Question

Consider the following implementation of a function to compute factorial: [1]

(define fac-tail
  (lambda (n)
    (define fac-tail-helper
      (lambda (n ac)
        (if (= 0 n)
            ac
            (fac-tail-helper (- n 1) (* n ac)))))
    (fac-tail-helper n 1)))

I attempted to rewrite using let for the inner define:

(define fac-tail-2
  (lambda (n)
    (let ((fac-tail-helper-2
            (lambda (n ac)
              (if (= 0 n)
                  ac
                  (fac-tail-helper-2 (- n 1) (* n ac))))))
    (fac-tail-helper-2 n 1))))

There is no error at define time, but execution results in:

#;> (fac-tail-2 4)
Error: undefined variable 'fac-tail-helper-2'.
{warning: printing of stack trace not supported}

How can I make the let version work?

Scheme version is SISC v 1.16.6

[1] Based on the iterative version of factorial in section 1.2.1 of SICP http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-11.html#%_sec_1.2.1

Answer 1

How can I make the let version work?

Use letrec instead of let.