maybe.l

(defun fmap (f m)
  (funcall (get (si:*structure-definition-name (si:*structure-definition m)) 'fmap) f m))
(defun >>= (m f)
  (funcall (get (si:*structure-definition-name (si:*structure-definition m)) '>>=) m f))
(defun >> (m a)
  (funcall (get (si:*structure-definition-name (si:*structure-definition m)) '>>) m a))

(defstruct maybe
  has-value
  value)
(defun just (x)
  (make-maybe :has-value t :value x))
(defun nothing ()
  (make-maybe :has-value nil))

(setf
 (get 'maybe 'fmap) (lambda (f m)
                      (if (maybe-has-value m)
                          (just (funcall f (maybe-value m)))
                        (nothing)))
 (get 'maybe '>>=) (lambda (m f)
                     (if (maybe-has-value m)
                         (just (funcall f (maybe-value m)))
                       (nothing)))
 (get 'maybe '>>) (lambda (m a)
                    (if (maybe-has-value m)
                        (just (funcall a))
                      (nothing)))
 (get 'maybe 'return) #'just
 (get 'maybe 'fail) (lambda (s) (nothing)))






(defun parse-integer-maybe (s)
  (or (ignore-errors
        (multiple-value-bind (n b)
            (parse-integer s)
          (and b (= (length s) b) (just n))))
      (nothing)))

(>>=
 (parse-integer-maybe "12")
 (lambda (i) (1+ i)))
(>>=
 (parse-integer-maybe "aa")
 (lambda (i) (1+ i)))