or EVERYTHING
  > drop all
• excluding objects using BUT or EXCEPT , like:
  > wear everything except the bowler hat
• the use of THEM to refer to the multiple objects referenced in the last command, e.g.
  > remove the hat and the scarf then drop them

The reference to multiple objects (or actors) in a position is, of course, only allowed if the adventure author has allowed it by using a multiple indicator in the syntax definition (see Syntax Definitions ). The variations above are built in and handled automatically by the run-time system.

The interpreter also automatically restricts parameter references to objects and actors at the current location. I.e. the player can only refer to objects and actors that are present in his input. The one single exception is if the syntax for the command uses the omnipotent '!' indicator, see Syntax Definitions for details. For hints on other ways to allow references to objects and actors that are not at the current location, refer to Distant & Imaginary Objects .

The use of ALL results in the execution of the appropriate verb for all objects at the current location, except the ones that does not pass all checks for the verb (see Verbs for further details on this).

Another restriction placed on the player input by the interpreter is that the words the player is allowed to use can only contain alphabetic characters. This must be kept in mind when naming verbs that use the default syntax (an explicit SYNTAX statement can always specify other player words to trigger the verb).

5.3 Run-time Contexts

When the player enters a command the Alan run-time system evaluates the various constructs from the adventure description (source) as described above. Depending on the player's command evaluation of different parts of the adventure may be triggered. These parts all have different conditions under which they are evaluated and also have different contexts. Four different execution contexts can be identified:

• execution of a verb, during the execution of a verb (the syntax and verb checks and the verb bodies), which is the result of the player entering a command that was not a directional command, parameters are defined and may be referenced in the statements and expressions. Also the ACTOR is set to the hero and LOCATION to the location where the hero is ( HERE refers to the location of the hero).
• execution of descriptions, these are started as the response to a directional command, a LOOK or DESCRIBE statement, or a LOCATE statement operating on the hero. During this no parameters are defined, ACTOR is set as above, and LOCATION of course to the current (or new) location. The description clauses for objects and locations as well as the DOES part of locations are evaluated in this context. DOES -parts are executed for all actors entering a location with ACTOR set to the current actor.
• execution of actors and rules, each actor performs his step and after each actor all rules are executed. In these contexts no parameters are defined but ACTOR is set to the actor that is executing or was executing immediately preceding the rules. So you could say that rules are run for each actor, and LOCATION is set to that of the executing actor ( HERE refers to where the executing actor is).
• execution of events, no parameters and no actor is defined. The location is set to where the event was scheduled to be executed (see also Event Statements ).

So the execution of various parts of the adventure source can also be said to have a number of different focuses, meaning where the action is considered to take place:

• the hero - the actions of the player are always focused on the hero and the actions performed are always related to where the hero is
• an actor - steps executed by an actor are always focused where the actor is
• an event - code executed in events are focused where the event was specified to take place (see Event Statements ).
• a rule - rules are executed once after each actor (including the hero) with the focus set to where that actor is

5.4 Moving Actors

The main way to move actors are the exits (see Locations ). They, of course, only apply to the hero, but are executed if the player inputs a directional command, i.e. a word defined as the name for an exit in any location. First the current location is investigated for an exit in the indicated direction, if there is none an error message is output. Otherwise that exit is examined for CHECK s which are run according to normal rules (see Verbs ). If no CHECK was present or if the check passed the statements in the body (the DOES -part) is executed. The hero is then located at the location indicated in the exit header, which will result in the description of the location (by executing the DESCRIPTION -clause of the location) and any objects or actors present (by executing their DESCRIPTION s).

When any actor (including the hero) is located at a location, the DOES -clause of that location is executed as if the actor had moved into that LOCATION . The actor which was moved will be the ACTOR even though the movement was not caused by himself (but the result of an event, for example). So this is also the last step in the sequence of events caused by locating the hero somewhere.