# Adding the Bird and the Nest

We have already defined one object in the game that isn't a room: the me object that defines the player character. This is a slightly unusual object because it has to act as the player character, so it's necessary to define several properties on it you wouldn't normally need to use elsewhere. That's why it's predefined for you in the template file you copied when first setting up your source files for the Heidi game, so that you wouldn't have to worry about it. We might take a quick look at it, however, before going on to define the objects that will represent the bird and the nest:

    + me: Thing 'you;;heidi'
        isFixed = true
        proper = true
        ownsContents = true
        person = 2
        contType = Carrier
    ;

Note that the definition starts with a + sign. This indicates that the object that's about to be defined is initially located within the immediately preceeding object that has one fewer plus signs (in this case, that means no plus signs at all, so the room where me starts out is beforeCottage, the starting location). The programmatic name of the object is me, which comes next. Following the colon is Thing, the class to which the me object belongs. You might think it's odd that the player character should be a mere Thing, but adv3Lite makes this possible (it wouldn't be in adv3, for example) provided we go on to define the properties that follow.

The first property that's defined is the vocab property, though this is done implicitly via a template (i.e. we don't need to explicitly write vocab = 'you;;heidi'). We'll explain this property in a bit more detail below, but for now we'll just say that defining it this way means that the me object is called 'you' in text that appears in the interpreter and can also be referred to as 'heidi' by the player. The next definition, isFixed = true, means that me object isn't something that can be picked up and carried around (although, obviously, this doesn't stop it moving around of its own accord, as we've already seen: moving the me object around is precisely what happens when the player character moves from one room to another). Defining isFixed = true also has the effect of preventing the me object appearing as an item in room listings: you don't want your players to see "You can see you here" and defining the isFixed property as true prevents that from happpening. The next line, proper = true, ensures that the me object is never referred to with an article (we don't want the game producing text like "You see a you here" or "You can't do that to the you"; such text probably wouldn't occur anyway, but it's best to make sure). Then comes ownsContents = true, which means that the me object can be regarded as the owner of anything located within it (so that, for example, the player can refer to "my bag" and the parser should be able to tell which bag is meant). Then comes person = 2 which tells the parser that we're going to tell the game from a second-person perspective, so that, for example, the game will generate messages like "You can't do that" rather than "I can't do that" or "Heidi can't do that." If you wanted to write a game in the first person or the third person you could change person to 1 or 3, but this isn't an option we'll be exploring any further here. Finally we define contType = Carrier, which means that the me object is to be regarded as carrying any objects located within in it (for example if we had gone on to define ++ bag: Thing 'bag'; on the next line, Heidi would be carrying the bag).

Don't worry if you're entirely sure you've grasped all of that just yet. As we said above, the me object is something of a special case. We'll now go on to define a couple of objects that are rather more typical of the things you'll normally be defining in a game, namely the bird and its nest. We'll start with the bird, which should look like this:

    + bird: Thing 'baby bird;;nestling'
        "Too young to fly, the nestling tweets helplessly. "
    ;

Type this code in immediately after the definition of the forest room, so that bird starts out in the middle of the forest.

Now let's look at this definition a little more closely. Once again bird is the programmatic name of the object (the name by which we can refer to it elsewhere in our code), and Thing is the class to which it belongs (which defines a whole bunch of standard behaviour that may be far from apparent at first sight, as we'll soon see). The two properties we've then gone on to define are the vocab property and the desc property. It's exactly the same as if we'd written:

    + bird: Thing
        vocab = 'baby bird;;nestling'
        desc = "Too young to fly, the nestling tweets helplessly. "
    ;

But since we're likely to define these two properties on virtually every Thing we create in our game, the library template allows us the shortcut of omitting the explicit property names, as shown above, so we'll use the short-form definition for every Thing we create from now on (or at least, wherever we can). Note, however, that when we use a template, we do have to define the properties in the right order. Neither of the following definitions would work:

    + bird: Thing
        "Too young to fly, the nestling tweets helplessly. "
    ;

    + bird: Thing  "Too young to fly, the nestling tweets helplessly. "
        'baby bird;;nestling'
    ;

The first won't work because we've missed out the vocab property altogether. The second won't work because we've tried to define the desc property before the vocab property, and that won't match the template. As it so happens, though, you can define vocab without desc, so the following would be legal (though not normally advisable):

    + bird: Thing 'baby bird;;nestling'
    ;

But what do the vocab and desc properties actually do? The desc property is probably the simpler to explain; it simply defines the description the game will show when the player types EXAMINE BIRD or the equivalent:

    >x bird
    Too young to fly, the nestling tweets helplessly.

The vocab property is a bit more complex, because it actually provides a way of defining several properties at once. The format of the vocab property in full is:

       vocab = 'article name; additional adjectives; additional nouns; pronoun'

But we don't need to define all these sections if we don't need them all. In the case of the bird we've defined the name ('baby bird') and an additional noun ('nestling'). The name is the name by which the parser will refer to this object in text displayed to the player (e.g. "You see a baby bird here."). At the same time the library assumes that since this object is called 'baby bird', the player should be able to use 'baby', 'bird' and 'baby bird' to refer to it (in fact any combination of the words 'baby' and 'bird' will work; if the player insists on typing X BIRD BABY BABY BIRD BIRD, the parser will still know what is meant). However, by default the parser will take 'baby' to be an adjective and 'bird' to be a noun, and will prefer a noun match to a purely adjectival match. That means, for example, that if you had the bird object in the same room as a baby object (with a name of 'young baby', say), the command X BABY would select the (human) baby rather than the baby bird.

Since the description of the bird uses the word 'nestling', the player could use the word 'nestling' to refer to the bird, and we need to ensure that the parser will recognize it. We therefore add it as an additional noun (i.e. a noun that can be used to refer to the bird but doesn't form part of its name). We do so here by leaving the additional adjectives section blank (by simply typing two semicolons together) and then adding 'nestling'. If we wanted to add a pronoun (such as 'it') we could then type a further semicolon and the word 'it', but since the adv3Lite library assumes every Thing is an 'it' by default we don't need to. If we wanted to define a feminine, masculine or plural object we could define the pronoun as 'her', 'him' or 'them' respectively, and all the appropriate properties would be set for us.

If you want the full story on the vocab property, which looks pretty complex at first, but which can save you a lot of time in the long run, you can find it in the adv3Lite library manual.

Now let's define the nest. This starts out in the clearing, so you should type the following immediately after the definition of the clearing room:

    + nest: Thing 'bird\'s nest; carefully woven; moss twigs'
        "The nest is carefully woven of twigs and moss. "

        contType = In
    ;

Much of this should be starting to become familiar. The programmatic name of the nest object is nest. Once again, it's of class Thing. The name of the object when the parser displays a message about it to the player is "bird's nest" and the player can refer to it as "bird's nest", "nest" or "bird's". Since the description of the nest refers to moss and twigs we add these as additional nouns in the vocab property. The description also talks about them being "carefully woven", so we define these two words as additional adjectives (yes, I know "carefully" is actually an adverb, but we can treat it as an adjective for the purpose of this object definition, right?); this will allow the player to refer to a "carefully woven nest" or whatever.

Note the backslash (\) before the apostrophe in 'bird\'s nest'. If we had simply written 'bird's nest; carefully woven; moss twigs', then it would have appeared to the compiler that the vocab property ended immediately after 'bird' and it wouldn't have known what to do with s nest; carefully woven; moss twigs', and a whole spate of compiler errors would have resulted. When we want to use an apostrophe or single-quote mark inside a single-quoted string, as here, we thus have to escape it with the escape character, which in TADS 3 is the backslash (\). The backslash can also be used to escape a double-quote mark (") inside a double-quoted string, as well as a number of other special characters.

Finally, note that we have given the contType property the value of In. Think of contType as standing for "containment type". Defining contType = In means that anything located within the nest (using the + syntax or its various equivalents) is to be regarded as being inside the nest (as opposed to being on it, under it, behind it, attached it or whatever). The definition contType = In thus makes the nest a container. We need the nest to be a container since Heidi needs to be able to put the bird back in its nest.

If you now compile and run the game you should find you can do quite a bit more than you could before. You can now pick up the nest and the bird and carry them around. You should also be able to put the bird in the nest. You can also drop the bird and the nest once you're carrying them. If you type the INVENTORY command (I for short) while holding the bird or the nest you'll see them included in the list of things you're carrying. This may not seem like much; the novelty will soon wear off; but remember that you really haven't done much to define any of this behaviour. You're getting it all for free, as it were, as it's all predefined for you on the Thing class.

adv3Lite Library Tutorial
Table of Contents | Heidi: our first adv3Lite game > Adding the Bird and the Nest
Prev: Defining the Game's Locations Next: Adding the Tree and the Branch