Lisp/Scheme/Racket: How to define a function with ellipsis

Question

I want to define a function in Racket with undefined number of arguments so I use ellipsis, but it doesn't work:

(define (f x ...) (printf x ...))
(f "~a ~a" "foo" "bar")

Error:

Arity mismatch

How to define a function in Racket with ellipsis?

Answer 1

There are two halves of this:

1. accepting an arbitrary number of inputs
2. passing an arbitrary number of arguments to another function

To accept an arbitrary number of inputs, instead of a ... after x, put a single . before x. This declares x as the "rest" argument, and the arguments will be collected into a list for x.

Example:

> (define (f . x) x)
> (f "~a ~a" "foo" "bar")
(list "~a ~a" "foo" "bar")

To pass an arbitrary number of arguments, you can use the apply function, which accepts a list as the last argument.

Example:

> (apply printf (list "~a ~a" "foo" "bar"))
foo bar

Putting these together:

> (define (f . x) (apply printf x))
> (f "~a ~a" "foo" "bar")
foo bar