ctford-lens.clj

;; You are right that the f function passed to update gets called only once.
;; However the value passed to it may be composite.

;; multifocals
=> (defn swap [[a b]] [b a])
#'enliven.core.lenses/swap

=> (update {:a "A" :b "B" :c "C"}
     (juxt :a :b) swap)
{:a "B", :b "A", :c "C"}

;; Lenses such as the ones created by lens-map or juxt (see below) are what I call multifocals:
;; the focused value is a composite of each member lens focus.
;; So it somehow gathers reads and scatters writes.

;; (tangentially related: see also focus and autofocus (af) functions which wrap a function
;; in lenses (most of them being read-only))

;; Implicit iteration (à la fmap-based lens) didn't seem sensible to me in this design.
;; If you want to iterate you focus on the collection and iterate in user code (eg the function
;; passed to update or even by having discrete fetch and putback calls). The same goes for Maybe
;; handling.

;; juxt, from a local branch:
(defn juxt ; TODO make safe version w/ dependent lenses
  "Beware of dependent lenses!"
  [& lenses]
  (let [lenses (vec lenses)]
    (reify Lens
      (-fetch [_ x]
        (mapv #(fetch x %) lenses))
      (-putback [_ x y]
        (reduce-kv (fn [x idx l]
                     (putback x l (nth y idx)))
          x lenses)))))