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<head>
<title>Clojure</title>
<meta http-equiv="Content-type" content="text/html; charset=iso-8859-1"/>
<meta name="description" content="Clojure is a dynamic programming language for the JVM" />
<meta name="keywords" content="Rich Hickey, Lisp, Java, software transactional memory, functional programming, persistent data structures"
/>
<meta name="author" content="Rich Hickey" />
<link rel="stylesheet" type="text/css" media="screen" href="clojure.css">
</head>
#Clo*j*ure
## A Dynamic Programming Language for the JVM
Copyright (c) Rich Hickey. All rights reserved.
The use and distribution terms for this software are covered by the [Common Public License 1.0][cpl], which can be found in the file CPL.TXT at the root of this distribution. By using this software in any fashion, you are agreeing to be bound by the terms of this license. You must not remove this notice, or any other, from this software.
[cpl]:http://www.opensource.org/licenses/cpl1.0.php
##Contents
###Introduction
Clojure is a dynamic programming language that targets the [Java Virtual Machine][jvm]. It is designed to be a general-purpose language, combining the approachability and interactive development of a scripting language with an efficient and robust infrastructure for multithreaded programming. Clojure is a compiled language - it compiles directly to JVM bytecode, yet remains completely dynamic. *Every* feature supported by Clojure is supported at runtime. Clojure provides easy access to the Java frameworks, with *optional* type hints and type inference, to ensure that calls to Java can avoid reflection.
Clojure is a dialect of Lisp, and shares with Lisp the code-as-data philosophy and a powerful macro system. Clojure is predominantly a [functional programming][fp] language, and features a rich set of immutable, [persistent data structures][pd]. When mutable state is needed, Clojure offers a [software transactional memory][stm] system that ensures clean, correct, multithreaded designs.
I hope you find Clojure's combination of facilities elegant, powerful, practical and fun to use.
[jvm]:http://java.sun.com/docs/books/jvms/
[fp]: http://en.wikipedia.org/wiki/Functional_programming
[pd]: http://en.wikipedia.org/wiki/Persistent_data_structure
[stm]:http://en.wikipedia.org/wiki/Software_transactional_memory
Rich Hickey
---
This documentation is continually being updated to reflect the current and new features of Clojure. Check back frequently for additions and corrections.
###[Setup](#setup)
###[Quick Start](#quickstart)
###[Reader](#reader)
###[Evaluation](#evaluation)
###[Special Forms](#specialforms)
###[Macros](#macros)
###[Other Useful Functions](#other)
###[Data Structures](#datastructures)
###[Metadata](#metadata)
###[Sequences](#sequences)
###[Namespaces](#namespaces)
###[Vars and the Global Environment](#vars)
###[Refs and Transactions](#refs)
###[Differences with other Lisps](#lisp)
<h2 id="setup">Setup</h2>
Clojure is [hosted on SourceForge][sf].
Feedback and discussion should occur on the [Clojure Google Group][cgg].
Clojure is delivered in a zip file containing a single .jar, `clojure.jar`, a readme, the CPL license and the source code in a `src` subdirectory. It uses the [ASM 3.0 bytecode library][asm], and the current alpha distribution includes it. [Java][jdk] 1.5 or greater is required.
[asm]: http://asm.objectweb.org/
[jdk]: http://java.sun.com/javase/downloads/index.jsp
[sf]: http://sourceforge.net/project/showfiles.php?group_id=137961
[cgg]: http://groups.google.com/group/clojure
<h2 id="quickstart">Quick Start</h2>
In the directory in which you expanded `clojure.zip`, run:
<pre><code>
java -cp clojure.jar clojure.lang.Compiler src/boot.clj
</code></pre>
This will bring up a simple read-eval-print loop (REPL). Much of Clojure is defined in Clojure itself (in the `boot.clj` file included in the `src` directory of distribution), so the command-line argument is needed to load it.
When boot.clj is loaded you will have the language as described herein fully available.
Try:
<pre><code>
user=> (+ 1 2 3)
6
user=> (. javax.swing.JOptionPane (showMessageDialog nil "Hello World"))
</code></pre>
<h2 id="reader">Reader</h2>
Clojure is a [homoiconic][hicon] language, which is a fancy term describing the fact that Clojure programs are represented by Clojure data structures. This is a very important difference between Clojure (and Common Lisp) and most other programming languages - Clojure is defined in terms of the evaluation of a data structure and **not** in terms of the syntax of character streams/files. It is quite common, and easy, for Clojure programs to manipulate, transform and produce other Clojure programs.
That said, most Clojure programs begin life as text files, and it is the task of the *reader* to parse the text and produce the data structure the compiler will see. This is not merely a phase of the compiler. The reader, and the Clojure data representations, have utility on their own in many of the same contexts one might use XML or JSON etc.
One might say the reader has syntax defined in terms of characters, and the Clojure language has syntax defined in terms of symbols, lists, vectors, maps etc. The reader is represented by the function `read`, which reads the next form (not character) from a stream, and returns the object represented by that form.
Since we have to start somewhere, we might as well start where evaluation starts, with the reader forms. This will inevitably entail talking about data structures whose descriptive details, and interpretation by the compiler, will follow.
[hicon]:http://en.wikipedia.org/wiki/Homoiconicity
### Reader forms
* Symbols
Symbols begin with a non-numeric character and can contain alphanumeric characters and *, +, !, -, _, and ? (other characters will be allowed eventually, but not all macro characters have been determined). '/' has special meaning, it can be used once in the middle of a symbol to separate the [namespace](#namespaces) from the name, e.g. `my-namespace/foo`. '/' by itself names the division function. '.' has special meaning - it can be used one or more times in the middle of a symbol to designate a fully-qualified class name, e.g. `java.util.BitSet`. Symbols beginning with '.' are reserved by Clojure.
* Literals
* Strings - Enclosed in `"double quotes"`. Standard Java escape characters.
* Numbers - as per Java, plus indefinitely long integers are supported, as well as ratios, e.g. `22/7`
* Characters - preceded by a backslash: `\c`.
`\newline`, `\space` and `\tab` yield the corresponding characters.
* `nil` - represents null and logical false
* Keywords
Keywords are like symbols, except:
* They can and must begin with a colon, e.g. `:fred`.
* They cannot contain '.' or name classes.
* Lists
Lists are zero or more forms enclosed in parentheses:
`(a b c)`
* Vectors
Vectors are zero or more forms enclosed in square brackets:
`[1 2 3]`
* Maps
Maps are zero or more key/value pairs enclosed in braces:
`{:a 1 :b 2}`
Commas are considered whitespace, and can be used to organize the pairs:
`{:a 1, :b 2}`
Keys and values can be any forms.
### Macro characters
The behavior of the reader is driven by a combination of built-in constructs and an extension system called the read table. Entries in the read table provide mappings from certain characters, called macro characters, to specific reading behavior, called reader macros. Unless indicated otherwise, macro characters cannot be used in user symbols.
* Quote (')
`'form` => `(quote form)`
* Character (\\)
As per above, yields a character literal.
* Comment (;)
Single-line comment, causes the reader to ignore everything from the semicolon to the end-of-line.
* Meta (^)
`^form` => `(meta form)`
* Deref (@, @!)
`@form` => `(deref form)`
`@!form` => `(deref! form)`
* Dispatch (#)
The dispatch macro causes the reader to use a reader macro from another table, indexed by the character following #:
* Metadata (#^)
Symbols, Lists, Vector and Maps can have metadata, which is a map associated with the object. The metadata reader macro first reads the metadata and attaches it to the next form read:
`#^{:a 1 :b 2} [1 2 3]` yields the vector [1 2 3] with a metadata map of {:a 1 :b 2}.
A shorthand version allows the metadata to be a simple symbol or keyword, in which case it is treated as a single entry map with a key of :clojure/tag and a value of the symbol provided, e.g.:
`#^String x` is the same as `#^{:clojure/tag String} x`
Such tags can be used to convey type information to the compiler.
* Var-quote (#')
`#'x` => `(the-var x)`
* Syntax-quote (\`, note, the "backquote" character), Unquote (~) and Unquote-splicing (~@)
For all forms other than Symbols, Lists, Vectors and Maps, \`x is the same as 'x.
For Symbols, syntax-quote *resolves* the symbol in the current context, yielding a fully-qualified symbol (i.e. namespace/name or fully.qualified.Classname). If a symbol is non-namespace-qualified and ends with '#', it is resolved to a generated symbol with the same name to which '\_' and a unique id have been appended. e.g. `x#` will resolve to `x_123`. All references to that symbol within a syntax-quoted expression resolve to the same generated symbol.
For Lists/Vectors/Maps, syntax-quote establishes a template of the corresponding data structure. Within the template, unqualified forms behave as if recursively syntax-quoted, but forms can be exempted from such recursive quoting by qualifying them with unquote or unquote-splicing, in which case they will be treated as expressions and be replaced in the template by their value, or sequence of values, respectively.
For example:
<pre><code>
user=> (def x 5)
user=> (def lst '(a b c))
user=> `(fred x ~x lst ~@lst 7 8 :nine)
(user/fred user/x 5 user/lst a b c 7 8 :nine)
</code></pre>
The read table is currently not accessible to user programs.
<h2 id="evaluation">Evaluation</h2>
Evaluation can occur in many contexts:
* Interactively, in the REPL
* On a sequence of forms read from a stream, via `load` or `load-file`
* Programmatically, via `eval`
Clojure programs are composed of expressions. Every form not handled specially by a special form or macro is considered by the compiler to be an expression, which is evaluated to yield a value. There are no declarations or statements, although sometimes expressions may be evaluated for their side-effects and their values ignored.
In all cases, evaluation is the same - a single object is considered by the compiler, evaluated, and its result returned. If an expression needs to be compiled, it will be. There is no separate compilation step, nor any need to worry that a function you have defined is being interpreted. *Clojure has no interpreter*.
Strings, numbers, characters, `nil` and keywords evaluate to themselves.
A Symbol is *resolved*:
* If it is namespace-qualified, the value is the value of the binding of the global var named by the symbol. It is an error if there is no global var named by the symbol.
* Else, it is not namespace-qualified and the first of the following applies:
1. If it names a special form it is considered a special form, and must be utilized accordingly.
2. A lookup is done in the \*imports\* map to see if there is a mapping from the symbol to a fully qualified class name. If so, the symbol is considered to name a Java class. Note that class names are not first-class objects and are only valid in certain special forms.
3. If in a local scope (i.e. in a function definition), a lookup is done to see if it names a local binding (e.g. a function argument or let-bound name). If so, the value is the value of the local binding.
4. A lookup is done in the \*refers\* map to see if there is a mapping from the symbol to a global var. If so, the value is the value of the binding of the global var referred-to by the symbol.
5. A lookup is done to see if there is a global var with a namespace equal to the `*current-namespace*` and the same name as the symbol. If so, the value is the value of the binding of that global var.
6. It is an error.
If a Symbol has metadata, it may be used by the compiler, but will not be part of the resulting value.
Vectors and Maps yield vectors and (hash) maps whose contents are the *evaluated values* of the objects they contain. The same is true of metadata maps. If the vector or map has metadata, the *evaluated* metadata map will become the metadata of the resulting value.
<pre><code>
user=> (def x 1)
user=> (def y 2)
user=> #^{:x x} [x y 3]
#^{:x 1} [1 2 3]
</code></pre>
An empty list `()` evaluates to an empty list.
Non-empty Lists are considered *calls* to either special forms, macros, or functions. A call has the form `(operator operands*)`.
Special forms are primitives built-in to Clojure that perform core operations. If the operator of a call is a symbol that resolves to the name of a special form, the call is to that special form. Each form discussed individually under [Special Forms](#specialforms).
[Macros](#macros) are functions that manipulate forms, allowing for syntactic abstraction. If the operator of a call is a symbol that names a global var that is a macro function, that macro function is called and is passed the *unevaluated* operand forms. The return value of the macro is then evaluated in its place.
If the operator is not a special form or macro, the call is considered a function call. Both the operator and the operands (if any) are evaluated, from left to right. The result of the evaluation of the operator is then cast to IFn (the interface representing Clojure functions), and invoke() is called on it, passing the evaluated arguments. The return value of invoke() is the value of the call expression. If the function call form has metadata, it may be used by the compiler, but will not be part of the resulting value.
Note that special forms and macros might have other-than-normal evaluation of their arguments, as described in their entries under [Special Forms](#specialforms).
The above describes the evaluation of a single form. `load` and `load-file` will sequentially evaluate the set of forms contained in the stream/file. Such sets of forms usually have side effects, often on the global environment, defining functions etc. The loading functions occur in a temporary context, in which `*current-namespace*` has a fresh binding. That means that, should any form have an effect on that var (e.g. `in-namespace), the effect will unwind at the completion of the load.
<h2 id="specialforms">Special Forms</h2>
---
### (*def* symbol init?)
Creates or locates a global var with the name of `symbol` and a namespace of the value of `*current-namespace*`. If `init` is supplied, it is evaluated, and the root binding of the var is set to the resulting value. If `init` is not supplied, the root binding of the var is unaffected. `def` always applies to the root binding, even if the var is thread-bound at the point where def is called. `def` yields the var itself *(not its value)*. Throws an exception if symbol is in the `*refers*` map.
---
### (*if* test then else?)
Evaluates `test`. If not nil, evaluates and yields `then`, otherwise, evaluates and yields `else`. If `else` is not supplied it defaults to `nil`.
---
### (*do* exprs*)
Evaluates the expressions in order and returns the value of the last. If no expressions are supplied, returns `nil`.
---
### (*let* [bindings* ] exprs*)
binding => symbol init-expr
Evaluates the exprs in a context in which the symbols are bound to their respective init-exprs. The bindings are sequential, so each binding can see the prior bindings. The exprs are contained in an implicit `do`. If a binding symbol is annotated with a metadata tag, the compiler will try to resolve the tag to a class name and presume that type in subsequent references to the binding.
<pre><code
|
