transform.clj

(def-alias Monad 
  (TFn [[m :kind (TFn [[x :variance :covariant]] Any)]]
    '{:m-bind (All [x y]
                [(m x) [x -> (m y)] -> (m y)])
      :m-result (All [x]
                  [x -> (m x)])
      :m-zero Undefined
      :m-plus Undefined}))

(ann maybe-m (Monad 
               (TFn [[x :variance :covariant]] 
                 (U nil x))))
(defn maybe-m ...)

(ann maybe-t
     (All [[m :kind (TFn [[x :variance :covariant]] Any)]]
       [(Monad m) nil (U ':m-plus-default ':m-plus-from-base)
        -> (Monad (TFn [[y :variance :covariant]]
                    (m (U nil y))))]))
(defn maybe-t ...)

(ann seq-maybe-m (Monad 
                   (TFn [[x :variance :covariant]]
                     (Seqable (U nil x)))))
(def seq-maybe-m ((inst maybe-t 
                        (TFn [[x :variance :covariant]] 
                          (Seqable x)))
                    sequence-m nil :m-plus-default))

(domonad seq-maybe-m
  [^{:T AnyInteger} x  (map (fn> [[n :- AnyInteger]] (when (odd? n) n)) (range 10))]
  (inc x))
;=> (clojure.lang.Seqable (U nil AnyInteger))