Recursion in lambda in Elisp

Question

I have the following Python code:

def sum_fibonacci():
    '''Project Euler: 2
    Sum of even-valued terms of Fibonacci sequence < 4 mil
    '''
    def i(a, b, acc):
        if a >= 4000000: return acc
        if a % 2 == 0:
            return i(a + b, a, acc + a)
        else: return i(a + b, a, acc)
    return i(2, 1, 0)

I want to translate it to Emacs Lisp. In Higher-order functions in Elisp i was told not to use defun within defun, because it enables the function globally, so i wrote lambda instead. But i need to make lambda call itself recursively.

My code is:

(defun sum-fibonacci ()
  (let ((i
         (lambda (a b acc)
           (cond ((>= a 4000000) acc)
                 ((evenp a) (funcall i (+ a b) a (+ a acc)))
                 (t (funcall i (+ a b) a acc))))))
    (funcall i 2 1 0)))

However the function i is called before it is assigned with let, and i get an error - *** Eval error *** Symbol's value as variable is void: i

How do i do recursion in lambda in Elisp?

Answer 1

Per Recursing in a lambda function I rewrote it with labels:

(defun sum-fibonacci ()
  (labels ((i (a b acc)
              (cond ((>= a 4000000) acc)
                    ((evenp a) (i (+ a b) a (+ a acc)))
                    (t (i (+ a b) a acc)))))
    (i 2 1 0)))