A bit of the Writer monad.

writer-bit

(deftype Writer [value output])  ;; output must implement Monoid.

(extend-type Writer
  Functor
    (fun [this f]
      (monad [x this] (return this (f x))))
  Applicative
    (<*> [this m]
      (if (coll? m)
        (monad [f this vs (seqm m)] (return this (apply f vs)))
        (monad [f this x m] (return this (f x)))))
  Monad
  (>>= [m k]
    (let [w (k (eval-writer m))]
      (writer (eval-writer w)
        (mappend (exec-writer m) (exec-writer w)))))
  (return [this x]
    (writer x (mempty (.output this)))))