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Clojure Reactive Programming

You're reading from   Clojure Reactive Programming Design and implement highly reusable reactive applications by integrating different frameworks with Clojure

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Product type Paperback
Published in Mar 2015
Publisher Packt
ISBN-13 9781783986668
Length 232 pages
Edition 1st Edition
Languages
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Author (1):
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Leonardo Borges Leonardo Borges
Author Profile Icon Leonardo Borges
Leonardo Borges
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Table of Contents (19) Chapters Close

Clojure Reactive Programming
Credits
About the Author
Acknowledgments
About the Reviewers
www.PacktPub.com
Preface
1. What is Reactive Programming? FREE CHAPTER 2. A Look at Reactive Extensions 3. Asynchronous Programming and Networking 4. Introduction to core.async 5. Creating Your Own CES Framework with core.async 6. Building a Simple ClojureScript Game with Reagi 7. The UI as a Function 8. Futures 9. A Reactive API to Amazon Web Services The Algebra of Library Design Bibliography
Index

Finding the average of ages


In this section, we will explore a different use case for the Option Functor. We would like to, given a number of pirates, calculate the average of their ages. This is simple enough to do:

(defn avg [& xs]
  (float (/ (apply + xs) (count xs))))

(let [a (some-> (pirate-by-name "Jack Sparrow") age)
      b (some-> (pirate-by-name "Blackbeard") age)
      c (some-> (pirate-by-name "Hector Barbossa") age)]
  (avg a b c)) ;; 56.666668

Note how we are using some-> here to protect us from nil values. Now, what happens if there is a pirate for which we have no information?

(let [a (some-> (pirate-by-name "Jack Sparrow") age)
      b (some-> (pirate-by-name "Davy Jones") age)
      c (some-> (pirate-by-name "Hector Barbossa") age)]
  (avg a b c)) ;; NullPointerException   clojure.lang.Numbers.ops (Numbers.java:961)

It seems we're back at square one! It's worse now because using some-> doesn't help if we need to use all values at once, as opposed to threading them through a chain of function calls.

Of course, not all is lost. All we need to do is check if all values are present before calculating the average:

(let [a (some-> (pirate-by-name "Jack Sparrow") age)
      b (some-> (pirate-by-name "Davy Jones") age)
      c (some-> (pirate-by-name "Hector Barbossa") age)]
  (when (and a b c)
    (avg a b c))) ;; nil

While this works perfectly fine, our implementation suddenly had to become aware that any or all of the values a, b, and c could be nil. The next abstraction we will look at, Applicative Functors, fixes this.

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