Chapter 3 - Essential Lisp in Twelve Lessons

Lesson 7 - Essential Function Definition

DEFUN defines named functions

You can define a named function using the DEFUN form:

 (defun secret-number (the-number)
     (let ((the-secret 37))
        (cond ((= the-number the-secret) 'that-is-the-secret-number)
              ((< the-number the-secret) 'too-low)
              ((> the-number the-secret) 'too-high))))
 SECRET-NUMBER

We described LET, COND, and ' (a.k.a. QUOTE) in Lesson 3. The numeric comparison functions have the obvious meaning.

The DEFUN form has three arguments:

1. the name of the function: SECRET-NUMBER,
2. a list of argument names: (THE-NUMBER), which will be bound to the function's parameters when it is called, and
3. the body of the function: (LET ...).

Since all three of these should stand for themselves, DEFUN does not evaluate any of its arguments. (If it did, you'd face the inconvenience of having to quote each argument.)

DEFUN returns the name of the defined function, and installs a global definition using the name, parameter list, and body that you supplied. Once you create a function using DEFUN, you can use it right away:

 (secret-number 11)
 TOO-LOW

 (secret-number 99)
 TOO-HIGH

 (secret-number 37)
 THAT-IS-THE-SECRET-NUMBER

When you call the function, its parameter (e.g. 99 in the second example) is bound to the argument name (i.e. THE-NUMBER) you supplied in the definition. Then, the body of the function (i.e. (LET ...)) is evaluated within the context of the parameter binding. In other words, evaluating (SECRET-NUMBER 99) causes the body of the SECRET-NUMBER function definition to be executed with the variable THE-NUMBER bound to 99.

Of course, you can define a function of more than one argument:

 (defun my-calculation (a b c x)
     (+ (* a (* x x)) (* b x) c))
 MY-CALCULATION

 (my-calculation 3 2 7 5)
 92

When calling a function, parameters are bound to argument names in order. Lisp has several optional variations on the list of argument names. Formally, this list is called a lambda list -- we'll examine some of its other features in Chapter 21.

LAMBDA defines anonymous functions

At times you'll need a function in only one place in your program. You could create a function with DEFUN and call it just once. Sometimes, this is the best thing to do, because you can give the function a descriptive name that will help you read the program at some later date. But sometimes the function you need is so trivial or so obvious that you don't want to have to invent a name or worry about whether the name might be in use somewhere else. For situations like this, Lisp lets you create an unnamed, or anonymous, function using the LAMBDA form. A LAMBDA form looks like a DEFUN form without the name:

(lambda (a b c x)
   (+ (* a (* x x)) (* b x) c))

You can't evaluate a LAMBDA form; it must appear only where Lisp expects to find a function -- normally as the first element of a form:

 (lambda (a b c x)
     (+ (* a (* x x)) (* b x) c))
 Error

 ((lambda (a b c x)
      (+ (* a (* x x)) (* b x) c))
   3 2 7 5)
 92