Clean
Paradigmfunctional
Designed bySoftware Technology Research Group of Radboud University Nijmegen
First appeared1987 (1987)
Stable release
3.1 / 5 January 2022 (2022-01-05)
Typing disciplinestrong, static, dynamic
OSCross-platform
LicenseSimplified BSD[1]
Filename extensions.icl, .dcl, .abc
Websiteclean.cs.ru.nl
Influenced by
Lean, Miranda, Haskell
Influenced
Haskell, Idris[2]

Clean is a general-purpose purely functional computer programming language. It was called the Concurrent Clean System,[3] then the Clean System,[4][5] later just Clean. Clean has been developed by a group of researchers from the Radboud University in Nijmegen since 1987.[6]

Features

The language Clean first appeared in 1987.[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.

Examples

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.

How Clean works

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]

Compiling

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

The ABC code mentioned above is an intermediate representation for an abstract machine. Because machine code generation for ABC code is relatively straightforward, this makes it easy to support new architectures. The ABC machine is an imperative abstract graph rewriting machine.[12] 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.). 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, 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

Although Clean is typically used to generative 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.[13][14]

Platforms

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

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.

The availability of Clean per platform varies with each version:[15][16]

Version Date Linux macOS Oracle Solaris Windows Miscellaneous
IA-32 x86-64 AArch64 Motorola 68040 PowerPC x86-64 SPARC IA-32 x86-64
3.15 January 2022 YesYesYes NoNoYes No YesYes
3.02 October 2018 YesYesNo NoNoYes No YesYes
2.423 December 2011 YesYesNo NoNoYes No YesYes
2.322 December 2010 YesYesNo NoNoNo No YesYes
2.219 December 2006 YesYesNo NoYesNo Yes YesYes
2.1.131 May 2005 YesNoNo NoYesNo Yes YesNo
2.1.031 October 2003 YesNoNo NoYesNo Yes YesNo
2.0.212 December 2002 YesNoNo NoYesNo Yes YesNo
2.0.14 July 2002 YesNoNo NoYesNo Yes YesNo
2.021 December 2001 NoNoNo NoNoNo No YesNo
1.3.313 September 2000 YesNoNo NoYesNo Yes YesNo
1.3.21 July 1999 NoNoNo YesYesNo Yes YesNo
1.3.1January 1999 YesNoNo NoYesNo Yes YesNo
1.322 May 1998 YesNoNo NoYesNo Yes YesNo
1.2.4June 1997 NoNoNo YesYesNo No YesNo
1.2.3May 1997 NoNoNo YesYesNo No YesNo
1.213 January 1997 NoNoNo YesYesNo No NoNo
1.1.3October 1996 NoNoNo NoNoNo Yes NoNo OS/2 (i80386)
1.1.2September 1996 YesNoNo NoNoNo Yes NoNo SunOS 4 (SPARC)
1.1March 1996 YesNoNo YesNoNo No NoNo
1.0.2September 1995 YesNoNo YesNoNo Yes NoNo OS/2 (i80386); SunOS 4 (SPARC)
1.0May 1995 NoNoNo YesNoNo No NoNo OS/2 (i80386)
0.8.411 May 1993 YesNoNo YesNoNo No NoNo Experimental T800 transputer release
0.8.326 February 1993 NoNoNo YesNoNo No NoNo
0.8.119 October 1992 NoNoNo YesNoNo No NoNo
0.813 July 1992 NoNoNo YesNoNo No NoNo OS/2 (i80386); SunOS 3–4 (SPARC)
0.7May 1991 NoNoNo YesNoNo No NoNo SunOS 3–4 (SPARC)

Comparison to Haskell

A 2008 benchmark showed that Clean native code performs similarly to the Haskell (Glasgow Haskell Compiler (GHC)), depending on the benchmark.[17]

Syntactic differences

The syntax of Clean is very similar to that of Haskell, with some notable differences:[10]

Haskell Clean Remarks
[ 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
fun t@(Node l x r) = ...

fun t=:(Node l x r) = ...
as-patterns
if x > 10 then 10 else x

if (x > 10) 10 x
if

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

References

  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. 1 2 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. ↑ Koopman, Pieter (10 December 1990). Functional Programs as Executable Specifications (PhD). Katholieke Universiteit Nijmegen. p. 35. ISBN 90-9003689-X.
  13. ↑ "Clean and iTasks / ABC Interpreter · GitLab". Clean and iTasks on GitLab. Retrieved 13 April 2023.
  14. ↑ 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.
  15. ↑ "Release history". Clean. Retrieved 7 January 2022.
  16. ↑ "Index of /Clean". Retrieved 7 January 2022.
  17. ↑ Jansen, Jan Martin; Koopman, Pieter; Plasmeijer, Rinus (2008). "From Interpretation to Compilation" (PDF). Retrieved 21 May 2016. {{cite journal}}: Cite journal requires |journal= (help)
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