Definition
Clojure is a dynamic, general-purpose programming 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, 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 language, and features a rich set of immutable, persistent data structures. When mutable state is needed, Clojure offers a software transactional memory system and reactive Agent system that ensure clean, correct, multithreaded designs.
Why Clojure
Clojure combines of:
- A Lisp
- for Functional programming
- symbiotic with an established Platform
- designed for Concurrency
1. Lisp is a good thing
- Often emulated/pillaged, still not duplicated
- Lambda calculus yields an extremely small core
- Almost no syntax
- Core advantage still code-as-data and syntactic abstraction
- What about the standard Lisps (Common Lisp and Scheme)?
- Slow/no innovation post standardization
- Core data structures mutable, not extensible
- No concurrency in specs
- Good implementations already exist for JVM (ABCL, Kawa, SISC et al)
- Standard Lisps are their own platforms
- Clojure is a Lisp not constrained by backwards compatibility
- Extends the code-as-data paradigm to maps and vectors
- Defaults to immutability
- Core data structures are extensible abstractions
- Embraces a platform (JVM)
2. languages and platforms
- VMs, not OSes, are the platforms of the future, providing:
- Type system
- Dynamic enforcement and safety
- Libraries
- Abstract away OSes
- Huge set of facilities
- Built-in and 3rd-party
- Memory and other resource management
- GC is platform, not language, facility
- Bytecode + JIT compilation
- Abstracts away hardware
- Language as platform vs. language + platform
- Old way - each language defines its own runtime
- GC, bytecode, type system, libraries etc
- New way (JVM, .Net)
- Common runtime independent of language
- Language built for platform vs language ported-to platform
- Many new languages still take 'Language as platform' approach
- When ported, have platform-on-platform issues
- Memory management, type-system, threading issues
- Library duplication
- If original language based on C, some extension libraries written in C don’t come over
- Platforms are dictated by clients
- 'Must run on JVM' or .Net vs 'must run on Unix' or Windows
- JVM has established track record and trust level
- Now also open source
- Interop with other code required
- C linkage insufficient these days
- Java/JVM is language + platform
- Not the original story, but other languages for JVM always existed, now embraced by Sun
- Java can be tedious, insufficiently expressive
- Lack of first-class functions, no type inference, etc
- Ability to call/consume Java is critical
- Clojure is the language, JVM the platform
3. Concurrency and the multi-core future
- Immutability makes much of the problem go away
- Share freely between threads
- But changing state a reality for simulations and for in-program proxies to the outside world
- Locking is too hard to get right over and over again
- Clojure’s software transactional memory and agent systems do the hard part
4. Features
- Dynamic development
- Functional programming
- Lisp
- Runtime polymorphism
- Concurrent programming
- Hosted on the JVM
Dynamic development
First and foremost, Clojure is dynamic. That means that a Clojure program is not just something you compile and run, but something with which you can interact. Clojure is not a language abstraction, but an environment, where almost all of the language constructs are reified, and thus can be examined and changed. This leads to a substantially different experience from running a program, examining its results (or failures) and trying again. In particular, you can grow your program, with data loaded, adding features, fixing bugs, testing, in an unbroken stream.
Dynamic compilation
Clojure is a compiled language, so one might wonder when you have to run the compiler. You don’t. Anything you enter into the REPL or load using load-file is automatically compiled to JVM bytecode on the fly. Compiling ahead-of-time is also possible, but not required.
