Mercury programming language
Encyclopedia
Mercury is a functional
logic
programming language
geared towards real-world applications. It is developed at the University Of Melbourne
Computer Science department under the supervision of Zoltan Somogyi
. The first version was developed by Fergus Henderson
, Thomas Conway and Zoltan Somogyi and was released on April 8, 1995.
Mercury is a purely declarative
logic language
. It is related to both Prolog
and Haskell
. It features a strong, static, polymorphic type system
, as well as a strong mode and determinism system.
The official implementation, the Melbourne Mercury Compiler, is available for most Unix
platforms, including Mac OS X
, as well as for Microsoft Windows
(in Windows, it requires one of the Cygwin
or MinGW
toolsets, and can be compiled either with GCC
or Microsoft Visual C++
).
. It has the same syntax, and the same basic concepts such as the SLD resolution
algorithm. It can ostensibly be viewed as a pure subset of Prolog with strong types and modes. As such, it is often compared to its predecessor, both in terms of features, and run-time efficiency.
The language is designed with software engineering
principles in mind. Unlike the original implementations of Prolog, it has a separate compilation
phase, rather than being directly interpreted, which allows a much wider range of errors to be caught before running a program. It features a sophisticated, strict type
and mode system, which its authors claim makes it much easier to write robust software. Mercury's module system enables division into self-contained units, a problem for past logic programming languages.
Due to the use of information obtained at compile time (such as type and mode information), programs written in Mercury typically perform significantly faster than equivalent programs written in Prolog. The authors claim that Mercury is the fastest logic language in the world, by a wide margin.
Mercury is a purely declarative
language, unlike Prolog
, since it lacks "extra-logical" Prolog statements such as "cut" and imperative
I/O
. This enables advanced program optimization
, but can make certain programming constructs (such as a switch over a number of options, with a default) harder to express. (Note that while Mercury does allow impure functionality, it is not necessary in most software, and serves primarily as a way of calling foreign language code. Also, all impure code must be explicitly marked.)
Operations which would typically be impure (such as input/output
) are expressed using pure constructs in Mercury using linear types
, by threading a dummy "world" value through all relevant code.
Notable programs written in Mercury include the Mercury compiler itself and the Prince XML
formatter. Mission Critical IT http://www.missioncriticalit.com/, a software company, has also been using Mercury since 2000 to develop enterprise applications.
Production level:
Beta quality:
Alpha quality (may not work well, or even be completely broken):
Past back-ends:
This makes Mercury a useful high-level language for targeting multiple platforms, or for linking with code written using multiple back-ends.
Mercury also has a strong foreign language interface, allowing code in other languages (depending on the chosen back-end) to be linked with Mercury code. The following foreign languages are possible:
Other languages can then be interfaced to by calling them from these languages. However, this means that foreign language code may need to be written several times for the different backends, otherwise portability between backends will be lost.
The most commonly used back-end is the original low-level C back-end. As both C backends are considered production quality, this means that you will not lose a great deal of portability using foreign-language C code.
:
Calculating the 10th Fibonacci number
(in the most obvious way):
The current release is 11.01 (April 2011).
Previously releases were numbered 0.12, 0.13, etc. and the period between stable releases was very large (3 years).
The Mercury project also makes a snapshot release daily. The snapshot release is the latest development snapshot of the system and has all the latest features and bug fixes added to the last stable release.
Functional programming
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
logic
Logic programming
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
geared towards real-world applications. It is developed at the University Of Melbourne
University of Melbourne
The University of Melbourne is a public university located in Melbourne, Victoria. Founded in 1853, it is the second oldest university in Australia and the oldest in Victoria...
Computer Science department under the supervision of Zoltan Somogyi
Zoltan Somogyi
Dr. Zoltan Somogyi is a senior lecturer at the University of Melbourne's department of Computer Science and Software Engineering, who currently leads the team responsible for the Mercury programming language....
. The first version was developed by Fergus Henderson
Fergus Henderson (computer scientist)
Fergus Henderson is a computer scientist who was one of the original developers of the Mercury programming language. He is the maintainer for Distcc.He now works for Google.- External links :* * *...
, Thomas Conway and Zoltan Somogyi and was released on April 8, 1995.
Mercury is a purely declarative
Declarative programming
In computer science, declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow. Many languages applying this style attempt to minimize or eliminate side effects by describing what the program should accomplish, rather than...
logic language
Logic programming
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
. It is related to both Prolog
Prolog
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
and Haskell
Haskell (programming language)
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...
. It features a strong, static, polymorphic type system
Type system
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
, as well as a strong mode and determinism system.
The official implementation, the Melbourne Mercury Compiler, is available for most Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
platforms, including Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, as well as for Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
(in Windows, it requires one of the Cygwin
Cygwin
Cygwin is a Unix-like environment and command-line interface for Microsoft Windows. Cygwin provides native integration of Windows-based applications, data, and other system resources with applications, software tools, and data of the Unix-like environment...
or MinGW
MinGW
MinGW , formerly mingw32, is a native software port of the GNU Compiler Collection and GNU Binutils for use in the development of native Microsoft Windows applications; MinGW can function either as a cross compiler targeting Windows or as a native toolchain run on Windows itself...
toolsets, and can be compiled either with GCC
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
or Microsoft Visual C++
Visual C++
Microsoft Visual C++ is a commercial , integrated development environment product from Microsoft for the C, C++, and C++/CLI programming languages...
).
Overview
Mercury is based on the logic programming language PrologProlog
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
. It has the same syntax, and the same basic concepts such as the SLD resolution
SLD resolution
SLD resolution is the basic inference rule used in logic programming. It is a refinement of resolution, which is both sound and refutation complete for Horn clauses.-The SLD inference rule:...
algorithm. It can ostensibly be viewed as a pure subset of Prolog with strong types and modes. As such, it is often compared to its predecessor, both in terms of features, and run-time efficiency.
The language is designed with software engineering
Software engineering
Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
principles in mind. Unlike the original implementations of Prolog, it has a separate compilation
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
phase, rather than being directly interpreted, which allows a much wider range of errors to be caught before running a program. It features a sophisticated, strict type
Data type
In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...
and mode system, which its authors claim makes it much easier to write robust software. Mercury's module system enables division into self-contained units, a problem for past logic programming languages.
Due to the use of information obtained at compile time (such as type and mode information), programs written in Mercury typically perform significantly faster than equivalent programs written in Prolog. The authors claim that Mercury is the fastest logic language in the world, by a wide margin.
Mercury is a purely declarative
Declarative programming
In computer science, declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow. Many languages applying this style attempt to minimize or eliminate side effects by describing what the program should accomplish, rather than...
language, unlike Prolog
Prolog
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
, since it lacks "extra-logical" Prolog statements such as "cut" and imperative
Imperative programming
In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
. This enables advanced program optimization
Optimization (computer science)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
, but can make certain programming constructs (such as a switch over a number of options, with a default) harder to express. (Note that while Mercury does allow impure functionality, it is not necessary in most software, and serves primarily as a way of calling foreign language code. Also, all impure code must be explicitly marked.)
Operations which would typically be impure (such as input/output
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
) are expressed using pure constructs in Mercury using linear types
Linear type system
A linear type system is a particular form of type system used in a programming language. Linear type systems allow references but not aliases. To enforce this, a reference goes out of scope after appearing on the right-hand side of an assignment, thus ensuring that only one reference to any object...
, by threading a dummy "world" value through all relevant code.
Notable programs written in Mercury include the Mercury compiler itself and the Prince XML
Prince XML
Prince is a proprietary software program that converts XML and HTML documents into PDF files by applying Cascading Style Sheets . It is developed by YesLogic, a small company based in Melbourne, Australia.-History:...
formatter. Mission Critical IT http://www.missioncriticalit.com/, a software company, has also been using Mercury since 2000 to develop enterprise applications.
Back-ends
Mercury has several back-ends, which means it is possible to compile Mercury code into the following languages and code-styles:Production level:
- Low-level CC (programming language)C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
for GCCGNU Compiler CollectionThe GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
(the original Mercury back-end) - High-level C
- JavaJava (programming language)Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
bytecode for the JVMJava Virtual MachineA Java virtual machine is a virtual machine capable of executing Java bytecode. It is the code execution component of the Java software platform. Sun Microsystems stated that there are over 4.5 billion JVM-enabled devices.-Overview:...
Beta quality:
- Erlang
Alpha quality (may not work well, or even be completely broken):
- ILCommon Intermediate LanguageCommon Intermediate Language is the lowest-level human-readable programming language defined by the Common Language Infrastructure specification and is used by the .NET Framework and Mono...
for Microsoft's .NET.NET FrameworkThe .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability... - AssemblyAssembly languageAn assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
via the GCC back-end
Past back-ends:
- Aditi, a deductive database system also developed at the University of MelbourneUniversity of MelbourneThe University of Melbourne is a public university located in Melbourne, Victoria. Founded in 1853, it is the second oldest university in Australia and the oldest in Victoria...
. Mercury-0.12.2 is the last version of Mercury that will support Aditi.
This makes Mercury a useful high-level language for targeting multiple platforms, or for linking with code written using multiple back-ends.
Mercury also has a strong foreign language interface, allowing code in other languages (depending on the chosen back-end) to be linked with Mercury code. The following foreign languages are possible:
| Back-end | Foreign language(s) |
|---|---|
| C (both levels) | C C (programming language) C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.... |
| Java | Java Java (programming language) Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities... |
| Erlang | Erlang |
| IL | IL Common Intermediate Language Common Intermediate Language is the lowest-level human-readable programming language defined by the Common Language Infrastructure specification and is used by the .NET Framework and Mono... or C# |
Other languages can then be interfaced to by calling them from these languages. However, this means that foreign language code may need to be written several times for the different backends, otherwise portability between backends will be lost.
The most commonly used back-end is the original low-level C back-end. As both C backends are considered production quality, this means that you will not lose a great deal of portability using foreign-language C code.
Examples
Hello WorldHello world program
A "Hello world" program is a computer program that outputs "Hello world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to...
:
:- module hello.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
main(!IO) :-
io.write_string("Hello, World!\n", !IO).
Calculating the 10th Fibonacci number
Fibonacci number
In mathematics, the Fibonacci numbers are the numbers in the following integer sequence:0,\;1,\;1,\;2,\;3,\;5,\;8,\;13,\;21,\;34,\;55,\;89,\;144,\; \ldots\; ....
(in the most obvious way):
:- module fib.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module int.
:-func fib(int) = int.
fib(N) = (if N =< 2 then 1 else fib(N - 1) + fib(N - 2)).
main(!IO) :-
io.write_string("fib(10) = ", !IO),
io.write_int(fib(10), !IO),
io.nl(!IO).
% Could instead use io.format("fib(10) = %d\n", [i(fib(10))], !IO).
Release schedule
The Mercury project has a new 6 monthly release cycle. Releases are named according to the year and month of the release.The current release is 11.01 (April 2011).
Previously releases were numbered 0.12, 0.13, etc. and the period between stable releases was very large (3 years).
The Mercury project also makes a snapshot release daily. The snapshot release is the latest development snapshot of the system and has all the latest features and bug fixes added to the last stable release.
See also
- AliceAlice (programming language)Alice ML is a functional programming language designed by the at Saarland University. It is a dialect of Standard ML, augmented with support for lazy evaluation, concurrency and constraint programming.-Overview:Alice extends Standard ML in a number of ways that distinguish it from its predecessor...
programming language - LogtalkLogtalkLogtalk is an object-oriented logic programming language that extends the Prolog language with a feature set suitable for programming in the large. It provides support for encapsulation and data hiding, separation of concerns and enhanced code reuse...
- Oz/MozartOz (programming language)Oz is a multiparadigm programming language, developed in the Programming Systems Lab at Université catholique de Louvain, for programming language education. It has a canonical textbook: Concepts, Techniques, and Models of Computer Programming....
programming language and compiler - Visual PrologVisual PrologVisual Prolog, also formerly known as PDC Prolog and Turbo Prolog, is a strongly typed object-oriented extension of Prolog. As Turbo Prolog it was marketed by Borland, but it is now developed and marketed by the Danish firm Prolog Development Center that originally developed it...
programming language