Mercury (programming language)

Mercury

Paradigm	Logic, functional
Designed by	Zoltán Somogyi
Developer	University of Melbourne
First appeared	1995
Stable release	10.04.1 / 30 August 2010
Typing discipline	Strong, static, polymorphic
OS	Cross-platform (Unix, Mac OS X, Windows)
License	GPL for compiler, LGPL for standard library
Website	http://www.mercury.csse.unimelb.edu.au/
Major implementations
Melbourne Mercury Compiler
Influenced by
Prolog, Haskell

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++).

Overview

Mercury is based on the logic programming language Prolog. 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. (Modern versions of Prolog normally support compilation as well as providing an interpreter. An executable produced by a modern Prolog compiler can also be very efficient.) 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. (But note that several Prolog implementations now also support modules.)

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[1], a software company, is also 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 C for GCC (the original Mercury back-end)
• High-level C
• Java bytecode for the JVM

Beta quality:

• Erlang

Alpha quality (may not work well, or even be completely broken):

• IL for Microsoft's .NET
• Assembler via the GCC back-end

Past back-ends:

• Aditi, a deductive database system also developed at the University of Melbourne. 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
Java	Java
Erlang	Erlang
IL	IL 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 World:

 :- 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 (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 10.04 (April 2010) and the next planned release is 10.10 (October 2010). 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

• Alice programming language
• Logtalk
• Oz/Mozart programming language and compiler
• Visual Prolog programming language

External links

• Official Mercury Homepage
• Literate Programs (examples) in Mercury
• Papers And Presentations About Mercury