Clojure Overview

I love Clojure because its powerful, flexible and fun.

Here are a selection of features which appealed to me when I first looked at Clojure

Dynamic Language & runtime environment (REPL)

  • quickly explore your problem domain by evaluating code on the fly in the REPL
  • new code is compiled as evaluated, no seperate compile cycle or wait time
  • very small language syntax, extensible via macros

A lot of the best programmers and the most productive programmers I know are writing everything in Clojure and swearing by it, and then just producing ridiculously sophisticated things in a very short time. And that programmer productivity matters. Adrian Cockcroft - Battery ventures, formally Cloud Architect, Netflix

Pragmatic Functional Programming

  • encourages an immutable approach that helps keep the code simple
  • minimising state changes makes scaling your application easy through parallelism
  • built-in persistent data structures (List, Map, Vector, Set) give an efficient way of modifying data without side effects
  • Concurrency becomes trivial with pure functions & immutable data

Managed State Changes

  • Reference types atoms & refs for mutable state
  • Changes are done safely within Software Transactional Memory (STM), like having an in-memory ACID database managing all state changes under the covers.

Hosted language with consise interoperabilty

  • Clojure can run on the Java Virtual Machine (JVM), Microsoft Common Language Runtime (CLR) or JavaScript engines via ClojureScript.
  • Clojure is compiled to bytecode, giving very high performance (close to Java, C++, etc.)
  • Simple syntax to call any other code that runs on the JVM (Java, Scala, JRuby, Jython, etc) or Microsoft CLR.

A modular / component approach to design

A typical approach with Clojure is to break a big problem space into small libraries, each with a specific perpose. This helps deconstruct complex systems into smaller, easier to understand code. This also helps make these Clojure libraries reusable in many other projects, reducing the development effort.

Clojure is

  • a general purpose language that compiles code that runs on the Java Virtual Machine (JVM).
  • a functional programming language with a data centric approach (mostly pure)
  • a very small syntax (12 primitives, 4 of which were added for Java interoperability)
  • a dynamic language in terms of type inference and runtime (REPL)
  • a fast-feedback approach for development, helping you quickly explore a problem domain
  • a modern implementation of LISP
  • a highly extensible language via macros
  • an efficient way to manage state changes via persistent data structures & software transactional memory

In functional programming we avoid changing state as much as possible. If a function does not change state it is referentially transparent, always returning the same result when given the same input (arguments). These are refered to as Pure Functions. Pure functions are truely modular as they do not affect any other part of the system and do not require complex threading for scalability.


ClojureScript is Clojure that compiles to to JavaScript and runs in the browser (JavaScript Engine). Most of the code and libraries available for Clojure works seamlessly when compiled to JavaScript.

ClojureScript is based heavily on the work done for Google Closures and there is a strong focus on Reactive client side apps.

There is a common file extension, .cljc that signifies Clojure code that will run on the JVM and JavaScript engines.

Clojure Community

There is a vibrant and highly active community around Clojure. In London alone there is a regular monthly talk and 3 or 4 coding dojo every month.

Community resources

Clojure. tv and Planet Clojure are the tip of the iceburg to a large amount of Clojure resources available via the Internet.

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