Variable-Arity Procedures

Summary: We consider how and why to write procedures that may take different numbers of parameters.

Contents:

• Introduction
• Creating Your Own Variable-Arity Procedures
  • An Example: Displaying a Line
• Enforcing a Minimum Arity
  • An Example: Displaying a Line with Separators
  • An Example: Set Difference
  • Requiring More than One Parameter

Introduction

A procedure's arity is the number of parameters it takes. For instance, the arity of the cons procedure is 2 and the arity of the predicate char-uppercase? is 1. You'll probably have noticed that while some of Scheme's built-in procedures always take the same number of parameters others have variable arity -- that is, they can take any number of parameters. All one can say about the arity of some procedures -- such as list, +, or string-append -- is that it is some non-negative integer.

Still other Scheme procedures, such as map and display, require at least a certain number of parameters, but will accept one or more additional parameters if they are provided. For example, the arity of map is 2 or more, and the arity of display is 1 or 2. These procedures, too, are said to have variable arity, because their arity varies from one call to another.

Creating Your Own Variable-Arity Procedures

All of the procedures you've written so far have had a fixed arity. Is it possible to define your own new new variable-arity procedures in Scheme? Yes! You can create variable-arity procedures by using alternate forms of the lambda-expression. The simplest variant takes the following form

(lambda args 
  body)

In all of the programmer-defined procedures that we have seen so far, the keyword lambda has been followed by a list of parameters: names for the values that will be supplied by the caller when the procedure is invoked. If, instead, what follows lambda is a single identifier (not a list, but a simple identifier that is not enclosed in parentheses) then the procedure denoted by the lambda-expression will accept any number of parameters, and the identifier following lambda will name a list of all the parameters supplied in the call.

An Example: Displaying a Line

Here's a simple example: We'll define a display-line procedure that takes any number of parameters and prints out each one (by applying the display procedure to it), then terminates the output line (by invoking newline). Note that in the lambda-expression, the identifier paramters denotes a list of all the items to be printed:

;;; Procedure: 
;;;   display-line
;;; Parameters:
;;;   val1 ... valn, 0 or more values.
;;; Purpose:
;;;   Displays the strings terminated by a carriage return.
;;; Produces:
;;;   [Nothing]
;;; Preconditions:
;;;   0 or more values given as parameters.
;;; Postconditions:
;;;   All of the values have been displayed.
;;;   The output is now at the beginning of a new line.
(define display-line
  (lambda parameters
    (let kernel ((rest parameters))
      (if (null? rest)
          (newline)
          (begin
            (display (car rest))
            (kernel (cdr rest)))))))

When display-line is invoked, however, the caller does not assemble the items to be printed into a list, but just supplies them as parameters:

> (display-line "+--" "Here is a string!" "--+")
+--Here is a string!--+

> (display-line "ratio = " 35/15)
ratio = 7/3

Enforcing a Minimum Arity

If the programmer wishes to require some fixed minimum number of parameters while permitting (but not requiring) additional ones, she can use yet another form of the lambda-expression, in which a dot is placed between the last two identifiers in the parameter list. This form looks like the following

(lambda (arg1 arg2 ... argn . remaining-args)
   body)

All the identifiers to the left of this dot correspond to required parameters. The identifier to the right of the dot designates the list of all of the remaining parameters, the ones that are optional.

An Example: Displaying a Line with Separators

For instance, we can define a procedure called display-separated-line that always takes at least one argument, separator, but may take any number of additional parameters. Display-separated-line will print out each of the additional parameters (by invoking display) and terminate the line, just as display-line does, but with the difference that a copy of separator will be displayed between any two of the remaining values. Here is some sample output:

> (display-separated-line "..." "going" "going" "gone")
going...going...gone
> (display-separated-line ":-" 5 4 3 2 1 'done)
5:-4:-3:-2:-1:-done
> (display-separated-line #\space "+--" "Here is a string!" "--+")
+-- Here is a string! --+
> (display-separated-line (integer->char 9) 1997 'foo 'wombat 'quux)
1997    foo     wombat  quux
; (integer->char 9) is the tab character.

And here is the definition of the procedure:

;;; Procedure:
;;;   display-separated-line
;;; Parameters:
;;;   separator, a string
;;;   val1 ... valn, 0 or more additional values.
;;; Purpose:
;;;   Displays the values separated by the separator and followed
;;;   by a carriage return.
;;; Preconditions:
;;;   The separator is a string.
;;; Postconditions:
;;;   All the values have been displayed.
;;;   The output is now at the beginning of a new line.
(define display-separated-line
  (lambda (separator . parameters)
    (if (null? parameters)
        (newline)
        (let kernel ((rest parameters))
          (display (car rest))
          (if (null? (cdr rest))
              (newline)
              (begin
                (display separator)
                (kernel (cdr rest))))))))

An Example: Set Difference

As another example, let's look at how we might write a set-difference procedure, which takes one or more lists l₁, l₂, ..., l_n as parameters and returns a list containing all of the elements of l₁ that are not also elements of any of l₂, ..., l_n. For example, the value of

> (set-difference (list 'a 'b 'c 'd 'e 'f 'g)
                  (list 'a 'e) (list 'b) (list 'a 'f 'h))
(c d g)

We get the result because the elements 'c, 'd, and 'g of the first argument are not elements of any of the subsequent list. The set-difference procedure allows the caller to supply any number of lists of values to be pruned out of the initial list.

Here's the definition:

;;; Procedure:
;;;   set-difference
;;; Parameters:
;;;   initial, a set
;;;   other-1 ... other-n, zero or more additional sets
;;; Purpose:
;;;   Removes elements of the additional sets from the original set.
;;; Produces:
;;;   newset, a set
;;; Preconditions:
;;;   All the sets are represented as lists.
;;; Postconditions:
;;;   All values in newset appear in initial.
;;;   No values in newset appear in other-1 ... other-n.
;;;   All values in initial appear in either newset or other-1...other-n.
;;;   Does not affect any of the parameters.  (Yes, this is a standard
;;;     postcondition, but I considered it important to restate since this
;;;     procedure is described as "removing" elements from its parameters;
;;;     it doesn't.)
(define set-difference
  (lambda (initial . others)
    (let kernel ((set initial)
                 (remaining others))
       (if (null? remaining) set
           (kernel (remove (right-section member (car remaining)) set)
                   (cdr remaining))))))

In English: Call the initial list initial and collect all of the other parameters into a list called others. Using list recursion, fold over others: In the base case, where remaining is null, just return set. In any other case, separate remaining into its car and its cdr, remove all elements in the car from the set, and then repeat with the cdr.

Requiring More than One Parameter

The dot notation can be used to specify any number of initial values. Thus, a parameter list of the form

(first-value second-value . remaining-values)

indicates that the first two parameters are required, while additional parameters will be collected into a list named remaining-values.