Clean (programming language)

The language Clean first appeared in 1987 and is still being further developed.[3] 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) for Microsoft Windows is included in the Clean distribution.

Clean deals with mutable state and I/O 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.[4]

Hello world:

 Start = "Hello, world!"

Factorial:

fac :: Int -> Int
fac 0 = 1
fac n = n * fac (n-1)

Start = fac 10

fac :: Int -> Int
fac n = prod [1..n] // The product of the numbers 1 to n

Start = fac 10

Fibonacci sequence:

fib :: Int -> Int
fib 0 = 1
fib 1 = 1
fib n = fib (n - 2) + fib (n - 1) 

Start = fib 7

fibs :: Int Int -> [Int]
fibs x_2 x_1 = [x_2:fibs x_1 (x_2 + x_1)]

fib :: Int -> Int
fib n = (fibs 1 1) !! n

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 StdEnv, the Clean standard library.

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 which use high abstraction run relatively fast according to the Computer Language Benchmarks Game.[5]

1. Source files (.icl) and definition files (.dcl) are translated into Core Clean, a basic variant of Clean, in Clean.
2. Core clean is converted into Clean's platform-independent intermediate language (.abc), implemented in C and Clean.
3. Intermediate ABC code is converted to object code (.o) using C.
4. Object code is linked with other files in the module and the runtime system and converted into a normal executable using the system linker (when available) or a dedicated linker written in Clean on Windows.

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

The SAPL system compiles Core Clean to JavaScript and does not use ABC code.

Clean is available for Microsoft Windows, Apple Macintosh, Solaris and Linux.

Some libraries are not available on all platforms, like ObjectIO which is only available on Windows and Mac. The feature to write dynamics to files is only available on Windows.

A 2008 benchmark showed that Clean native code performs roughly equally well as Haskell (GHC), depending on the benchmark.[7]

[ x | x <- [1..10] , isOdd x]

[ x \\ x <- [1..10] | isOdd x]

x:xs

[x:xs]

data Tree a
  = Empty
  | Node (Tree a) a (Tree a)

:: Tree a
  = Empty
  | Node (Tree a) a (Tree a)

(Eq a, Eq b) => ...

... | Eq a & Eq b

fun t@(Node l x r) = ...

fun t=:(Node l x r) = ...

if x > 10 then 10 else x

if (x > 10) 10 x

• IRC channel: #cleanlang on freenode

• Clean Wiki
• Cloogle: Clean function search engine