bifocal.clj

;; bifocal is the lens library which I may spin off the enliven codebase

;; a lens is the generalization of associative access/update
;; fetch/putback are the pendant of get/assoc

;; symbols, strings, numbers, keywords, regex are lenses on their own

=> (fetch {:a 1 :b 2} :a)
1
=> (putback {:a 1 :b 2} :a 6)
{:a 6, :b 2}
=> (update {:a 1 :b 2} :a inc)
{:a 2, :b 2}

;; lenses can be compounded:
=> (fetch {:a {:b 2} :c 3} (compound :a :b))
2

;; fetch and putback implicitely call #'lens on their lens argument and
;; #'lens converts sequential collections of lenses into compound lenses

=> (fetch {:a {:b 2} :c 3} [:a :b])
2

;; lenses can be wrapped around functions:
=> (def inc-count (af inc :count))
#'enliven.core.lenses/inc-count
=> (inc-count {:count 41})
{:count 42}

;; af stands for autofocus, you can manually focus:
=> (def inc-count (focus :count inc :count))
#'enliven.core.lenses/inc-count
=> (inc-count {:count 41})
{:count 42}

;; focused fns can take additional args
=> ((focus :c + :a :b) {:a 1 :b 2})
{:c 3, :a 1, :b 2}
=> ((af + :acc :x) {:acc 3 :x 6})
{:x 6, :acc 9}

;; things to come: automatic simplification when composing focused fns
;; so that
(comp (af inc :c) (focus :c + :a :b)) becomes (focus c (comp inc +) :a :b)