= Rationale
Rich Hickey
2015-01-01
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:navlinktext: Rationale
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Customers and stakeholders have substantial investments in, and are comfortable with the performance, security and stability of, industry-standard platforms like the JVM. While Java developers may envy the succinctness, flexibility and productivity of dynamic languages, they have concerns about running on customer-approved infrastructure, access to their existing code base and libraries, and performance. In addition, they face ongoing problems dealing with concurrency using native threads and locking. Clojure is an effort in pragmatic dynamic language design in this context. It endeavors to be a general-purpose language suitable in those areas where Java is suitable. It reflects the reality that, for the concurrent programming future, pervasive, unmoderated mutation simply has to go.

Clojure meets its goals by: embracing an industry-standard, open platform - the JVM; modernizing a venerable language - Lisp; fostering functional programming with immutable persistent data structures; and providing built-in concurrency support via software transactional memory and asynchronous agents. The result is robust, practical, and fast.

Clojure has a distinctive approach to <<state#,state and identity>>.

== Why Clojure?

Why did I write yet another programming language? Basically because I wanted:

* A Lisp
* for Functional Programming
* symbiotic with an established Platform
* designed for Concurrency

and couldn't find one. Here's an outline of some of the motivating ideas behind Clojure.

== 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)

== Functional programming is a good thing

* Immutable data + first-class functions
* Could always be done in Lisp, by discipline/convention
** But if a data structure _can_ be mutated, dangerous to presume it won't be
** In traditional Lisp, only the list data structure is structurally recursive
* Pure functional languages tend to strongly static types
** Not for everyone, or every task
* Clojure is a functional language with a dynamic emphasis
** All data structures immutable & persistent, supporting recursion
** Heterogeneous collections, return types
** Dynamic polymorphism

== 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

== Object Orientation is overrated

* Born of simulation, now used for everything, even when inappropriate
** Encouraged by Java/C# in all situations, due to their lack of (idiomatic) support for anything else
* Mutable stateful objects are the new spaghetti code
** Hard to understand, test, reason about
** Concurrency disaster
* Inheritance is _not_ the only way to do polymorphism
* "It is better to have 100 functions operate on one data structure than to have 10 functions operate on 10 data structures." - Alan J. Perlis
* Clojure models its data structures as immutable objects represented by interfaces, and otherwise does not offer its own class system.
* Many functions defined on few primary data structures (seq, map, vector, set).
* Write Java in Java, consume and extend Java from Clojure.

== Polymorphism is a good thing

* Switch statements, structural matching etc yield brittle systems
* Polymorphism yields extensible, flexible systems
* Clojure multimethods decouple polymorphism from OO and types
** Supports multiple taxonomies
** Dispatches via static, dynamic or external properties, metadata, etc

== 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

In short, I think Clojure occupies a unique niche as a functional Lisp for the JVM with strong concurrency support. Check out some of the <<features#,features>> or <<xref/../../guides/getting_started#,get started with Clojure>>.
