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Macros with Ellipses

As previously discussed, a macro is an expression that gets replaced with predetermined code during compile-time. This article examines the broader implications of macros when using the ellipses; denoted as . The ellipses is useful whenever a macro wishes to utilize an indefinite amount of parameters in its construction.

Ellipses

Suppose a macro adds two values together.

(define-syntax add
  (syntax-rules ()
    [(add p q)
     (+ p q)]))

However, this code is still limited by two arguments. To extend beyond it, the ellipses is used.

(define-syntax add
  (syntax-rules ()
    [(add p ...)
     (+ p ...)]))

The symbolizes zero or more arguments, and must always be pair with the named parameter to the left of it. The following uses of add are now valid:

(add) ; => 0
(add 1) ; => 1
(add 1 2) ; => 3

Infinite Boolean Operators

Previously, macros were used to implement the or operator. One of the drawbacks of this approach is the parameter limit that my-or imposes; a limitation of two values.

(define-syntax my-or
  (syntax-rules ()
    [(my-or p q)
     (if p #t q)]))

; (my-or #t #t #t) ; => Bad syntax error

As illustrated above, a bad syntax error would occur if my-or receives more than two values. In order to prevent this, the macro needs to be able to receive an indefinite amount of parameters to perform its computation.

(define-syntax my-or
  (syntax-rules ()
    [(my-or p q) 
     (if p #t q)]
    [(my-or p q ...)
     (my-or p (my-or q ...))]))

(my-or #f #f #f #t) ; => #t

By using the ellipses in this example, my-or is able to take any amount of boolean expressions. The reader might’ve noticed that macro pattern-matching is recursive within its definition. This allows macros to infinitely expand until a base case is reached. The same approach can be used to extend my-and.

(define-syntax my-and
  (syntax-rules ()
    [(my-and p q)
     (if (not p) #f q)]
    [(my-and p q ...)
     (my-and p (my-and q ...))]))

(my-and #t #t #t) ; => #t
(my-and #f #t #t) ; => #f

Summary

Ellipses allow macros to have multiple parameters in its expansion.

Published 7 Dec 2015