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 la ...
. 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
Radboud University (abbreviated as RU, nl, Radboud Universiteit , formerly ''Katholieke Universiteit Nijmegen'') is a public research university located in Nijmegen, the Netherlands. The university bears the name of Saint Radboud, a 9th century D ...
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
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lang ...
:
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) withou ...
,
list comprehension
A list comprehension is a Syntax of programming languages, syntactic construct available in some programming languages for creating a list based on existing list (computing), lists. It follows the form of the mathematical ''set-builder notation'' ( ...
,
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 ...
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 recyclable m ...
,
higher order functions
In mathematics and computer science, a higher-order function (HOF) is a function that does at least one of the following:
* takes one or more functions as arguments (i.e. a procedural parameter, which is a parameter of a procedure that is itself ...
,
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 that ...
and
lazy evaluation
In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing).
The b ...
. 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
An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of at least a source code editor, build automation tools a ...
(IDE) for
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
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 C ...
:
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) \t ...
:
Fibonacci sequence
In mathematics, the Fibonacci numbers, commonly denoted , form a integer sequence, sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start ...
:
Infix
An infix is an affix inserted inside a word stem (an existing word or the core of a family of words). It contrasts with ''adfix,'' a rare term for an affix attached to the outside of a stem, such as a prefix or suffix.
When marking text for int ...
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
Linker or linkers may refer to:
Computing
* Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
(when available) or a dedicated linker written in Clean on
Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
.
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 very ...
, 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
In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact: "either it will or will not be ...
. 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 t ...
, 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
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
(
IA-32
IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation of ...
and
X86-64
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
),
macOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
(
X86-64
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
), and
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
(
IA-32
IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation of ...
,
X86-64
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...
, 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, AR ...
).
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
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lang ...
(
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
In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
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