Clojure - Amusing and Hard parts for a Java developer

By: Kannan Ramamoorthy On: Sat 10 August 2019
In: Clojure
Tags: #Clojure #FP #Programming


I’ve been experimenting with Clojure for recent months. As I was learning, there were moments of amusement and difficulties. This is intended to share those moments to any fellow Java developers.

This is not a Clojure tutorial.

The ‘Amusing parts’ is intended to give you a glimpse of what Clojure can do even if you don’t have prior experience in Clojure.

The ‘Hard parts’ is intended to give a heads-up if you are starting with Clojure. Since the reader is assumed to be new to Clojure, this section might help the user know what it takes to get comfortable with Clojure and not to give up soon.

Please do let me know if the above-mentioned sections don’t meet what it claims.

To quickly try out the snippets in this document you can use


Throughout this article, there would be lots of comparison between Clojure and Java. The comparison is in no way to demean Java. Java is ubiquitous and IMHO it will continue to be so for a few more decades.

I’m comparing with Java because it is the only language which I’m well versed with currently. I’m still working in Java and think most of the solutions in Java.

Clojure Primer:

To get a feel of what I realized, the reader might need a quick intro to Clojure. I’ll try to get some basics set here.

The ‘Clojure Primer’ part is meant just to give a quick mental model on Clojure. That section is oversimplified and I might have used some non-standard terms to solve the purpose. Hence, this is not meant as a reference in anyways.

To define, Clojure is a dynamically typed, functional programming language in Lisp dialect and runs on JVM.

(Over)simplified Syntax:

Any Clojure expressions that you write follows the syntax,

 (operator <argument1> <argument2> ..)

Where ‘operator’ is a term that I gave here for a ‘function/macro/special forms’, and not a standard term used in the Clojure world. You will get to know about it shortly.

For ex,

To add 3 numbers you would say

(+ 1 2 3)


+ is the 'operator', 1, 2 and 3 are parameters

To print the biggest of 2 numbers, you would say,

(if (> 2 1) (print 2) (print 1))


if is 'operator', (> 2 1), (print 2) , (print 1) are its parameters. Where the first parameter is the condition, the second parameter is the path when the condition succeeds, the third if it fails.

Again, within each of them, > and print are 'operators' along with their respective parameters.

  • And that’s all about the syntax of the language. Rest of the things that you need to know is different 'operators' that you might need and how it work*


A function is the same as a method in Java. It accepts 0 or more parameters does something and might return a result.


In programs, macros look the same as a function. 'But', instead of getting executed it accepts a parameter and returns a Clojure code to be executed.

This allows you to plugin a transformation logic, which transform the parameter of a Macro into some other code.

Amusing Parts:

Simple Syntax:

As mentioned in the primer, the syntax is so simple and standard.

Some examples,

Expression Meaning

(+ 1 2)

'Add' 1 and 2.

(= v1 v2)

'Equality check' for values of 'v1' and 'v2'.

(ns new-name-space)

'Switches namespace' to “new-name-space”. (Namespace is equivalent to Java’s package)

(println “Test String”)

'Prints' the given string.

(def v 1)

'Define' a variable ‘v’ with value 1 (in the current namespace)

(defn prn [x] (prinln x))

'Define' a function named 'prn' that accepts a parameter and prints it.

Less Special Constructs:

In a language “Special constructs“ can be defined as something that you are not sure how it is happening until you dig deep. Usually, there will be a set of limited constructs on which the whole language is built.

For example, take the below Java excerpt of a factorial implementation,

// This logic is not perfect. But it is intended to prove the point.
public static int fact(int a)
	if ((a == 1) || (a == 0) )
		return 1;
		return a * fact(--a);

It says if a is '1' or '0' then return 1 or multiply a with the fact(a -1).

As a typical Java developer on the meanings (implementation/definition) of the constructs like public, static, int, if .. else .., || , return , * , and -- you have to take it as is or dig deep into the JVM/JDK to know how it is implemented. You’re not sure how it is implemented and you cannot implement something like that on your own.

In the case of Clojure, the equivalent of the above snippet can be,

(defn fact [a]
(if (or (= a 1) (= a 0))
(* a
  (fact (dec a)))))

In the above snippet, except if everything else is a function/macro. Which means you can (re)define it yourself. For now, don’t worry about what function/macro means. You will get to know about it shortly.

The special constructs in Clojure are so less and the rest of things are just build on top of those special constructs. Which means most of the Clojure code can be seen or even developed by yourself(if you wish).

Infinite/lazy sequence as a normal sequence:

As a Java developer, we hardly heard about infinite sequence before Java 8 streams(eg, Stream.iterate). Even after Java 8, we don’t use it often. The primary reason is that the infinite/lazy sequence is offered by Streams. Where Streams and Collections are kept apart by hierarchy and are not interchangeable.

In the case of Clojure, the lazy/infinite sequence is so commonly used. Primarily because there is not much difference in the way 'lazy' and 'normal' sequence is handled.

For ex, look at the below expressions,

(print (take 2 (list 0 1 2 3 4)))
; prints (0 1)

(print (take 2 (range)))
; Prints (1 2)

(print (take 10 (range)))
; Prints (0 1 2 3 4 5 6 7 8 9)

It is apparent that take n takes the first n elements in the given sequence. 2 main inferences in the above examples are,

range virtually returns an infinite sequence which you can take (lazily) as you need.
  1. The normal list and lazy sequence are treated in the same way and can be used interchangeably (in most cases). Thus providing you an opportunity to do a lazy evaluation whenever you normally use a sequence.

Lazy sequence (mostly) behaves the same way as a normal sequence. So you can defer the evaluation whenever possible.

Program as Data:

Before getting into detail, I would like to introduce you 2 constructs,


List as in any other languages is a sequential data structure. As seen in the above examples, you can create a list like,

(list 1 2 3)

In short form, you can also write it as,

‘(1 2 3)


If you imagine 'Clojure evaluator' as something that evaluates the expression that you give. You can ask it not to evaluate some expressions by quoting it.

For ex,

(quote (+ 1 2))
; Returns (+ 1 2)

In short, you can also write it as,

‘(+ 1 2)

Does the above 2 rings a bell? No?

If you noted any of the above Clojure snippets, doesn’t it resemble the list definition without a quote? Yeah. All your Clojure program is just a nested-list (i.e., tree) that is evaluated by the Clojure evaluator. If you want to have just a list datastore without being evaluated then you just quote it.

Let it be!

Why should we care if the program is a list/not-a-list?

This property of a programming language is called homoiconicity. One main advantage of this property is it lets do meta-programming such as macros, which we’ll see in a moment.

Your Clojure program is nothing but a nested Clojure list.


We had an intro about macro in the primer section.

To elaborate, a macro lets you create syntactic abstraction. i.e., you can write macros that take anything that doesn’t even look like Clojure but returns a Clojure code for evaluation.

Some of the common logical operators in other languages such as and, or are macros in Clojure. Other cool examples are list-iteration (for), thread-first macro (), thread-last macro (→>) and a lot more.

For example, while learning about macro, I tried implementing the or (Java equivalent ||) operator.

Macros let you have syntactic abstraction and do meta-programming. In other words, Macro is a program that writes(returns) a program.


Think that you have a function and when you call it with some parameters, the function will be evaluated and the result will be returned. And if you call the function with the same parameters, it won’t do the evaluation but just returns the result that we had earlier.

What’s so special about it? We do this in Java most of the times, you maintain a cache that stores the result of an evaluation. On consecutive calls, we just return it. Yeah, we do that.

But I was amused that we don’t have to deal with the caching ourself and memoization works can be used with almost all the functions(if needed) in Clojure because of referential transparency, a property of pure functions.

Lets you wrap your function so that the wrapper takes care of caching the results for a given set of parameters, without you having to deal with caching.


core.async library provides CSP style programming that involves threads communicating to each other using channels. This is not much different from using threads and blocking queues that we have in Java.

One thing that I found fascinating is alts! function, which lets you wait on multiple channels at once and gets the result from whichever comes first.

For example, this program searches a keyword in 2 search engines and prints from which we had the fastest result.

core.async lets you write clean inter-communicating threads.


Not many of us would have heard about Constraint Logic Programming(CLP) in mainstream programming. We’ll see it what it is with an example using Clojure’s core.logic.

Remember Sudoku?

To reiterate, you will have a 9X9 grid, which itself is subdivided into 9 3X3 sub-grids.

  • You have to place numbers from 1 to 9 in each column, row and the sub-grid.

  • So that the numbers must be unique in each of the row, column, and sub-grid.

  • Incidentally, you have to place all the 9 numbers in each row, column, and sub-grid.

Solving Sudoku means, filling up all the numbers in the row/column/sub-grid, also satisfying the above mentioned "Constraints".

Think, if you can have a (generic)program that takes all constraints(of any such problems) and spits out the solution. core.logic is one such module in Clojure that lets you do ‘Constraint Logic Programming’.

Below is an excerpt from a sudoku solver in Clojure using core.logic,

   (run 1 [q]
 ;; Solve for the lvar q
     (== q vars) ;; q should unify with the sequence of the lvars
     (everyg #(fd/in % (fd/domain 1 2 3 4 5 6 7 8 9)) vars) ;; For every entry in vars the range must be in 1 to 9
     (init vars hints) ;; Creates the goals to match the vars with the known hints.
     (everyg fd/distinct rows) ;; Every entry in rows must be unique.
     (everyg fd/distinct cols) ;; Every entry in cols must be unique.
     (everyg fd/distinct sqs)) ;; Every entry in sqs must be unique.

Problems like “Time-table creation”, “n-queens” are some basic candidates for core.logic. CLP is a whole new area to explore as a mainstream programmer. It is good that you can do it with Clojure using core.logic.

core.logic lets you do prologue style CLP.

Isomorphic counterpart - Clojurescript:

Similar to Isomorphic Javascripts, which can run on both browser and server, Clojure has an isomorphic counterpart, Clojurescript. Wherein you can write Clojure code that gets compiled to Javascript.

With a glimpse, its cool to know that the goodness of Clojure will be available to be compiled as Javascript to be executed in the browser.

With CLJS, you may run your code on the server as well as in the browser.

core.match :

core.match library of Clojure lets you write a concise conditional statement, which you might need nested and complicated if..else.

For example, with core.match you can write like,

  (match [x y z]
    [_ 1 2] 1
    [1 2 _ ] 2
    [_ _ 1] 3
    [_ _ 2] 4
    :else 5)

Which you might write in Java as,

if (y == 1 && z ==2)
  reutrn 1;
else if(x == 1 && y == 2)
  return 2;
else if( z == 1)
  return 3;
else if(z == 2)
  return 4;
  return 5;

Not just for simple variables.

  • core.match works for data structures like vector, map, etc.

  • There are other sophisticated features like wildcard match, additional functional application, etc.

  • Backed by this paper, the matches are optimized.

core.match lets you write concise conditional statements with sophistications such as wildcard, function application, etc.


One thing that primarily amuses other programmers is conciseness of Clojure. Just to give a feel of it, I’m adding some of the examples. Clojure has a lot of such constructs, but I’m just mentioning a few here. If you’re interested also take a look into a detailed analysis on conciseness of Clojure using a particular example.

List iteration:

List iterator is familiar for Python-dev, but surely it is something new for a Java developer. It is a constructor that lets you parallelly iterate through multiple sequences and produce a single result from it.

For example to do a cartesian product of 2 lists the below 1-liner would do,

(for [x '(1 2 3) y '(1 2 3)] [x y])


slurp is a Clojure function to read files. And the nice thing about this function is that you can use this to read a file from local disc, HTTP(s), FTP.

For example, you can use slurp as below,

(slurp "/Users/me/sample.txt")
(slurp "")
(slurp "ftp://localhost:2121/sample.txt")

A Fibonacci solution that you can tweet:

The below snippet gives you the Fibonacci series up to any number that you want, just using built-in functions/macros. To understand you need some hands-on. But this snippet sure won’t fail to amuse you.

(def fib
  (lazy-cat [0 1] (map + (rest fib) fib)))
(take 10 fib)

Java Interop:

Along with all other goodness of Clojure, Clojure works well with Java. You can use the ocean of libraries from Java in Clojure.


(println (System/currentTimeMillis))
; prints Current time in milliseconds. Equivalent of calling System.currentTimeMillis()

(doto (new java.util.HashMap) (.put "one" 1) (.put "two" 2))
; Creates a new hashmap
; Calls put(“one”, 1) and put(“two”,2) on the map.

Further amusements:

There are lot more amusements that I didn’t include, as to realize the importance of those some basic hands-on is needed. While starting, it is worth to have a look at Tail-recursion, Software Transaction Management(STM), Transducers, multi-methods, protocols and a lot more.

Hard Parts:

Usage of parentheses:

I’m fine with too many parentheses. But initially, I was not clear about the usage of parentheses. Like in Java, you cannot use an arbitrary number of parentheses. Everything that you put into parentheses is evaluated assuming the first element within parentheses is an operator.

Hence (+ 1 2 3) works whereas, (+ 1 (2 3)) won’t work. Because after ( there must be an operator, where 2 is not one.

Shift to s-expression:

With S-expression (the expression syntax of Clojure and other Lisp), we need a switch from Java style of expression, especially the logical/arithmetic expressions.

For example, 1 + 2 + 3 will be written as (+ 1 2 3). But it was a bit hard to convert logical operations such as (2 > 1) in Java to (> 2 1) in Clojure. Because the operator’s nose is pointing to 2 gives an impression that check means if 2 is smaller.

But it got easy after reading the documentation of >, which says it will check if the given numbers are in ‘decreasing order’. i.e., you can also check if (> 2 1 0) is true.

Immutable Datastructures:

Almost all the Clojure’s core data-structures are immutable. For example, when you add, remove an element in a list, the operation will result in a new list but won’t change the source list. This was a deliberate decision to keep concurrency in place.

In Java, having used to adding/removal/changing the Collections element so commonly, this was initially 'very' difficult. Had to practice a lot to get over. In Clojure, we typically use recursion for most of the problems that typically require modification in the data.

Dynamic programming:

With dynamic programming, you divide your problem into much smaller problems and use the solutions of the already solved ones, to make the problem solving faster.

One problem that I stuck for a long time was Level-15 of Project Euler. I tried an initial solution which worked well for small numbers, but even for number asked in the question (which is not so large), my program didn’t finish even after a whole day. Because this required dynamic-programming approach and Clojure doesn’t support mutability for basic data structures.

It took nearly 2-weeks for me to know and try memoization and came with a solution using the dynamic programming approach which took only a few seconds to solve the problem.

Lazy sequence:

The lazy sequence was both amusing and hard to imagine. When I tried, it took a couple of hours to understand before I come up with a solution on my own for 'van-eck generator' with a lazy sequence.


Q: You say core data structures are not mutable. So how a very basic use case of addition or deletion of the element from a data structure is addressed?

Clojure does support those operations. But it doesn’t change the original data structure. For ex,

(def a [1 2]) ; Define a vector with 1,2

(println (conj a 3)) ; Append 3 to it
;; Prints [1 2 3]

(println a)
;; Prints [1 2]

Q: What?! Do you create a new collection for each change that we do? Aren’t you filling up the memory unnecessarily?

Nope. Clojure uses a mechanism called Persistent Data structure, which doesn’t actually create a new object but reuses the existing one and still having the variables immutable.

Q: It is nice that the programs are concise. But anyhow it has to execute so many lines behind the scenes right? What is so nice about conciseness?

Programming with less code could probably mean more productivity. But surely means less testing, if you are building on top of existing and pre-tested code. In that way, conciseness matters.


  • 17-Aug-2019 - Added description on core.match.

This is expected to be an ongoing article. Will update here as get to see more amusing or hard things.

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