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Clean (programming language)

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Designed bySoftware Technology Research Group of Radboud University Nijmegen
First appeared1987; 37 years ago (1987)
Stable release
3.1 / 5 January 2022; 2 years ago (2022-01-05)
Typing disciplinestrong, static, dynamic
LicenseSimplified BSD[1]
Filename extensions.icl, .dcl, .abc
Influenced by
Lean, Miranda, Haskell
Haskell, Idris[2]

Clean is a general-purpose purely functional programming language. Originally called the Concurrent Clean System[3] or the Clean System,[4][5] it has been developed by a group of researchers from the Radboud University in Nijmegen since 1987.[6][7] Although development of the language has slowed, some researchers are still working in the language.[8] In 2018, a spin-off company was founded that uses Clean.[9]


Clean shares many properties and syntax with a younger sibling language, Haskell: referential transparency, list comprehension, guards, garbage collection, higher order functions, currying, and lazy evaluation. However, Clean deals with mutable state and input/output (I/O) through a uniqueness type 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 at any point during the execution of the program, only one reference can exist to a value with a unique type. Therefore, a unique value can be changed in place.[10]

An integrated development environment (IDE) for Microsoft Windows is included in the Clean distribution.


Hello world:

 Start = "Hello, world!"


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.

How Clean works[edit]

Computing is based on graph rewriting and reduction. Constants such as numbers are graphs and functions are graph rewriting formulas. This, combined with compiling to native code, makes Clean programs which use high abstraction run relatively fast according to The Computer Language Benchmarks Game.[11] A 2008 benchmark showed that Clean native code performs similarly to the Glasgow Haskell Compiler (GHC), depending on the benchmark.[12]


Compilation of Clean to machine code is performed as follows:

  1. Source files (.icl) and definition files (.dcl) are translated into Core Clean, a basic variant of Clean, by the compiler frontend written in Clean.
  2. Core clean is converted into Clean's platform-independent intermediate language (.abc), by the compiler backend written in Clean and C.
  3. Intermediate ABC code is converted to object code (.o) by the code generator written in 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 versions of the Clean compiler were written completely in C, thus avoiding bootstrapping issues.

The ABC machine[edit]

The ABC code mentioned above is an intermediate representation for an abstract machine. Because machine code generation for ABC code is relatively straightforward, it is easy to support new architectures. The ABC machine is an imperative abstract graph rewriting machine.[13] It consists of a graph store to hold the Clean graph that is being rewritten and three stacks:

  • The A(rgument)-stack holds arguments that refer to nodes in the graph store.
  • The B(asic value)-stack holds basic values (integers, characters, reals, etc.). Although these values could be nodes in the graph store, a separate stack is used for efficiency.
  • The C(ontrol)-stack holds return addresses for flow control.

The runtime system, which is linked into every executable, builds a Start node in the graph store and pushes it on the A-stack. It then begins printing it, evaluating it as needed.

Running Clean in the browser[edit]

Although Clean is typically used to generate native executables, several projects have enabled applications in web browsers. The now abandoned SAPL project compiled Core Clean to JavaScript and did not use ABC code. Since 2019, an interpreter for ABC code, written in WebAssembly, is used instead.[14][15]


Clean is available for Microsoft Windows (IA-32 and X86-64), macOS (X86-64), and Linux (IA-32, X86-64, and AArch64).[citation needed]

Some libraries are not available on all platforms, like ObjectIO which is only available on Windows. Also the feature to write dynamics to files is only available on Windows.[citation needed]

The availability of Clean per platform varies with each version:[16][17]

Version Date Linux macOS Oracle Solaris Windows Miscellaneous
IA-32 x86-64 AArch64 Motorola 68040 PowerPC x86-64 SPARC IA-32 x86-64
3.1 5 January 2022 Yes Yes Yes No No Yes No Yes Yes
3.0 2 October 2018 Yes Yes No No No Yes No Yes Yes
2.4 23 December 2011 Yes Yes No No No Yes No Yes Yes
2.3 22 December 2010 Yes Yes No No No No No Yes Yes
2.2 19 December 2006 Yes Yes No No Yes No Yes Yes Yes
2.1.1 31 May 2005 Yes No No No Yes No Yes Yes No
2.1.0 31 October 2003 Yes No No No Yes No Yes Yes No
2.0.2 12 December 2002 Yes No No No Yes No Yes Yes No
2.0.1 4 July 2002 Yes No No No Yes No Yes Yes No
2.0 21 December 2001 No No No No No No No Yes No
1.3.3 13 September 2000 Yes No No No Yes No Yes Yes No
1.3.2 1 July 1999 No No No Yes Yes No Yes Yes No
1.3.1 January 1999 Yes No No No Yes No Yes Yes No
1.3 22 May 1998 Yes No No No Yes No Yes Yes No
1.2.4 June 1997 No No No Yes Yes No No Yes No
1.2.3 May 1997 No No No Yes Yes No No Yes No
1.2 13 January 1997 No No No Yes Yes No No No No
1.1.3 October 1996 No No No No No No Yes No No OS/2 (i80386)
1.1.2 September 1996 Yes No No No No No Yes No No SunOS 4 (SPARC)
1.1 March 1996 Yes No No Yes No No No No No
1.0.2 September 1995 Yes No No Yes No No Yes No No OS/2 (i80386); SunOS 4 (SPARC)
1.0 May 1995 No No No Yes No No No No No OS/2 (i80386)
0.8.4 11 May 1993 Yes No No Yes No No No No No Experimental T800 transputer release
0.8.3 26 February 1993 No No No Yes No No No No No
0.8.1 19 October 1992 No No No Yes No No No No No
0.8 13 July 1992 No No No Yes No No No No No OS/2 (i80386); SunOS 3–4 (SPARC)
0.7 May 1991 No No No Yes No No No No No SunOS 3–4 (SPARC)

Comparison to Haskell[edit]

The syntax of Clean is very similar to that of Haskell, with some notable differences. In general, Haskell has introduced more syntactic sugar than Clean:[10]

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


  1. ^ "Download Clean". Clean. Retrieved 23 July 2019.
  2. ^ "Idris - Uniqueness Types". Retrieved 20 November 2018.
  3. ^ "Clean 0.7: Readme". Archived from the original on 24 May 2019.
  4. ^ "Clean 1.0: Readme". Archived from the original on 5 May 2019.
  5. ^ "Clean 1.3: Readme". Archived from the original on 27 April 2019.
  6. ^ "Radboud University Nijmegen: Department of Software Science: Software".
  7. ^ "FAQ". Clean. Retrieved 26 November 2021.
  8. ^ "Publications". Clean. Retrieved 26 November 2021.
  9. ^ "Home". TOP Software Technology. Retrieved 26 November 2021.
  10. ^ a b ftp://ftp.cs.ru.nl/pub/Clean/papers/2007/achp2007-CleanHaskellQuickGuide.pdf
  11. ^ "Which programming languages are fastest?". Computer Language Benchmarks Game. Archived from the original on 28 June 2011.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  12. ^ Jansen, Jan Martin; Koopman, Pieter; Plasmeijer, Rinus (2008). "From Interpretation to Compilation" (PDF). Retrieved 21 May 2016. {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ Koopman, Pieter (10 December 1990). Functional Programs as Executable Specifications (PhD). Katholieke Universiteit Nijmegen. p. 35. ISBN 90-9003689-X.
  14. ^ "Clean and iTasks / ABC Interpreter · GitLab". Clean and iTasks on GitLab. Retrieved 13 April 2023.
  15. ^ Staps, Camil; van Groningen, John; Plasmeijer, Rinus (15 July 2021). "Lazy interworking of compiled and interpreted code for sandboxing and distributed systems". Proceedings of the 31st Symposium on Implementation and Application of Functional Languages. pp. 1–12. doi:10.1145/3412932.3412941. ISBN 9781450375627. S2CID 202751977.
  16. ^ "Release history". Clean. Retrieved 7 January 2022.
  17. ^ "Index of /Clean". Retrieved 7 January 2022.

External links[edit]