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Green Eggs and Transducers

A quick tour of Clojure Transducers with core.async with Dr. Seuss as a guide.

Follow along at home by:

  • lein new green-eggs
  • modify your project.clj to include the following:
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(defproject green-eggs "0.1.0-SNAPSHOT"
  :description "try them"
  :url "http://en.wikipedia.org/wiki/Green_Eggs_and_Ham"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.7.0-alpha1"]
                 [org.clojure/core.async "0.1.338.0-5c5012-alpha"]])
  • Start up a repl and hack in!

Green Eggs and Ham

Transducers are a new feature of Clojure 1.7. Instead of trying to explain them with words, let’s take a look of them in action. First we need some data. Let’s def a vector of all the places you could try green eggs and ham.

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(ns green-eggs.core
  (:require [clojure.core.async :as async]))

(def green-eggs-n-ham
  ["in the rain"
   "on a train"
   "in a box"
   "with a fox"
   "in a house"
   "with a mouse"
   "here or there"
   "anywhere"])

Next, let’s create a function that will transform the places into a “I would not eat them …” sentence.

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(defn i-do-not-like-them [s]
  (format "I would not eat them %s." s))

(i-do-not-like-them "in the rain")
;; -> "I would not eat them in the rain."

We also need a function to take this result and actually try the green eggs and ham.

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(defn try-them [s]
  (clojure.string/replace s  #" not" ""))

(try-them "I would not eat them in the rain.")
;; -> "I would eat them in the rain."

Now we have two transformations that we can apply to the vector of green-eggs-n-ham strings. One of the really nice things about transducers is that you can describe and compose this transformation without a data structure present.

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(def sam-i-am-xform
  (comp
   (map i-do-not-like-them)
   (map try-them)))

We can run the transformation of the transducers against the data in a few ways.

  • into: Non-lazy turn the transformation into a collection
  • sequence: Same thing but lazy
  • transduce: Acts like reduce on the all the transformed elements
  • With core.async channels doing the transformations.

Let’s look at the green eggs and ham example for each one of these ways:

Into

Into takes a transducer and collection to work on and returns the vector we asked for:

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(into [] sam-i-am-xform green-eggs-n-ham)
;; -> ["I would eat them in the rain."
;;     "I would eat them on a train."
;;     "I would eat them in a box."
;;     "I would eat them with a fox."
;;     "I would eat them in a house."
;;     "I would eat them with a mouse."
;;     "I would eat them here or there."
;;     "I would eat them anywhere."]

Sequence

Sequence takes similar arguments, but as promised, returns a lazy sequence that we can interact with.

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(class (sequence sam-i-am-xform green-eggs-n-ham))
;; -> clojure.lang.LazyTransformer

(take 1 (sequence sam-i-am-xform green-eggs-n-ham))
;; -> ("I would eat them in the rain.")

Transduce

If we want to finally arrange all our sentences in the vectors into one string, we would use reduce. The way to do this with transducers is to use transduce. It takes a function of two arguments to perform the reduce, as well as an initial data input.

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(transduce sam-i-am-xform #(str %1 %2 " ") "" green-eggs-n-ham)
;; -> "I would eat them in the rain.
;;     I would eat them on a train.
;;     I would eat them in a box.
;;     I would eat them with a fox.
;;     I would eat them in a house.
;;     I would eat them with a mouse.
;;     I would eat them here or there.
;;     I would eat them anywhere."

;;_note: In  1.7.0-alpha2, transduce changed and you need to use a
;;(transduce sam-i-am-xform (completing #(str %1 %2 " ")) "" green-eggs-n-ham)
;;instead.

Core.async

Core.async has a really nice way to define channels with a transducer that will transform each element on the channel.

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(def sam-i-am-chan (async/chan 1 sam-i-am-xform))

Let’s define another channel to reduce the results of the sam-i-am-chan to a string.

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(def result-chan (async/reduce #(str %1 %2 " ") "" sam-i-am-chan))

Finally, let’s actually put the green-eggs-n-ham data onto the sam-i-am-chan and let the data transformations flow….

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(async/onto-chan sam-i-am-chan green-eggs-n-ham)

At last, we can get our result off the result channel and revel in the beauty of asynchronous data transducers.

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(def i-like-them (async/<!! result-chan))

i-like-them
;; -> "I would eat them in the rain.
;;     I would eat them on a train.
;;     I would eat them in a box.
;;     I would eat them with a fox.
;;     I would eat them in a house.
;;     I would eat them with a mouse.
;;     I would eat them here or there.
;;     I would eat them anywhere."

Transducers are elegant and powerful, just like the rest of Clojure. Try them, you will like them :)

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