Clean (programming language)

Clean

File:Cleanlang logo.jpg
Usual file extensions	.icl, .dcl, .abc, .obj
Paradigm	functional
Appeared in	1987
Designed by	Software Technology Research Group of Radboud University Nijmegen
Stable release	2.2 (December 18, 2006)
Typing discipline	strong, static, dynamic
Influenced by	Lean, Haskell

In computer science, Clean is a general-purpose purely functional computer programming language.

Features

The language Clean first appeared in 1987 and is still further developed; it shares many properties with Haskell: referential transparency, list comprehension, guards, garbage collection, higher order functions and currying and lazy evaluation.

An integrated development environment (IDE) is included in the Clean distribution.

Clean's method for dealing with mutable state and I/O is done through a uniqueness typing system, in contrast to Haskell's use of monads. "The uniqueness type system also allows the Clean compiler to generate efficient code because uniquely attributed data structures can be destructively updated."

Examples

Hello world:

module hello

Start = "Hello, world!"

Factorial:

 module factorial

 fac 0 = 1
 fac n = n * fac (n-1)
 
 // find the factorial of 10
 Start = fac 10

Fibonacci sequence:

 module fibonacci

 fib 0 = 0 
 fib 1 = 1 
 fib n = fib (n - 2) + fib (n - 1) 

 Start = fib 7

Infix operator:

 (^) infixr 8 :: Int Int -> Int 
 (^) x 0 = 1
 (^) x n = x * x ^ (n-1) 

The type declaration states that the function is a right associative infix operator with priority 8: this states that x*x^(n-1) is equivalent to x*(x^(n-1)) as opposed to (x*x)^(n-1); this operator is pre-defined in the Clean standard environment.

How Clean works

Computation is based on graph rewriting and reduction. Constants such as numbers are graphs and functions are graph rewriting formulas. This, combined with compilation to native code, makes Clean programs relatively fast, even with high abstraction.

Compiling

1. Source files (.icl) and project files (.dcl) are converted into Clean's platform-independent bytecode (.abc), implemented in C and Clean.
2. Bytecode is converted to object code (.obj) using C.
3. object code is linked with other files in the module and the runtime system and converted into a normal executable in Clean.

Earlier Clean system versions were written completely in C, thus avoiding bootstrapping issues.

Platforms

Clean is available for Microsoft Windows. It is also available with limited input/output capabilities and without the "Dynamics" feature for Apple Macintosh, Solaris and Linux.

License

Clean is dual licensed: it is available under the terms of the GNU LGPL, and also under a proprietary license.

Versus Haskell

Speed

Some state that Clean is faster than Haskell, but other research show that this depends on the kind of program that is tested.

Syntactic differences

The syntax of Clean is very similar to Haskell, with some notable differences.

Haskell	Clean	Remarks
(a -> b) -> [a] -> [b]	(a -> b) [a] -> [b]	higher order function
f . g	f o g	function composition
-5	~5	unary minus

See also

• Haskell programming language
• List of functional programming topics

External links

• Clean homepage
  • Clean Language Reports
  • An introductory textbook to Clean
• Clean in FOLDOC
• Clean in The Computer Language Benchmarks Game
• Clean vs. Haskell discussion (haskell-cafe)
• Clean FAQ