Learn Clojure Quick

Clojure is a modern programming language. Depending on your skill level, you can create any type of software with Clojure. This includes games, web apps, mobile apps, and desktop applications.

The first step in programming Clojure is to install it and other things needed for creating Clojure programs.

Continue reading “Hello Clojure” →

Installing Clojure with Lein

On: February 18, 2021February 19, 2021 By: cljdude

Most of the Clojure tutorials and Clojure examples on this site require at the minimum Clojure to be installed. Usually, you want Leiningen installed too.

Easy peasy.

Seriously, do it once and ignore it for the rest of your learning experience. Here are the steps for getting Lein and Clojure up and running.

Continue reading “Installing Clojure with Lein” →

Clojure Docker Web Apps

On: February 9, 2021March 4, 2021 By: cljdude

If you’re interested in making a commercial grade Clojure web app that deploys as a Docker container, this is the tutorial for you.

Prerequisites: You will need Leiningen and Docker installed.

First step needed is to create a Clojure web app template project. I used the following command.

lein new luminus mywebapp +reitit +http-kit +mysql +swagger +auth

1	lein new luminus mywebapp +reitit +http-kit +mysql +swagger +auth

This creates a Clojure web app using the Luminus Framework, Reitit for mapping of incoming requests to the proper handlers, HTTP Kit for HTTP handling, MySQL for JDBC connections to our MySQL database, Swagger for API support, and Buddy for authentication middleware. Your new web app is in the directory, mywebapp .

Continue reading “Clojure Docker Web Apps” →

JAVA_CMD for lein

On: February 4, 2021February 4, 2021 By: cljdude

Sometimes you need to configure lein to use a custom JDK. On Linux, set the environment variable JAVA_CMD to point to the java executable you want lein to use.

For example, you might add the following lines to your .bash_profile

#used by LEIN

JAVA_CMD=$HOME/installs/jdk-11.0.10+8/bin/java

export JAVA_CMD

#used by LEIN

JAVA_CMD=$HOME/installs/jdk-11.0.10+8/bin/java

export JAVA_CMD

Once you update your bash profile, you can start using your configuration by using the following command.

source ~/.bash_profile

1	source ~/.bash_profile

Clojure Record Heirarchy

On: January 6, 2021January 6, 2021 By: cljdude

In Clojure, if you want a basic OOP behavior without killing yourself to fully implement Java classes and interfaces in Clojure, the preferred technique is to use records. Sometimes implementing protocols in records suits your needs, mostly due to speed of code execution. However, if you want polymorphism with your records, then multimethods are where it’s at.

Admittedly, multimethods are very flexible, so the technique I’ll show you is one of many techniques you could use. The basic pattern I use for inheritance with records, is to use the derive command with keywords that are kept as a value in the record. For example,

(defrecord food [
                  ^Keyword type
                  ^String name
                  ^String color
                  ^boolean yummy
                  ])
(defn construct-food [name color yummy]
  (->food ::food name color yummy))


(derive ::fruit ::food)
(defrecord fruit [
                  ^Keyword type
                  ^String name
                  ^String color
                  ^boolean yummy
                  ^boolean needs-peeling
                   ])
(defn construct-fruit [name color yummy needs-peeling]
  (->fruit ::fruit name color yummy needs-peeling))

(defrecord food [

^Keyword type

^String name

^String color

^boolean yummy

])

(defn construct-food [name color yummy]

(->food ::food name color yummy))

(derive ::fruit ::food)

(defrecord fruit [

^Keyword type

^String name

^String color

^boolean yummy

^boolean needs-peeling

])

(defn construct-fruit [name color yummy needs-peeling]

(->fruit ::fruit name color yummy needs-peeling))

type is used to track isa relationships of ::food and ::fruit . derive is used to set ::food as the parent of ::fruit . Notice the use of custom constructors to set default values for type in the records.

Continue reading “Clojure Record Heirarchy” →

Clojure Agents

On: December 31, 2020December 31, 2020 By: cljdude

Most of the time, Clojure does not need locks like Java and other languages. This is because of atom s, 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.

(ns agentexample.core)

(def x (agent 0))

(defn mod-x
  [z]
  (Thread/sleep 500)
  (inc z)
  )

(defn foo
  []
  (println "Starting send-offs for agent")
  (dotimes [_  10]
      (send-off x mod-x)
      (println "x is now" @x))
  (Thread/sleep 8000)
  (println "final value ... " @x)
  (shutdown-agents))


(foo)

(ns agentexample.core)

(def x (agent 0))

(defn mod-x

[z]

(Thread/sleep 500)

(inc z)

)

(defn foo

[]

(println "Starting send-offs for agent")

(dotimes [_ 10]

(send-off x mod-x)

(println "x is now" @x))

(Thread/sleep 8000)

(println "final value ... " @x)

(shutdown-agents))

(foo)

A few things are going on here, so let’s look at them.

Continue reading “Clojure Agents” →

Quick OOP Example

On: December 13, 2020January 6, 2021 By: cljdude

Want some Object Oriented Programming in Clojure? Here’s a quick example.

Let’s create a dog object.

(ns temp2020toss.dogo)

(defprotocol Bark
  "Just a protocol"
  (barking [this] "bark")
  (hungry [this] "hangry")
  (sleeping[this] "zzzz"))

(defrecord Dog [breed ^Integer age color]
  Bark
  (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)))
  )

(ns temp2020toss.dogo)

(defprotocol Bark

"Just a protocol"

(barking [this] "bark")

(hungry [this] "hangry")

(sleeping[this] "zzzz"))

(defrecord Dog [breed ^Integer age color]

Bark

(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.

(ns temp2020toss.core
  (:gen-class)
  (:require [temp2020toss.dogo]
            [temp2020toss.dogo :refer :all])
  (:import (temp2020toss.dogo Dog)))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (println "Hello, World!")

  (let [my-dog (Dog. "chow" 8 "green")]
    (println "-->" (:breed my-dog))
    (println "-->" (barking my-dog))))

(ns temp2020toss.core

(:gen-class)

(:require [temp2020toss.dogo]

[temp2020toss.dogo :refer :all])

(:import (temp2020toss.dogo Dog)))

(defn -main

"I don't do a whole lot ... yet."

[& args]

(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.

Razvan’s ‘defrecord’ Example

On: December 12, 2020December 13, 2020 By: cljdude

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"))

;;retrieve values
(:firstname person1)
(:city (:address person1))

;;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"))

;;retrieve values

(:firstname person1)

(: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!