Something

(define-syntax iterate
  (syntax-rules ()

    ; No final expression
    ((iterate loop (fold-vars ...) ((state-var init) ...) body)
     (iterate loop (fold-vars ...) ((state-var init) ...) body
	     (values state-var ...)))

    ; Weird degenerate case with no iteration variables.
    ((iterate loop () ((state-var init) ...) body final)
     (let loop ((state-var init) ...) body))

    ; All there
    ((iterate loop
	      ((type value-var args ...) ...)
	      ((state-var init) ...)
	      body
	      final)
     ((folder ((type value-var args ...) ...) (state-var ...) loop body)
      (lambda (state-var ...)
	final)
      args ... ...
      init ...))))