Clean

Usual file extensions	.icl, .dcl, .abc, .obj
Paradigm	functional
Appeared in	1987
Designed by	Software Technology Research Group of Radboud University Nijmegen
Latest release	2.2/ 2006-12-18; 811 days ago
Typing discipline	strong, static, dynamic
Major implementations	Clean
Influenced by	Lean, Haskell

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

Contents 1 Features 2 Examples 3 How Clean works 4 Compiling 5 Platforms 6 License 7 References 8 Versus Haskell 8.1 Speed 9 Syntactic differences 10 See also 11 External links

Features

The language Clean is heavily influenced by Haskell, and has therefore shares 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 has an alternative to monads, which are regular in the Haskell language; a uniqueness typing system deals with unduplicatable resources such as input and output.

Examples

Hello world (Store as hello.icl):

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.

References

Versus Haskell

Speed

Some state that Clean is faster than Haskell [1], but other researches show that this depends on the kind of program that is tested [2].

Syntactic differences

Although the syntax of Clean and Haskell is almost the same, these are some differences (not complete)[3]

Haskell	Clean	Remarks
(x:xs)	[x:xs]	list construction
(a -> b) -> [a] -> [b]	(a -> b) [a] -> [b]
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