The program we'll create is a small text adventure game. This program will simulate a small world, describing the current situation to the user. The user can then type in commands (like "look" and "get key") to make things happen. First we will implement "look" and "quit" commands; we will then add "get object" and "drop object" commands; and then we'll add "go direction" (north, south, etc.) commands. We'll want to be flexible so we can easily add other commands (like "open") and have different operations do different things (for example, perhaps you can get some objects but not others).
Given those simple requirements, what should the basic program design be? Let's try to break it down into logical groupings. An adventure game involves manipulating different things, of different but related types, performing possibly different actions when given the same commands. It sounds like an object-oriented approach would be appropriate for this kind of problem, so we'll probably have a set of different object type definitions. We'll figure out how to define them in a moment, but we'll probably need to define a "root" type of all these different things, since they'll probably share many of the same operations. For the moment, let's assume that we'll define a package called "Things" that defines the root tagged type called Thing:
package Things is type Thing is tagged ...; -- We'll figure this out later. type Thing_Access is access all Thing'Class; -- Usual OO definition. -- Operations to be determined ... end Things;
We also need to take a line of text and attempt to execute it. We might want that functionality for other reasons (for example, perhaps objects can have lines of text that are executed, or perhaps monsters can accept commands), so let's separate that capability into a separate package rather than incorporating it inside package Things. So let's make a package called "Parser" and a procedure inside it called "Execute"; Execute will take one parameter (a line of text), parse that line, and execute it:
package Parser is procedure Execute(Command : in Unbounded_String); end Parser;
Finally, we'll need to kick off this whole program, so we'll create a main procedure that gets everything going. In general, it's good for main procedures to be relatively small; in this case the main procedure will run a simple loop - wait for input, and then send that input to Execute to execute the command. Let's call the main procedure (and thus the program) "Small", since it's a small text adventure game:
procedure Small is begin -- Repetitively get a command, then send it to Parser.Execute. end Small;
We already have another package called "Ustrings" that provides a routine to read a text line into an Unbounded_String; that will be useful for implementing procedure Small.
But wait - how will the main procedure know when "quit" has been commanded? There might be other reasons a player would stop the game (for example, if the player is "killed"). Probably Execute should return a Boolean value, which says whether or not more commands should be accepted. So we'll go back to Execute and change it to:
procedure Execute(Command : in Unbounded_String; Quit : out Boolean);
This particular change is only one example of many changes - I actually restructured this example several times as I was designing it. This is normal. You should try to break the system down, and then carefully examine the design to repair omissions in the interface between components. You should also strive to have a general structure so that changes you can anticipate can be easily handled.
Note that we're only worrying about the major design components and making sure their interfaces are sufficient for the job. In Ada, that means that you generally define the package declarations (specifications) first, and then once you think you've got them (basically) right you then implement the bodies. You'll notice that I haven't written any bodies yet - why bother, since I'm still working on the program's basic structure? You'll find that implementing bodies is easy if the specifications are right.
How many program units have we broken this problem into so far?
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David A. Wheeler (email@example.com)
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