User-Defined Classes and Methods#

In this chapter, we show the most basic techniques for writing object-oriented code in Dylan. We define a class, make instances of the class, initialize slots of the instances, and get and set the values of slots. We define methods, and call them on the instances. One method returns multiple values — and that is an extremely useful technique. Another method uses local variables.

In this chapter, we start to develop an example of a library that represents different kinds of time. A library is a complete unit of code that can be used by many different clients, where a client can be another library or an application program. In Chapters Class Inheritance and Multimethods, we expand and refine the example that we begin in this chapter. A Simple Library, shows the result: a complete and working library.

Requirements of the time and position classes and methods#

Our eventual goal in this book is to develop a sample application based on an airport theme. The sample application handles the scheduling of aircraft that are arriving into and departing from an airport. For more information, see Design of the Airport Application.

We know that, for our airport application, we need to represent time. There are several ways to represent time. We could say that an event happened 2 hours ago (a time offset). We could say that an event happened at 21:30 (a time of day). We must represent both kinds of time in our time library, and we must provide a way to print representations of both. In this chapter, we define a class named <time-of-day>, and we define a method that prints a representation of <time-of-day>. In Class Inheritance, we define the <time-offset> class, and a method that prints a representation of <time-offset>.

The airport application also requires us to represent physical objects (such as aircraft), and the positions (locations) of physical objects. In Modularity, we define classes that represent physical objects and positions.

Eventually, we need to be able to add times, to compare times for similarity, and to determine which of two times is greater than the other. We implement those operations in Multimethods.

We package the result of all our work into a complete and working library, in A Simple Library. Later, we refine this library to achieve greater modularity and extensibility. The final result is given in Four Complete Libraries.

User-defined classes#

A user-defined class is like a structure or a record type in other languages. When you define a class, you specify its name, its direct superclasses, and its slots. A slot has a name and a type. Normally, each instance stores its own value for the slot. A class inherits the slots defined by its superclasses, and it can define more slots if it needs them.

The <time-of-day> class#

We start by defining a class to represent the concept of a time of day, such as 21:30. The definition of the <time-of-day> class is as follows:

// A specific time of day from 00:00 (midnight) to below 24:00 (tomorrow)
define class <time-of-day> (<object>) // 1
  slot total-seconds :: <integer>; // 2
end class <time-of-day>; // 3

The top line is a comment. The // characters begin a comment, which continues to the end of the line. We also provide comments that number the lines of code after the first comment. The line numbers are useful only for discussing the code examples in the book, and would not be used in source files. You can also have multi-line comments that start with /* and end with */.

On line 1, the words define class start the class definition. The name of the class is <time-of-day>. The list following the name of the class is a list of the direct superclasses of this class. The <time-of-day> class has one direct superclass, which is the class <object>. Each user-defined class must have at least one direct superclass. If no other class is appropriate, the class must have <object> as its superclass.

Line 2 contains the only slot definition of this class. This class has one slot, named total-seconds. The slot’s type constraint is <integer>. The double colon, ::, specifies the type constraint of a slot, just as it specifies the type constraint of a module variable or of a method’s parameter.

Line 3 is the end of the class definition. The text after the word end and before the semicolon is an optional part of the definition; it documents which definition is ending. Any text appearing after the end must match the definition ending, such as end class <time-of-day>, or end class. You do not need to put any text after the end — however, such text is useful for long or complex definitions, where it can be difficult to see which language construct is ending.

The type constraint of a slot#

The type constraint of the total-seconds slot is <integer>. This slot can hold instances of <integer>, and cannot hold any other kind of object.

The type constraint of a slot is optional. Specifying a slot with no explicit type constraint is equivalent to specifying <object> as the type constraint. A slot whose type constraint is <object> can hold any object. The ability to have slots with the type constraint <object> provides flexibility that can be valuable; for more information, see Performance and Flexibility.

Use of make to create an instance#

We want to make an instance of <time-of-day>, but first we need a place to store it. We define a module variable called *my-time-of-day*, and initialize it to contain a new instance of <time-of-day>:

? define variable *my-time-of-day* = make(<time-of-day>);

The make function creates an instance of <time-of-day>. The argument to make is the class to create. The make function returns the new instance.

The instance stored in *my-time-of-day* has a total-seconds slot with no value. The next logical step is to store a value in that slot.

Getters and setters of slot values#

We can store a value in the total-seconds slot of the <time-of-day> instance by using the assignment operator, :=, as follows:

? *my-time-of-day*.total-seconds := 180;
=> 180

We can examine the value of the slot in the instance:

? *my-time-of-day*.total-seconds;
=> 180

Although these expressions may look like they are accessing the slots directly, they are not. They are abbreviations for function calls to a getter and a setter. A getter is a method that retrieves the current value of a slot in an object. A setter is a method that stores a value in a slot. Each slot in a class automatically has a getter and a setter defined for it. You can see the function-call syntax, and other information about getters and setters, in Slots.

Initialization of slots when instances are made#

So far, we have made an instance and set the value of its slot. We might like to combine those two steps and to set the slot’s value while making the instance — in other words, to initialize the slot when we make the instance. One way to do that is to provide a keyword argument to make. (Dylan offers several techniques for initializing slots; see Slots.)

Keyword arguments in function calls#

We would like to be able to call make as follows:

? make(<time-of-day>, total-seconds: 120);

We will be able to make this call after we have done a bit of homework, as we shall show in Init keywords: Keywords that initialize slots. In the preceding call to make, we provided a keyword argument, consisting of a keyword, total-seconds:, followed by a value, 120. The <time-of-day> instance returned by make has its total-seconds slot set to 120.

A keyword argument consists of a keyword followed by the keyword’s value. A keyword is a name followed by a colon, such as total-seconds:. The colon after a keyword is not a convention; it is a required part of the keyword. There must be no space between the name and the colon.

You can define functions to accept keyword arguments. When a function accepts keyword arguments, you can provide them in any order. Keyword arguments can be useful for functions that take many arguments — when you call the function, you do not need to remember the order of the arguments. Keyword arguments are optional arguments, so they are useful for parameters that have a default value that you may want to override at times. For more information about keyword arguments, see Parameter lists.

How does make know that the value of the total-seconds: keyword should be used to initialize the total-seconds slot? The keyword and the slot happen to have the same name, but that is not how it knows. Before you can use the total-seconds: keyword argument to make, you must associate that keyword with the total-seconds slot in the class definition.

Init keywords: Keywords that initialize slots#

The total-seconds: keyword is an init keyword — a keyword that we can give to make to provide an initial value for a slot. To make it possible to give an init keyword to make, we need to use the init-keyword: slot option when we define the class. A slot option lets us specify a characteristic of a slot. Slot options appear after the optional type specifier of a slot.

Here, we redefine the <time-of-day> class to use the init-keyword: slot option:

1 // A specific time of day from 00:00 (midnight) to below 24:00 (tomorrow)
2 define class <time-of-day> (<object>)
3   slot total-seconds :: <integer>, init-keyword: total-seconds:;
4 end class <time-of-day>;

The preceding definition redefines the class <time-of-day>. That is, this new definition of <time-of-day> replaces the old definition of <time-of-day>.

In line 3, the init-keyword: slot option defines total-seconds: as a keyword parameter that we can give to make when we make an instance of this class. Now that we have defined total-seconds: as an init keyword, we can provide the keyword argument as follows:

? *my-time-of-day* := make(<time-of-day>, total-seconds: 120);
=> {instance of <time-of-day>}

The preceding expression creates a new instance of <time-of-day>, and stores that instance in the variable *my-time-of-day*. The value of the total-seconds slot of this instance is initialized to 120. The assignment operator returns the new value stored; in the preceding call, the new value is the newly created instance of <time-of-day>, which the listener displays as {instance of <time-of-day>}.

We can use the getter to verify that the slot has an initial value:

? *my-time-of-day*.total-seconds;
=> 120

If you call make and provide a keyword that has not been declared as a valid keyword for the class, you get an error; for example,

? make(<time-of-day>, seconds: 120);
=> ERROR: seconds: is not a valid keyword argument to make for {class <time-of-day>}

Methods for handling time#

We decided to represent the time of day with a single slot named total-seconds. An alternate choice would be to give the class three slots, named hours, minutes, and seconds. People naturally think of time in terms of hours, minutes, and seconds. We chose to store the total seconds instead, because we envisioned needing to operate on times, such as adding a time of day to a time offset. For example, if it is 9:00 now, and a meeting is to be held 2.5 hours from now, then the meeting will be held at 11:30. It is easier to operate on a single value, rather than on three values of hours, minutes, and seconds. On the other hand, it is convenient to see times expressed as hours, minutes, and seconds. We can represent the instances with a single slot, and can provide methods that let users create and see <time-of-day> instances as being hours, minutes, and seconds.

Method for encode-total-seconds#

We can provide a method that converts from hours, minutes, and seconds to total seconds:

1 define method encode-total-seconds
2     (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
3  => (total-seconds :: <integer>)
4   ((hours * 60) + minutes) * 60 + seconds;
5 end method encode-total-seconds;

Line 2 contains the parameter list of the method encode-total-seconds. The method has three required parameters, named hours, minutes, and seconds, each of type <integer>. This method is invoked when encode-total-seconds is called with three integer arguments.

Line 3 contains the value declaration, which starts with the characters =>. It is a list declaring the values returned by the method. Each element of the list contains a descriptive name of the return value and the type of the value (if the type is omitted, it is <object>). In this case, there is one value returned, named total-seconds, which is of the type <integer>. The name of a return value is used purely for documentation purposes. Although methods are not required to have value declarations, there are advantages to supplying those declarations. When you provide a value declaration for a method, the compiler signals an error if the method tries to return a value of the wrong type, can check receivers of the results of the method for correct type, and can usually produce more efficient code. These advantages are significant, so we use value declarations throughout the rest of this book. For more information about value declarations, see Value declarations.

Line 4 is the only expression in the body. It uses arithmetic functions to convert the hours, minutes, and seconds into total seconds. All methods return the value of the expression executed last in the body. This method returns the result of the arithmetic expression in line 4.

In line 5, we could have simply used end;. We provided end method decode-total-seconds; for documentation purposes. Throughout the rest of this book, we provide the extra words after the end of a definition.

We can call encode-total-seconds with arguments representing 8 hours, 30 minutes, and 59 seconds:

? encode-total-seconds(8, 30, 59);
=> 30659

We find it convenient to call encode-total-seconds to initialize the total-seconds slot when we create an instance of <time-of-day>, or when we store a new value in that slot. Here, for example, we create a new instance:

? define variable *your-time-of-day*
  = make(<time-of-day>, total-seconds: encode-total-seconds(8, 30, 59));

We examine the value of the total-seconds slot:

? *your-time-of-day*.total-seconds;
=> 30659

The result reminds us that it would be useful to convert in the other direction as well — from total seconds to hours, minutes, and seconds.

Method for decode-total-seconds#

We define decode-total-seconds to convert in the other direction — from total seconds to hours, minutes, and seconds:

1define method decode-total-seconds
2    (total-seconds :: <integer>)
3 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
4  let (total-minutes, seconds) = truncate/(total-seconds, 60);
5  let (hours, minutes) = truncate/(total-minutes, 60);
6  values(hours, minutes, seconds);
7end method decode-total-seconds;

We can use decode-total-seconds to see the value of the total-seconds slot:

? decode-total-seconds(*your-time-of-day*.total-seconds);
=> 8
=> 30
=> 59

The value declaration on line 3 specifies that decode-total-seconds returns three separate values: the hours, minutes, and seconds. This method illustrates how to return multiple values, and how to use let to initialize multiple local variables. We describe these techniques in Sections Multiple return values and Use of let to declare local variables.

Multiple return values#

The method for decode-total-seconds returns three values: the hours, the minutes, and the seconds. To return the three values, the method uses the values function as the expression executed last in the body. The values function simply returns all its arguments as separate values. The ability to return multiple values allows a natural symmetry between encode-total-seconds and decode-total-seconds, as shown in symmetry-of-encode-decode.

Symmetry of encode-total-seconds and decode-total-seconds.#



Return value(s)


hours, minutes, seconds




hours, minutes, seconds

Lines 4 and 5 of the decode-total-seconds method contain calls to truncate/. The truncate/ function is a built-in Dylan function. It takes two arguments, divides the first by the second, and returns two values: the result of the truncating division, and the remainder.

Use of let to declare local variables#

When a function returns multiple values, you can use let to store each returned value in a local variable, as shown in lines 2 and 3 of the decode-total-seconds method in Method for decode-total-seconds. On line 2, we use let to declare two local variables, named total-minutes and seconds, and to initialize their values to the two values returned by the truncate/ function. Similarly, on line 3, we use let to declare the local variables hours and minutes.

The local variables declared by let can be used within the method until the method’s end. Although there is no begin to define explicitly the beginning of a body for local variables, define method begins a body, and its end finishes that body. Local variables are scoped within the smallest body that surrounds them, so you can use begin and end within a method to define a smaller body for local variables, although doing so is usually not necessary.

Second method for decode-total-seconds#

The decode-total-seconds method is called as follows:

? decode-total-seconds(*your-time-of-day*.total-seconds);

If we envision calling decode-total-seconds frequently to see the hours, minutes, and seconds stored in a <time-of-day> instance, we can make it possible to decode <time-of-day> instances, as well as integers. For example, we can make it possible to make this call:

? decode-total-seconds(*your-time-of-day*);

We can implement this behavior easily, by defining another method for decode-total-seconds, which takes a <time-of-day> instance as its argument:

define method decode-total-seconds
    (time :: <time-of-day>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
end method decode-total-seconds;)

The decode-total-seconds generic function and its methods shows the two methods for the decode-total-seconds generic function.

The decode-total-seconds generic function and its methods.

// Method on <integer>
define method decode-total-seconds
    (total-seconds :: <integer>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
  let (total-minutes, seconds) = truncate/(total-minutes, 60);
  values(hours, minutes, seconds);
end method decode-total-seconds;

// Method on <time-of-day>
define method decode-total-seconds
    (time :: <time-of-day>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
end method decode-total-seconds;

Looking at The decode-total-seconds generic function and its methods, we analyze what happens in this call:

? decode-total-seconds(*your-time-of-day*);
  1. The argument is an instance of <time-of-day>, so the method on <time-of-day> is called.

  2. The body of the method on <time-of-day> calls decode-total-seconds on an instance of <integer>, the value of the total-seconds slot of the <time-of-day> instance. In this call, the argument is an integer, so the method on <integer> is called.

  3. The method on <integer> returns three values to its caller — the method on <time-of-day>. The method on <time-of-day> returns those three values.

The purpose of the method on <time-of-day> is simply to allow a different kind of argument to be used. The method extracts the integer from the <time-of-day> instance, and calls decode-total-seconds with that integer.

Method for say-time-of-day#

We can provide a way to ask an instance of <time-of-day> to describe the time in a conventional format, such as 8:30. For the application that we are planning, there is no need to view the seconds. We want the method to print the description in a window on the screen. We define a method named say-time-of-day:

1 define method say-time-of-day (time :: <time-of-day>) => ()
2   let (hours, minutes) = decode-total-seconds(time);
3   format-out
4     ("%d:%s%d", hours, if (minutes < 10) "0" else "" end, minutes);
5 end method say-time-of-day;

On line 1, we provide an empty value declaration, which means that this method returns no values.

On line 2, we use let to initialize two local variables to the first and second values returned by decode-total-seconds. Remember that decode-total-seconds returns three values (the third value is the seconds). For the application that we are planning, the say-time-of-day method does not need to show the seconds, so we do not use the third value. It is not necessary to receive the third value of decode-total-seconds; here we do not provide a local variable to receive the third value, so that value is simply ignored.

On line 4, we use if to print a leading 0 for the minutes when there are fewer than 10 minutes, such as 2:05.

We can call say-time-of-day:

? say-time-of-day(*your-time-of-day*);
=> 8:30

? say-time-of-day(*my-time-of-day*);
=> 0:02

The listener displays the output (printed by format-out), but displays no values, because say-time-of-day does not return any values.


In this chapter, we covered the following:

  • We defined a class (with define class).

  • We created an instance (with make).

  • We read the value of a slot by calling a getter.

  • We set the value of a slot by using :=, the assignment operator.

  • We defined a method that returns multiple values (with values), and showed how to initialize multiple local variables (with let).

  • We showed the syntax of some commonly used elements of Dylan; see Syntax of Dylan elements..

Syntax of Dylan elements.#

Dylan element

Syntax example

calling a getter


calling a setter

*my-time-of-day*.total-seconds := 180;



single-line comment

// Text of comment

multiline comment

/* Text of comment that spans more than one line */

value declaration

=> (total-seconds :: <integer>)