Clean (programming language)

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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; 452805361 ago)
Typing discipline strong, static, dynamic
Influenced by Lean, Haskell

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



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."[1]


Hello world:

module hello
Start = "Hello, world!"


 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.


  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.


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.


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

Versus Haskell


Some state that Clean is faster than Haskell,[2] but other research show that this depends on the kind of program that is tested.[3]

Syntactic differences

The syntax of Clean is very similar to Haskell, with some notable differences.[4]

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


External links

cs:Clean de:Clean (Programmiersprache) nl:Clean (programmeertaal) ja:Concurrent Clean pt:Clean ru:Clean fi:Clean (ohjelmointikieli) zh:Concurrent Clean

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