Why defining a recursive-display procedure in Guile causes infinite loop?

Question

The following procedure causes an infinite loop:

(define (recursive-display . args)
  (if (null? args)
      #t
      (begin
        (display (car args))
        (recursive-display (cdr args)))))

I'm still learning about Guile so I'm a bit confused why that's the case.

Answer 1

Use display to print the value of args and look at the output:

(define (recursive-display . args)
  (if (null? args)
      #t
      (begin
        (display args)
        (recursive-display (cdr args)))))

> (recursive-display 1 2 3)
(1 2 3)
((2 3))
(())
(())
(())
...

Your function recursive-display has a variable number of arguments. When you call something like (recursive-display 1 2 3), all arguments passed to this function are wrapped in the list, so variable args has a value '(1 2 3).

Next, you call (recursive-display (cdr args)). The same rule applies again- all arguments are wrapped in the list, so args has now value '((2 3)).

In all following iterations, args has a value '(()).

When you add apply, it will work as expected:

(define (recursive-display . args)
  (if (null? args)
      #t
      (begin
        (display (car args))
        (apply recursive-display (cdr args)))))

> (recursive-display 1 2 3)
123#t