Like most other Lisps, Clojure's syntax is built on
S-expressions that are first parsed into data structures by a reader before being compiled.[21][17] Clojure's
reader supports literal syntax for
maps, sets and
vectors along with lists, and these are compiled to the mentioned structures directly.[21] Clojure treats
code as data and has a
Lisp macro system.[22] Clojure is a
Lisp-1 and is not intended to be code-compatible with other dialects of Lisp, since it uses its own set of data structures incompatible with other Lisps.[22]
Clojure advocates
immutability and
immutable data structures and encourages programmers to be explicit about managing identity and its states.[23] This focus on programming with immutable values and explicit progression-of-time constructs is intended to facilitate developing more robust, especially
concurrent, programs that are simple and fast.[24][25][17] While its type system is entirely
dynamic, recent efforts have also sought the implementation of a
dependent type system.[26]
The language was created by
Rich Hickey in the mid-2000s, originally for the Java platform; the language has since been ported to other platforms, such as the
Common Language Runtime (.NET). Hickey continues to lead development of the language as its
benevolent dictator for life.
History
Rich Hickey is the creator of the Clojure language.[19] Before Clojure, he developed dotLisp, a similar project based on the
.NET platform,[27] and three earlier attempts to provide interoperability between Lisp and
Java: a Java foreign language interface for
Common Lisp (jfli),[28] A Foreign Object Interface for Lisp (FOIL),[29] and a Lisp-friendly interface to Java Servlets (Lisplets).[30]
Hickey spent about two and a half years working on Clojure before releasing it publicly in October 2007,[31] much of that time working exclusively on Clojure with no outside funding. At the end of this time, Hickey sent an email announcing the language to some friends in the Common Lisp community.
Clojure's name, according to Hickey, is a
word play on the programming concept "
closure" incorporating the letters C, L, and J for
C#,
Lisp, and
Java respectively—three languages which had a major influence on Clojure's design.[18]
Clojure's approach to
state is characterized by the concept of identities,[23] which are represented as a series of immutable states over time. Since states are immutable values, any number of workers can operate on them in parallel, and concurrency becomes a question of managing changes from one state to another. For this purpose, Clojure provides several mutable
reference types, each having well-defined semantics for the transition between states.[23]
Clojure runs on the
Java platform and as a result, integrates with
Java and fully supports calling Java code from Clojure,[36][17] and Clojure code can be called from Java, too.[37] The community uses tools such as Clojure
command-line interface (CLI)[38] or
Leiningen for project automation, providing support for
Maven integration. These tools handle project package management and dependencies and are configured using Clojure syntax.
As a Lisp dialect, Clojure supports
functions as
first-class objects, a
read–eval–print loop (REPL), and a macro system.[6] Clojure's
Lisp macro system is very similar to that of
Common Lisp with the exception that Clojure's version of the
backquote (termed "syntax quote") qualifies symbols with their
namespace. This helps prevent unintended name capture, as binding to namespace-qualified names is forbidden. It is possible to force a capturing macro expansion, but it must be done explicitly. Clojure does not allow user-defined reader macros, but the reader supports a more constrained form of syntactic extension.[39] Clojure supports
multimethods[40] and for
interface-like abstractions has a
protocol[41] based polymorphism and data type system using
records,[42] providing high-performance and dynamic polymorphism designed to avoid the
expression problem.
Clojure 1.7 introduced reader conditionals by allowing the embedding of Clojure, ClojureScript and ClojureCLR code in the same namespace.[49][21] Transducers were added as a method for composing transformations. Transducers enable higher-order functions such as
map and
fold to generalize over any source of input data. While traditionally these functions operate on
sequences, transducers allow them to work on channels and let the user define their own models for transduction.[50][51][52]
Extensible Data Notation
Extensible Data Notation, or edn,[53] is a subset of the Clojure language intended as a data transfer format. It can be used to serialize and deserialize Clojure data structures, and Clojure itself uses a superset of edn to represent programs.
edn is used in a similar way to
JSON or
XML, but has a relatively large list of built-in elements, shown here with examples:
In addition to those elements, it supports extensibility through the use of tags, which consist of the character # followed by a symbol. When encountering a tag, the reader passes the value of the next element to the corresponding handler, which returns a data value. For example, this could be a tagged element: #myapp/Person {:first "Fred" :last "Mertz"}, whose interpretation will depend on the appropriate handler of the reader.
This definition of extension elements in terms of the others avoids relying on either convention or context to convey elements not included in the base set.
Alternative platforms
The primary platform of Clojure is
Java,[20][36] but other target implementations exist. The most notable of these is ClojureScript,[54] which compiles to
ECMAScript 3,[55] and ClojureCLR,[56] a full port on the
.NET platform, interoperable with its ecosystem.
Other implementations of Clojure on different platforms include:
Babashka,[57] Native Clojure scripting language leveraging
GraalVM native image and Small Clojure Interpreter
Tooling for Clojure development has seen significant improvement over the years. The following is a list of some popular
IDEs and
text editors with plug-ins that add support for programming in Clojure:[69]
In addition to the tools provided by the community, the official Clojure
command-line interface (CLI) tools[38] have also become available on
Linux,
macOS, and
Windows since Clojure 1.9.[83]
Development
The development process is restricted to the Clojure core team, though issues are publicly visible at the Clojure
JIRA project page.[84] Anyone can ask questions or submit issues and ideas at ask.clojure.org.[85] If it's determined that a new issue warrants a JIRA ticket, a core team member will triage it and add it. JIRA issues are processed by a team of screeners and finally approved by Rich Hickey.[86][87]
Impact
With continued interest in functional programming, Clojure's adoption by software developers using the Java platform has continued to increase. The language has also been recommended by software developers such as Brian Goetz,[88][89][90] Eric Evans,[91][92]James Gosling,[93]Paul Graham,[94] and
Robert C. Martin.[95][96][97][98]ThoughtWorks, while assessing functional programming languages for their Technology Radar,[99] described Clojure as "a simple, elegant implementation of Lisp on the JVM" in 2010 and promoted its status to "ADOPT" in 2012.[100]
In the "JVM Ecosystem Report 2018" (which was claimed to be "the largest survey ever of Java developers"), that was prepared in collaboration by Snyk and Java Magazine, ranked Clojure as the 2nd most used programming language on the JVM for "main applications".[101] Clojure is used in industry by firms[102] such as
Apple,[103][104]Atlassian,[105]Funding Circle,[106]Netflix,[107]Nubank,[108]Puppet,[109] and
Walmart[110] as well as government agencies such as
NASA.[111] It has also been used for creative computing, including visual art, music, games, and poetry.[112]
In the 2023 edition of
Stack Overflow Developer Survey, Clojure was the fourth most admired in the category of programming and scripting languages, with 68.51% of the respondents who have worked with it last year saying they would like to continue using it. In the desired category, however it was marked as such by only 2.2% of the surveyed, whereas the highest scoring
JavaScript was desired by 40.15% of the developers participating in the survey.[113] It could possibly be attributed to limited professional opportunities for Clojure developers with just over 50 search results for "Clojure developer" at the
U.S. section of
Indeed, as of May 2024.[114]
New syntax for keyword argument invocation, new clojure.math namespace, namespace aliasing without loading, and new helper functions added to clojure.core