Clojure for Java Developers: Transition smoothly from Java to the most widely used functional JVM-based language – Clojure

One particular thing that is important to note is the nREPL protocol; Someday it might grant us the power to go into a machine running 100 million miles away.

When you fire up your REPL, the first thing you see is:

nREPL server started on port 55995 on host 127.0.0.1 - nrepl://127.0.0.1:55995
REPL-y 0.3.5, nREPL 0.2.6

What it is saying is that there's a Clojure process running an nREPL server on port 55995. We have connected to it using a very simple client that allows us to interact with the Clojure process.

The really interesting bit is that you can connect to a remote host just as easily; let's try attaching an REPL to the same process by simply typing the following command:

lein repl :connect localhost:55995

Most IDEs have a good integration with Clojure and most of them use this exact mechanism, as clients that work a little more intelligently.

Now that we are inside the REPL, (any of the two) let's try writing our first expression, go on and type:

"Hello world"

You should get back a value from the REPL saying Hello world, this is not really a program, and it is the Hello world value printed back by the print phase of the REPL.

Let's now try to write our first Lisp form:

(println "Hello world")

This first expression looks different from what we are used to, it is called an S-expression and it is the standard Lisp way.

There are a couple of things to remember with S-expressions:

So we are asking for the string Hello world to be printed, but if we look a bit closer at the output, as shown in the following screenshot, there is a nil that we weren't expecting:

The reason for this is that the println function returns the value nil (Clojure's equivalent for null) after printing Hello world.

As we saw, the Leiningen nREPL client prints help text; but how does that work? Let's explore some of the other utilities that we have.

Try each of them to get a feeling of what it does with the help of the following table:

Let's check how these functions work:

user=> (javadoc java.util.List)
;; Should open the javadoc for java.util.List

user=> (doc doc)
-------------------------
clojure.repl/doc
([name])
Macro
  Prints documentation for a var or special form given its name
nil

user=> (source doc)
(defmacro doc
"Prints documentation for a var or special form given its name"
  {:added "1.0"}
  [name]
  (if-let [special-name ('{& fn catch try finally try} name)]
    (#'print-doc (#'special-doc special-name))
    (cond
      (special-doc-map name) `(#'print-doc (#'special-doc '~name))
      (find-ns name) `(#'print-doc (#'namespace-doc (find-ns '~name)))
      (resolve name) `(#'print-doc (meta (var ~name))))))
nil

What you are seeing here is metadata pertaining to the doc function; Clojure has the ability to store metadata about every function or var you use. Most of the Clojure core functions include a doc string and the source of the function and this is something that will become very handy in your day to day work.

Besides these functions, we also get easy access to the latest three values and the latest exceptions that happened in the REPL, let's check this out:

user=> 2
2
user=> 3
3
user=> 4
4
user=> (* *1 *2 *3) ;; We are multiplying over here the last three values
24 ;;We get 24!
user=> (/ 1 0) ;; Let's try dividing by zero
ArithmeticException Divide by zero clojure.lang.Numbers.divide (Numbers.java:156)
user=> *e
#<ArithmeticException java.lang.ArithmeticException: Divide by zero>

user=> (.getMessage *e)
"Divide by zero"

You can imagine the possibilities of being able to execute and introspect code with this, but what about the tools that we are already used to? How can we use this with an IDE?

Let's check now how to create a new Clojure project, we'll use Leiningen from the command line to understand what is happening.

Leiningen is similar to other Java development tools; it uses a similar convention and allows for heavy customizations in the project.clj file.

If you are familiar with Maven or Gradle, you can think of it as pom.xml or build.gradle respectively.

The following screenshot is the project structure:

As you can see in the preceding screenshot, there are four main folders:

Once your project is created, you can build and run a Java standalone command-line app quite easily, let's try it now:

lein uberjar
java -jar target/uberjar/getting-started-0.1.0-SNAPSHOT-standalone.jar
# Hello, World!

As you can see, it is quite easy to create a standalone app and it is very similar to using Maven or Gradle.

You can check the full documentation for Cursive at their website (https://cursiveclojure.com/), it is still under development but it is quite stable and a great aid when writing Clojure code.

We are going to use the latest IntelliJ Community Edition release, which at the time of this writing is version 14.

You can download IntelliJ from here https://www.jetbrains.com/idea/download/.

Installing Cursive Clojure is very simple, you need to add a repository for IntelliJ. You'll find the instructions to your specific IntelliJ version here: https://cursiveclojure.com/userguide/.

After you have installed Cursive Clojure, we are ready to go.

Now, we are ready to import our getting started project into Cursive Clojure.

Here is how you will do it:

Now, we are ready to go, you can use the Cursive Clojure as your main development tool. There are a few more things to do with your IDE but I recommend you to look for them; they are important and will come in handy:

One important part of Clojure programming is that it can modify and reevaluate code in runtime. Check the manual of your current version of Clojure and check for the structural editing section (https://cursiveclojure.com/userguide/paredit.html). It is one of the most useful functionalities of Clojure IDEs and a direct consequence of the Clojure syntax.

I recommend you to check other functionalities from the manual. I really recommend checking the Cursive Clojure manual, it includes animations of how each functionality works.

You will use the last two key bindings quite a lot, so it is important to set them up correctly. There is more information about keybindings at https://cursiveclojure.com/userguide/keybindings.html.

Clojure is based around "forms" or lists. In Clojure, same as every Lisp, the way to denote a list is with parentheses, so here are some examples of lists in the last code:

(println parameter)
(dotimes [x times] (println parameter))
(defn some-function [times parameter] (dotimes [x times] (println parameter)))

Lists are one data type in Clojure and they are also the way to express code; you will learn later about all the benefits of expressing code as data. The first one is that it is really simple, anything you can do must be expressed as a list! Let's look at some other examples of executable code:

(* 1 2 3)
(+ 5 9 7)
(/ 4 5)
(- 2 3 4)
(map inc [1 2 3 4 5 6])

I encourage you to write everything into the REPL, so you get a good notion of what's happening.

In Clojure, MOST of the executable forms have this structure:

(op parameter-1parameter-2 ….)

op is the operation to be executed followed by all the parameters it needs, let's analyze each of our previous forms in this new light:

(+ 1 2 3)

We are asking to execute the + (addition) operation with the parameters 1, 2, and 3. The expected result is 6.

Let's analyze something a bit more complicated:

(map inc [1 2 3 4 5 6])

In this, we are asking to execute the clojure.core/map function with two parameters:

Map applies the inc function to each member of the passed collection and returns a new collection, what we expect is a collection containing [2 3 4 5 6 7].

Now let's check how a function definition is essentially the same as the previous two forms:

(defn some-function [times parameter]
"Prints a string certain number of times"
  (dotimes [x times]
    (println parameter)))

The defn is the operation that we are asking for. It has several parameters, such as:

The defn calls a function into existence. After this piece of code is executed, some-function exists in the current namespace and you can use it with the defined parameters.

The defn is actually written in Clojure and supports a few nice things. Let's now define a multi-arity function:

(defn hello
  ([] (hello "Clojure"))
  ([name] (str "Hello " name)))

Over here we are defining a function with two bodies, one of them has no arguments and the other one has one argument. It is actually pretty simple to understand what's happening.

Try changing the source in your project's core.clj file similar to the following example:

(ns getting-started.core
  (:gen-class))

(defn hello
  ([] (hello "Clojure"))
  ([name] (str "Hello " name)))

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

Now run it, you'll get three different Hello outputs.

As you can see, Clojure has a very regular syntax and even if it's a little strange for newcomers, it is actually quite simple.

Here, we have used a few data types that we haven't properly introduced; in the next section we'll take a look at them.

In every language you need primitive types; you use them in everyday life as they represent numbers, strings, and Booleans. These primitive types are called scalars in the Clojure world.

Clojure has a couple of very interesting types like ratios and keywords. In the following table, you get to know the different types of scalars, how they compare to Java and a simple example of how to use each of them.

In Clojure there are two types of collections: sequential and associative collections. Sequential are things you can iterate, such as lists. Associative collections are maps, sets, and things you can access by a certain index. Clojure's collections are fully compatible with Java and it can even implement the java.util interfaces, such as java.util.List and java.util.Map.

One of the main characteristics of collections in Clojure is that they are immutable; it has a lot of benefits that we'll see later.

Let's have a look at the characteristics of each collection data type available in Clojure and compare them with Java with the help of a sample (in Clojure) and its description.