monad.lisp

(defstruct monad lift bind)

(defconstant +id-monad+
  (make-monad
   :lift #'identity
   :bind (lambda (x f) (funcall f x))))

(defconstant +list-monad+
  (make-monad
   :lift #'list
   :bind (lambda (xs f) (mapcan f xs))))

(defconstant +nil-monad+
  (make-monad
   :lift #'identity
   :bind (lambda (mx f) (if mx (funcall f mx) mx))))

(defvar *monad* +id-monad+)

(defun ret (x)
  (funcall (monad-lift *monad*) x))