Clean (programming language)

Paradigm functional
Designed by Software Technology Research Group of Radboud University Nijmegen
First appeared 1987 (1987)
Stable release
2.4 / December 23, 2011 (2011-12-23)
Typing discipline strong, static, dynamic
OS Cross-platform
License GNU LGPL, Simplified BSD, commercial software
Filename extensions .icl, .dcl, .abc, .o, .sapl
Influenced by
Lean, Miranda, Haskell

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


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.

On Windows, 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]


Hello world:

 module hello
 Start :: {#Char}
 Start = "Hello, world!"


  module factorial
  import StdEnv
  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] // The product of the numbers 1 to n

  // find the factorial of 6
  Start = fac 6

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 StdEnv, the Clean standard library.

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]


  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 bytecode (.abc), implemented in C and Clean.
  3. Bytecode 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 in Clean.

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.

The ABC-Machine

To close the gap between Core Clean, a high-level functional language, and machine code, the ABC-machine is used. This is an imperative abstract graph rewriting machine.[4] Generating concrete machine code from abstract ABC code is a relatively small step, so by using the ABC-machine it is much easier to target multiple architectures for code generation.

The ABC-machine has a uncommon memory model. It has a graph store to hold the Clean graph that is being rewritten. The A(rgument)-stack holds arguments that refer to nodes in the graph store. This way, a node's arguments can be rewritten, which is needed for pattern matching. The B(asic value)-stack holds basic values (integers, characters, reals, etc.). While not strictly necessary (all these elements could be nodes in the graph store as well), using a separate stack is much more efficient. The C(ontrol)-stack holds return addresses for flow control.

The runtime system, which is linked into every executable, has a print rule which prints a node to the output channel. When a program is executed, the Start node is printed. For this, it has to be rewritten to root normal form, after which its children are rewritten to root normal form, etc., until the whole node is printed.


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.


Clean is dual licensed: it is available under the terms of the GNU LGPL, and also under a proprietary license. For the libraries, runtime system and examples, not the GNU LGPL but the Simplified BSD License applies.

Versus Haskell


A benchmark from 2008 shows that Clean is faster than Haskell in most cases:[5]

Speed comparison of five compilers (time in seconds)
Language Pri Sym Inter Fib Match Ham Twi Qns Kns Parse Plog Qsort Isort Msort
SAPL Int 6.1 17.6 7.8 7.3 8.5 15.7 7.9 6.5 47.1 4.4 4.0 16.4 9.4 4.4
SAPL Bas 4.3 13.2 6.0 6.5 5.9 9.8 5.6 5.1 38.3 3.8 2.6 10.1 6.7 2.6
GHC 2.0 1.7 8.2 4.0 4.1 8.4 6.6 3.7 17.7 2.8 0.7 4.4 2.3 3.2
GHC -O 0.9 1.5 1.8 0.2 1.0 4.0 0.1 0.4 5.7 1.9 0.4 3.2 1.9 1.0
Clean 0.9 0.8 0.8 0.2 1.4 2.4 2.4 0.4 3.0 4.5 0.4 1.6 1.0 0.6

As can be seen, Clean outruns Haskell (GHC) on almost all test cases. Only parser combinators are faster in Haskell. Using GHC -O we get some optimisations, making pattern matching and higher order functions faster than in Clean as well. In most cases, however, Clean outperforms GHC -O or at least isn't slower.

The programs used were:

Syntactic differences

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

Haskell Clean Remarks
[ x | x <- [1..10] , isOdd x]
[ x \\ x <- [1..10] | isOdd x]
list comprehension
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
fun t@(Node l x r) = ...
fun t=:(Node l x r) = ...
if x > 10 then 10 else x
if (x > 10) 10 x

In general, Haskell has introduced more syntactic sugar than Clean.



  1. "FAQ - Clean".
  2. 1 2
  3. Boxplot Summary | Ubuntu : Intel Q6600 Computer Language Benchmarks Game
  4. Koopman, Pieter (December 10, 1990). Functional Programs as Executable Specifications. Meppel: Krips Repro. p. 35. ISBN 90-9003689-X.
  5. Jansen, Jan Martin; Koopman, Pieter; Plasmeijer, Rinus (2008). "From Interpretation to Compilation" (PDF). Retrieved 2016-05-21.

External links

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