Clojure: Combining Calls To Doseq And Let
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Join For FreeIf you've ever looked at the docs
for clojure's for macro, then you probably know about the :let, :when,
and :while modifiers. What you may not know is that those same modifiers
are available in doseq.
I was recently working with some code that had the following form.
(doseq [[printer {:keys [id sub-ids]}] {(partial println "sub1") {:id :A :sub-ids [11 12 13 14]} (partial println "sub2") {:id :B :sub-ids [21 22 23 24]}}] (let [id-str (name id)] (doseq [sub-id sub-ids] (printer id-str sub-id)))) ;;; -output- ;;; sub1 A 11 ;;; sub1 A 12 ;;; sub1 A 13 ;;; sub1 A 14 ;;; sub2 B 21 ;;; sub2 B 22 ;;; sub2 B 23 ;;; sub2 B 24
Upon seeing this code, John Hume
asked if I preferred it to a single doseq with multiple bindings. He
sent over an example that looked similar to the following example.
(doseq [[printer {:keys [id sub-ids]}] {(partial println "sub1") {:id :A :sub-ids [11 12 13 14]} (partial println "sub2") {:id :B :sub-ids [21 22 23 24]}} sub-id sub-ids] (let [id-str (name id)] (printer id-str sub-id)))
That was actually the first time that I'd seen multiple bindings in a
doseq, and my immediate reaction was that I preferred the explicit
simplicity of having multiple doseqs. However, I always have a
preference for concise code, and I forced myself to starting using
multiple bindings instead of multiple doseqs - and, unsurprisingly, I
now prefer multiple bindings to multiple doseqs.
You might have noticed that the second version of the code slightly
changes what's actually being done. In the original version the 'name'
function is called once per 'id', and in the second version the 'name'
function is called once per 'sub-id'. Calling name significantly more
often isn't likely to have much impact on your program; however, if you
were calling a more expensive function this change could have a negative
impact. Luckily, (as I previously mentioned) doseq also provides
support for :let.
The second example can be evolved to the following code - which also
demonstrates that the let is only evaluated once per iteration.
(doseq [[printer {:keys [id sub-ids]}] {(partial println "sub1") {:id :A :sub-ids [11 12 13 14]} (partial println "sub2") {:id :B :sub-ids [21 22 23 24]}} :let [id-str (do (println id) (name id))] sub-id sub-ids] (printer id-str sub-id)) ;;; :A ;;; sub1 A 11 ;;; sub1 A 12 ;;; sub1 A 13 ;;; sub1 A 14 ;;; :B ;;; sub2 B 21 ;;; sub2 B 22 ;;; sub2 B 23 ;;; sub2 B 24
That's really the final version of the original code, but you can alter
it slightly for experimentation purposes if you'd like. Let's assume we
have another function we're calling in an additional let and it's
expensive, it would be nice if that only occurred when an iteration was
going to happen. It turns out, that's exactly what happens.
(doseq [[printer {:keys [id sub-ids]}] {(partial println "sub1") {:id :A :sub-ids [11 12 13 14]} (partial println "sub2") {:id :B :sub-ids []}} sub-id sub-ids :let [sub-id-times-2 (do (println "binding occured") (* sub-id 2))]] (printer id sub-id)) ;;; binding occured ;;; sub1 :A 11 ;;; binding occured ;;; sub1 :A 12 ;;; binding occured ;;; sub1 :A 13 ;;; binding occured ;;; sub1 :A 14 ;;; (nothing else was printed, no other binding occurred)
Whether you prefer multiple bindings or multiple doseqs, it's probably a good idea to get comfortable reading both.
Published at DZone with permission of Jay Fields, DZone MVB. See the original article here.
Opinions expressed by DZone contributors are their own.
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