Clean is a general-purpose purely functional programming language. Functional languages stress application of functions, as opposed to execution of commands, as in imperative languages.

Features

• Referential transparency - a function, given the same inputs, always gives the same output.
• Uniqueness typing system - easily and functionally deal with unduplicatable resources such as input and output without monads
• List comprehension - compact, powerful list generation syntax
• Guard statements for clear, concise conditionals
• Easy-to-use Clean IDE.
• Portability is almost always a simple recompile to support another platform, due to the high level of abstraction.
• Automatic garbage collection - one less thing to worry about
• Higher order functions and Currying
• Delayed evaluation support allows infinite data structures

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 equivelent 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

• Windows
• Macintosh
• Solaris
• Linux, but with limited input-output capabilities

License

Clean is licensed under the GNU LGPL, but can be used without the LGPL if bought for €495.

See also

• Haskell programming language
• List of functional programming topics

External links

• Clean homepage
  • Clean 2.0 Language Report in HTML
  • An introductory textbook to Clean

• Clean in FOLDOC