Let over lambda block-scanner in clojure
I have just start reading Let over lambda and I thought I would try and write a clojure version of the block-scanner in the closures chapter.
I have the following so far:
(defn block-scanner [trigger-string]
(let [curr (ref trigger-string) trig trigger-string]
(fn [data]
(doseq [c data]
(if (not (empty? @curr))
(dosync(ref-set curr
(if (= (first @curr) c)
(rest @curr)
trig)))))
(empty? @cu开发者_运维问答rr))))
(def sc (block-scanner "jihad"))
This works I think, but I would like know what I did right and what I could do better.
I would not use ref-set
but alter
because you don't reset the state to a completely new value, but update it to a new value which is obtained from the old one.
(defn block-scanner
[trigger-string]
(let [curr (ref trigger-string)
trig trigger-string]
(fn [data]
(doseq [c data]
(when (seq @curr)
(dosync
(alter curr
#(if (-> % first (= c))
(rest %)
trig)))))
(empty? @curr))))
Then it is not necessary to use refs since you don't have to coordinate changes. Here an atom is a better fit, since it can be changed without all the STM ceremony.
(defn block-scanner
[trigger-string]
(let [curr (atom trigger-string)
trig trigger-string]
(fn [data]
(doseq [c data]
(when (seq @curr)
(swap! curr
#(if (-> % first (= c))
(rest %)
trig))))
(empty? @curr))))
Next I would get rid of the imperative style.
- it does more than it should: it traverses all data - even if we found a match already. We should stop early.
- it is not thread-safe, since we access the atom multiple times - it might change in between. So we must touch the atom only once. (Although this is probably not interesting in this case, but it's good to make it a habit.)
- it's ugly. We can do all the work functionally and just save the state, when we come to a result.
(defn block-scanner
[trigger-string]
(let [state (atom trigger-string)
advance (fn [trigger d]
(when trigger
(condp = d
(first trigger) (next trigger)
; This is maybe a bug in the book. The book code
; matches "foojihad", but not "jijihad".
(first trigger-string) (next trigger-string)
trigger-string)))
update (fn [trigger data]
(if-let [data (seq data)]
(when-let [trigger (advance trigger (first data))]
(recur trigger (rest data)))
trigger))]
(fn [data]
(nil? (swap! state update data)))))
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