Most of the time, Clojure does not need locks like Java and other languages. This is because of
ref s, and
agent s. I’ll focus on Clojure agents in this short post.
Agents are handled in single-threaded queues. By lining up calls of a function in a queue, no locks are needed. When combined with
send-off , deadlocks won’t happen.
Here’s a quick example.
(def x (agent 0))
(println "Starting send-offs for agent")
(dotimes [_ 10]
(send-off x mod-x)
(println "x is now" @x))
(println "final value ... " @x)
A few things are going on here, so let’s look at them.
Continue reading “Clojure Agents”
Want some Object Oriented Programming in Clojure? Here’s a quick example.
Let’s create a dog object.
"Just a protocol"
(barking [this] "bark")
(hungry [this] "hangry")
(defrecord Dog [breed ^Integer age color]
(barking [this] (str "barking my breed ... " (:breed this)))
(hungry [this] (str "I'm so hungry I could age one whole year to ... " (str (inc (:age this)))))
(sleeping [this] (str "Really tired, and it has nothing to do with my fur color that is " (:color this)))
Now let’s use the dog object.
[temp2020toss.dogo :refer :all])
(:import (temp2020toss.dogo Dog)))
"I don't do a whole lot ... yet."
(println "Hello, World!")
(let [my-dog (Dog. "chow" 8 "green")]
(println "-->" (:breed my-dog))
(println "-->" (barking my-dog))))
See, OOP in Clojure is fairly simple. This sample uses protocols with records, but you can add polymorphism with just a little extra work. For polymorphism, use records and multimethods.
I absolutely love the user razvan’s Clojure
defrecord example over at stackoverflow, so I copied it here. I know I’ll never find it again if I don’t. Look at the original over on stackoverflow at this link.
Here’s the example …
Clojure defrecord example:
;;define Address record
(defrecord Address [city state])
;;define Person record
(defrecord Person [firstname lastname ^Address address])
;;buid the constructor
(defn make-person ([fname lname city state]
(->Person fname lname (->Address city state))))
;;create a person
(def person1 (make-person "John" "Doe" "LA" "CA"))
(:city (:address person1))
If you’re looking for more information on combining protocols with records in Clojure, then Matthew Boston has the blog for you!
NOTE: All the code in this tutorial can be quickly tested at the REPL using the command
lein repl to start a REPL to test with.
One of the first things you learn when learning clojure is
println . For example,
That’s probably the first Clojure program you ever wrote. Maybe you wrote this variation.
print are the first ways learned of displaying output in Clojure, most Clojure developers continue using them when debugging code. Nothing wrong with that. But sometimes it is better to use a more specialized tool.
In steps the
pr commands. These two commands are tailored for printing objects. At first glance, they appear to work the same as
println . However, they actually show more information in some instances.
println , the
\n in the following code quietly gets replaced by the action of a new line being printed.
prn are used, the two characters
n are displayed instead of an actual new line. This is very handy when hidden characters are fowling up your layout, or needed to be visible when debugging code.
Final note: The only difference between
prn , is that
prn prints a new line after the objects it is printing out.