Clean is a
general-purpose purely functional computer programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming l ...
. It was called the Concurrent Clean System, then the Clean System, later just Clean. Clean has been developed by a group of researchers from the
Radboud University in Nijmegen since 1987.
Features
The language Clean first appeared in 1987. Although development of the language itself has slowed down, some researchers are still working in the language. In 2018, a spin-off company was founded that uses Clean as well.
Clean shares many properties and syntax with a younger sibling language,
Haskell:
referential transparency
In computer science, referential transparency and referential opacity are properties of parts of computer programs. An expression is called ''referentially transparent'' if it can be replaced with its corresponding value (and vice-versa) witho ...
,
list comprehension
A list comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical ''set-builder notation'' (''set comprehension'') as distinct from the use of ...
,
guard
Guard or guards may refer to:
Professional occupations
* Bodyguard, who protects an individual from personal assault
* Crossing guard, who stops traffic so pedestrians can cross the street
* Lifeguard, who rescues people from drowning
* Prison gu ...
s,
garbage collection
Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
,
higher order functions,
currying
In mathematics and computer science, currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument. For example, currying a function f th ...
and
lazy evaluation. However, 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 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.
[ftp://ftp.cs.ru.nl/pub/Clean/papers/2007/achp2007-CleanHaskellQuickGuide.pdf]
An
integrated development environment (IDE) for
Microsoft Windows is included in the Clean distribution.
Examples
Hello world
''Hello'' is a salutation or greeting in the English language. It is first attested in writing from 1826. Early uses
''Hello'', with that spelling, was used in publications in the U.S. as early as the 18 October 1826 edition of the '' Norwich ...
:
Start = "Hello, world!"
Factorial
In mathematics, the factorial of a non-negative denoted is the product of all positive integers less than or equal The factorial also equals the product of n with the next smaller factorial:
\begin
n! &= n \times (n-1) \times (n-2) ...
:
Fibonacci sequence:
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
In computer science, graph transformation, or graph rewriting, concerns the technique of creating a new graph out of an original graph algorithmically. It has numerous applications, ranging from software engineering ( software construction and also ...
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.
Compiling
# Source files (.icl) and definition files (.dcl) are translated into Core Clean, a basic variant of Clean, in Clean.
# Core clean is converted into Clean's platform-independent intermediate language (.abc), implemented in
C and Clean.
# Intermediate ABC code is converted to object code (.o) using
C.
# 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.
The ABC machine
To close the gap between Core Clean, a high-level functional language, and
machine code
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a ve ...
, the ABC machine is used.
This is an
imperative abstract
graph rewriting
In computer science, graph transformation, or graph rewriting, concerns the technique of creating a new graph out of an original graph algorithmically. It has numerous applications, ranging from software engineering ( software construction and also ...
machine.
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 an 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
In computer programming, a runtime system or runtime environment is a sub-system that exists both in the computer where a program is created, as well as in the computers where the program is intended to be run. The name comes from the compile ...
, 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.
Platforms
Clean is available for
Microsoft Windows (
IA-32 and
X86-64),
macOS (
X86-64), and
Linux (
IA-32,
X86-64, and
AArch64
AArch64 or ARM64 is the 64-bit extension of the ARM architecture family.
It was first introduced with the Armv8-A architecture. Arm releases a new extension every year.
ARMv8.x and ARMv9.x extensions and features
Announced in October 2011, A ...
).
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:
Comparison to Haskell
A 2008 benchmark showed that Clean native code performs roughly equally well as
Haskell (
GHC), depending on the benchmark.
Syntactic differences
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.
References
{{Reflist
External links
Clean Wikiclean-lang.org public registry with Clean packages
cloogle.org a search engine to search in Clean packages
Functional languages
Term-rewriting programming languages
Free compilers and interpreters
Cross-platform free software