Transducers and Maps

Tue, 24 Sep 2019 20:14:24 +0000

Hi,

I'm learning more about transducers, but coming up with some deadends.

What I'm looking to understand, is whether they are appropriate to use (in my use-case, but also as a learning exercise) and whether what I am doing is right/efficient/maybe-there-is-a-better-way :-)

Let' say I have a data stucture thus:

 (def trip {:tripData 
            {:segments [{:dataPoints 
                         [{:location {:lat 1 :lng 2}} 
                          {:location {:lat 3 :lng 4}}]}]}})

There could be hundreds/thousands of dataPoints with each dataPoint having a single location.

I want to efficiently extract out only the lat and lng into a single collection and turn it into a string. I came up with this:

 (def xf
   (comp
    (mapcat :dataPoints)
    (map :location)
    (map (fn [{lat :lat lng :lng}] (str lat " " lng)))))

then evaluated via:

(def lat-lng (into [] xf (->> trip :tripData :segments)))

And I get back something like this:

["1 2" "3 4"]

Which I can then do (for the purposes of my exercise), this:

 (clojure.string/join ", " lat-lng)

to obtain, finally, this:

"1 2, 3 4"

Which is all fine and dandy :-)

However, given my inexperience with transducers, I'm left wondering if there is a different/better way. For example, a way, within the comp xf, to turn the data into the string and joined at the end instead of using clojure.string/join.

I also discovered, I can do this too, without the use of transducers:

 (def lat-lng-2 (->> trip
                     :tripData
                     :segments
                     (mapcat :dataPoints)
                     (map :location)
                     (map (fn [{lat :lat lng :lng}] (str lat " " lng)))))

Which, given the clojure.string/join, ends up with the same result.

However, it's my understanding that you can't use a map keyword (i.e., :tripData, :segments) as part of a comp as keywords are not transducers.

I'm at a loss on how to make this efficent/better whilst learning how I can use transducers.

I would appreciate some help/guidance/feedback!

Thank you.

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