# The Default Display Function

The T3 VM does not have a built-in input/output system; as far as the VM is concerned, input/output operations are part of the host environment, and not part of the VM. As a result, T3 doesn't know what to do with self-printing strings (strings enclosed in double quotes in a TADS program's source code) or embedded expressions (expressions contained in << >> sequences within double-quoted strings). To enable implicit display operations, the VM has a mechanism that lets the running program specify a function, defined in the program code, to call to display self-printing strings and embedded expressions. This is the "default display function." The program can also define a method to be called on the active self object at the time a string is to be displayed; this is the "default display method."

Note: if you're using the adv3 library, the library will take care of the VM-level display function registration. You'll generally want to work with the adv3 output manager instead of directly with the VM-level display mechanism.

The program can define a default display function and method simultaneously. The VM chooses whether to call the method or the function on a case-by-case basis, each time a value is to be displayed:

- If all of the following conditions are true, the VM calls the default display method:
  - A default display method has been defined (via the t3SetSay() function)
  - There is a valid self object that is displaying the string or embedded value
  - The self object defines or inherits the default display method
- If any of the necessary conditions for invoking the default display method aren't met, then the VM invokes the default display function instead. If no default display function has been defined (via the t3SetSay() function), the VM throws an error.

To set the default display function, call the t3SetSay() function in the t3vm function set, passing a function pointer argument (this must be a program-defined function, not an intrinsic function). To set the default display method, call t3SetSay() with a property pointer argument.

Most programs can simply set up the default display function and method once at program startup. However, some programs might wish to change the function from time to time. For example, a program might wish to use special output filtering at some times but not others; this can be achieved by switching between a version of the display function that performs the filtering, and another version that displays unfiltered output.

## Writing a display function

A default display function takes a single argument, which is the value to be displayed, and returns no value. The simplest implementation is to simply pass the value to the tadsSay() function (in the tads-io function set) to display the value on the console:

    myDispFunction(val)
    {
      tadsSay(val);
    }

To establish myDispFunction() as the default display function, you'd write a line of code like this:

    t3SetSay(myDispFunction);

t3SetSay() call in your main() routine, since this would establish the display function during start-up. Of course, you can also switch display functions at any time during program execution, which can be useful for certain special effects. For example, if you wanted to display text in all capitals at certain times (such as when the player is in a particular location), you could switch to a capitalizing display function:

    dispAllCaps(val)
    {
      tadsSay(val.toUpper());
    }

    // when entering all-caps room
    t3SetSay(dispAllCaps);

    // when leaving all-caps room
    t3SetSay(myDispFunction);

## Writing a display method

The default display method works just like the default display function, but it's a method on an object or class rather than a function. If you use a default display method, you will in most cases define one method in one of your classes that is near the root of your class hierarchy, so that most or all of your other objects and classes inherit the method.

Like the default display function, the default display method takes a single argument, which is a value to be displayed, and returns no value. The method should simply display the value using whatever mechanism you wish.

    class Item: object
      myDispMethod(val) { tadsSay(val); }
    ;

The benefit of using a default display method instead of (or in addition to) a display function is that the method can use properties of the "self" object to customize the display. For example, suppose that you defined a class of objects that all have a color attribute, and you wanted to create a mechanism that lets you write generic messages describing instances of the class, while still customizing the color name in the messages. You could do this by defining a substitution string - let's say it's "COLOR" - then looking for that string in display values and substituting the color attribute. Here's some code that would accomplish this.

    class ColorItem: Item
      myDispMethod(val)
      {
        /* substitute color placeholder strings */
        if (dataType(val) == TypeSString)
          val = rexReplace('COLOR', val, colorName, ReplaceAll);

        /* display the value */
        tadsSay(val);
      }
      sdesc = "COLOR item"
      ldesc = "It's a COLOR item. "
    ;

    redItem: ColorItem colorName='red';
    blueItem: ColorItem colorName='blue';

The implementation of myDispMethod() in the ColorItem class, which is inherited by redItem and blueItem, checks the datatype of the value to be displayed. If the value is a string, the method performs a replacement on the string, substituting the string in the self object's colorName property for any instance of the text "COLOR" in the original string.

## How the VM calls the display function

The VM calls the current display method or function each time your program evaluates a double-quoted string, and each time the program evaluates an expression embedded in a double-quoted string with the << >> syntax.

The argument to the display function can be of any type. When you evaluate a double-quoted string, the VM calls the display function with a single-quoted string containing the same text as the double-quoted string. However, when you evaluate an expression embedded in a double-quoted string using the << >> syntax, the VM calls the display function with the result of evaluating the expression. This value can be of any type.

Note that the display function should generally display nothing when called with a nil argument. This allows you to use expressions that have side effects, but which return no value, as embedded expressions. We will see an example of this a little later.

By way of explanation, we could rewrite any double-quoted string in the program as a call to the display function with the string value as the argument. So, we could rewrite this:

    f1() { "Hello!"; }

like this:

    f1() { myDispFunction('Hello!'); }

Similarly, any time there is an embedded expression in a string, we could rewrite the entire string as a series of calls to the display function. We could thus rewrite this:

    f2() { "Hello <<Me.nameString>>!  Your age is <<Me.age>>."; }

like so:

    f2()
    {
      myDispFunction('Hello ');
      myDispFunction(Me.nameString);
      myDispFunction('!  Your age is ');
      myDispFunction(Me.age);
      myDispFunction('.');
    }

When a default display method is in effect, and you display a string or embedded expression from an object that defines or inherits the display method, double-quoted strings are displayed by calls to the method. For example, assume that we have a class named DispItem that defines the current default display method. We could then rewrite this:

    obj1: DispItem
      sdesc = "My name is <<nameString>>."
    ;

as this:

    obj1: DispItem
      sdesc
      {
        self.myDispMethod('My name is ');
        self.myDispMethod(self.nameString);
        self.myDispMethod('.');

      }
    ;

When the VM calls the display method, self is the object that actually defines the property or method displaying the string. This applies even for embedded expressions. Consider this example:

    obj2: DispItem
      sdesc = "The other object is <<obj3.openDesc>>"
    ;

    obj3: DispItem
      openDesc { isOpen ? "open" : "closed"; }
      isOpen = true
    ;

This is a bit complicated, because we are evaluating a double-quoted string which has an embedded expression, which in turn evaluates a double-quoted string in a different object. So, what self object is in effect when we display "open" or "closed"?

This is easier to answer if we use our rewriting rules. We can rewrite the example like this:

    obj2: DispItem
      sdesc
      {
        self.myDispMethod('The other object is ');
        self.myDispMethod(obj3.openDesc);
      }
    ;

    obj3: DispItem
      openDesc
      {
        isOpen ? self.myDispMethod('open')
               : self.myDispMethod('closed');
      }
      isOpen = true
    ;

We can see that we start out in obj2.sdesc, and display the first fragment of the string ("The other object is "); self is clearly obj2 for this display method call. We then evaluate obj3.openDesc (using the normal TADS order of evaluation rules, we must evaluate a function's arguments before we can call the function). So, we find ourselves in obj3.openDesc. This method chooses to display either "open" or "closed", depending on its isOpen property value. Once again, the rewrite makes it fairly obvious what's going on: we call myDispMethod to display "open" or "closed", with self set to obj3. This method returns no value, which means that its effective return value is nil. Finally, we return back to where we came from in obj2.sdesc, where we call myDisplayMethod() with the nil return value from obj3.openDesc, using obj2 for the self object.

It should be noted that the compiler does not actually make the transformations above; the actual compiled representation is a lot more compact than this, since the T3 byte-code has dedicated instructions for displaying strings and expressions. The effect, however, is the same.

TADS 3 System Manual
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