List comprehensions, @darius style.

darius-for.el

;; An Emacs Lisp port of Darius Bacon's list comprehension macro.

(setq lexical-binding t)

(defmacro for (bindings body)
  (declare (indent defun))
  (expand-for bindings body '(lambda () ())))

(defun expand-for (bindings body rest-var)
  (if (null bindings)
      `(cons ,body (funcall ,rest-var))
    (case (car bindings)
      (:while
       `(if ,(cadr bindings)
            ,(expand-for (cddr bindings) body rest-var)
          '()))
      (:when
       `(if ,(cadr bindings)
            ,(expand-for (cddr bindings) body rest-var)
          (funcall ,rest-var)))
      (:let
       `(let (,(cadr bindings))
          ,(expand-for (cddr bindings) body rest-var)))
      (otherwise
       (let ((sub-var (gensym)))
         `(foldr-lazy (lambda (,(car bindings) ,sub-var)
                        ,(expand-for (cddr bindings) body sub-var))
                      ,rest-var
                      ,(cadr bindings)))))))

(defun  foldr-lazy (f z-thunk xs)
  (if (null xs)
      (funcall z-thunk)
    (funcall f (car xs) (lambda ()
                          (foldr-lazy f z-thunk (cdr xs))))))

;; Examples

(for (x '(1 2 3))
  x)
;; => (1 2 3)

(for (x '(1 2 3) :while (< x 2))
  x)
;; => (1)