Mercury (programming language)

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Mercury
Image:Mercury logo.jpg
Paradigm Logic, functional
Appeared in 1995
Designed by Zoltán Somogyi
Developer University of Melbourne
Latest release 0.13.1/ 1 December 2006
Latest unstable release Release of the day (daily)
Typing discipline Strong, static, polymorphic
Major implementations Melbourne Mercury Compiler
Influenced by Prolog, Haskell
OS Cross-platform
License GPL
Website http://www.cs.mu.oz.au/research/mercury/
This article is about the functional logic programming language. For other software with the same name, see Mercury.

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 Zoltán Somogyi. The first version was developed by Fergus Henderson, Thomas Conway and Zoltán Somogyi and was released on April 8, 1995.

Mercury has several features intended for better software engineering. It is compiled rather than interpreted, as is traditional for logic programming languages. It features a sophisticated, strict type and mode system. Its authors claim these features combined with logic programming's abstract nature speeds writing of reliable programs. 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.)

Mercury is a more declarative language than Prolog, since it lacks "extra-logical" Prolog statements such as "cut" (which prevents backtracking) and imperative I/O. This enables better program optimization, but makes coding sequential algorithms harder.

Mercury has a strong emphasis on purity. While it allows impure functionality, this must be explicitly marked, and at the top level all functionality must either be pure or the programmer must promise that it is pure. Often impure operations can be made pure by threading the I/O state through them, as is done in the Mercury standard library. Due to the optimizations enabled by the purity of the language, programs written in Mercury typically perform significantly faster than equivalent programs written in Prolog ([1], [2]).

Mercury is available for most Unix platforms, including Mac OS X, and Microsoft Windows (in Windows, it requires one of the Cygwin or MinGW toolsets, and can be compiled either with gcc or Microsoft Visual C++).

Notable programs written in Mercury include the Mercury compiler itself and the Prince XML formatter.

Contents

[edit] 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

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

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
IL IL or C#
Java Java

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 the only back-ends considered production quality, this means that you will not lose a great deal of portability using foreign-language C code.

[edit] 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:

 :- 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).

(adapted from Ralph Becket's Mercury tutorial).

[edit] See also

[edit] External links