zip two lists and foldl in one pass

zipfoldl.lfe

;;; Notes:
;; - Doesn't check arity of `F'
;; - Like #'lists:zip/3, throws a function_clause exception
;;   when given lists of unequal lengths

(defun zipfoldl
  ([f acc `(,ha . ,ta) `(,hb . ,tb)] (when (is_function f))
   (zipfoldl f (funcall f ha hb acc) ta tb))
  ([f acc () ()] acc))

(include-lib "lfe/include/clj.lfe")

(defun hamming-distance (a b)
  (flet ((f (x y sum) (cond-> sum (=/= x y) (+ 1))))
    (zipfoldl #'f/3 0 a b)))

The Fun in #'lists:mapfoldl/3 behaves similarly:

Fun = fun((A, AccIn) -> {B, AccOut})

It might be good to have f take (x y acc)...

f could also be defined:

(flet ((f (z x y) (+ z (if (=:= x y) 0 1))))
  ...)

Basically analogous to a single-pass version of:

(defun zip-then-foldl (f z xs ys) (lists:foldl f z (lists:zip xs ys)))