# Clean (programming language)

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Paradigm functional Software Technology Research Group of Radboud University Nijmegen 1987 2.4 / December 23, 2011 strong, static, dynamic .icl, .dcl, .abc, .obj http://clean.cs.ru.nl/ Lean, Miranda, Haskell

In computer science, Clean is a general-purpose purely functional computer programming language. For much of the language's active development history it was called Concurrent Clean and emphasized concurrency, but this was dropped at some point.

## Features

The language Clean first appeared in 1987 and is still being further developed.[1] It shares many properties with Haskell: referential transparency, list comprehension, guards, garbage collection, higher order functions, 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 compiler takes advantage of the uniqueness type system to generate more efficient code, because it knows that anything with a uniqueness type can only be used once. Therefore, a unique value can be changed in place. [2]

## Examples

``` module hello
Start :: {#Char}
Start = "Hello, world!"
```
 ``` module factorial fac 0 = 1 fac n = n * fac (n-1) // find the factorial of 10 Start = fac 10 ``` ``` module factorial2 import StdEnv fac 0 = 1 fac n = prod [1..n] // Generate a list that goes from 1 to n and returns the product of the elements // find the factorial of 6 Start = fac 6 ```
```  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 run relatively fast, even with high abstraction.[3]

## 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.

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

### Speed

Some[who?] state that Clean is faster than Haskell,[4] but other research show that this depends on the kind of program that is tested.[5]

### Syntactic differences

The syntax of Clean is very similar to Haskell, with some notable differences:[2]

```(a -> b) -> [a] -> [b]
```
```(a -> b) [a] -> [b]
```
higher order function
```f . g
```
```f o g
```
function composition
```-5
```
```~5
```
unary minus
```[ x | x <- [1..10] , isOdd x]
```
```[ x \\ x <- [1..10] | isOdd x]
```
list comprehension
```x:xs
```
```[x:xs]
```
cons operator
```data Tree a
= Empty
| Node (Tree a) a (Tree a)
```
```:: Tree a
= Empty
| Node (Tree a) a (Tree a)
```
algebraic data type
```(Eq a, Eq b) => ...
```
```... | Eq a & Eq b
```
class assertions and contexts