
We are pleased to announce the release of version 22.01 of the Mercury system.

Mercury is a modern general-purpose programming language, originally
designed and implemented by a small group of researchers at the University
of Melbourne, Australia.  Mercury is based on the paradigm of purely
declarative programming, and was designed to be useful for the
development of large and robust ``real-world'' applications.
It improves on existing logic programming languages by providing
increased productivity, reliability and efficiency, and by avoiding the
need for non-logical program constructs.  Mercury provides the
traditional logic programming syntax, but also allows the
syntactic convenience of user-defined functions, smoothly integrating
logic and functional programming into a single paradigm.

For a list of what's new in this release, see the NEWS file.

The main features of Mercury are:

     o  Mercury is purely declarative: predicates and functions
        in Mercury do not have non-logical side effects.

        Mercury does I/O through built-in and library predicates that
        take an old state of the world and some other parameters, and
        return a new state of the world and possibly some other
        results.  The language requires that the input argument
        representing the old state of the world be the last reference
        to the old state of the world, thus allowing the state of
        the world to be updated destructively.  The language also
        requires that I/O take place only in parts of the program where
        backtracking will not be needed.

        Mercury handles dynamic data structures not through Prolog's
        assert/retract but by providing several abstract data types in
        the standard Mercury library that manage collections of items
        with different operations and tradeoffs.

     o  Mercury is a strongly typed language.  Mercury's type system is
        based on many-sorted logic with parametric polymorphism, very
        similar to the type systems of modern functional languages such
        as ML and Haskell.  Programmers must declare the types they
        need using declarations such as

        :- type list(T) --->    [] ; [T | list(T)].
        :- type maybe(T) --->   yes(T) ; no.

        They must also declare the type signatures of the predicates and
        functions they define, for example

        :- pred append(list(T), list(T), list(T)).

        The compiler infers the types of all variables in the program.
        Type errors are reported at compile time.

     o  Mercury is a strongly moded language.  The programmer must
        declare the instantiation state of the arguments of predicates
        at the time of the call to the predicate and at the time of the
        success of the predicate.  Currently only a subset of the
        intended mode system is implemented.  This subset effectively
        requires arguments to be either fully input (ground at the time
        of call and at the time of success) or fully output (free at
        the time of call and ground at the time of success).

        A predicate may be usable in more than one mode.  For example,
        append is usually used in at least these two modes:

        :- mode append(in, in, out).
        :- mode append(out, out, in).

        If a predicate has only one mode, the mode information can be
        given in the predicate declaration.

        :- pred factorial(int::in, int::out).

        The compiler will infer the mode of each call, unification and
        other builtin in the program.  It will reorder the bodies of
        clauses as necessary to find a left to right execution order;
        if it cannot do so, it rejects the program.  Like type-checking,
        this means that a large class of errors are detected at
        compile time.

     o  Mercury has a strong determinism system.  For each mode of each
        predicate, the programmer should declare whether the predicate
        will succeed exactly once (det), at most once (semidet), at
        least once (multi) or an arbitrary number of times (nondet).
        These declarations are attached to mode declarations like
        this:

        :- mode append(in, in, out) is det.
        :- mode append(out, out, in) is multi.

        :- pred factorial(int::in, int::out) is det.

        The compiler will try to prove the programmer's determinism
        declaration using a simple, predictable set of rules that seems
        sufficient in practice (the problem in general is undecidable).
        If it cannot do so, it rejects the program.

        As with types and modes, determinism checking catches many
        program errors at compile time.  It is particularly useful if
        some deterministic (det) predicates each have a clause for each
        function symbol in the type of one of their input arguments,
        and this type changes; you will get determinism errors for all
        of these predicates, telling you to put in code to cover the
        case when the input argument is bound to the newly added
        function symbol.

     o  Mercury has a module system.  Programs consist of one or more
        modules.  Each module has an interface section that contains
        the declarations for the types, functions and predicates
        exported from the module, and an implementation section that
        contains the definitions of the exported entities and also
        definitions for types and predicates that are local to the
        module.  A type whose name is exported but whose definition is
        not, can be manipulated only by predicates in the defining
        module; this is how Mercury implements abstract data types.
        For predicates and functions that are not exported, Mercury
        supports automatic type, mode, and determinism inference.

     o  Mercury supports higher-order programming,
        with closures, currying, and lambda expressions.

     o  Mercury is very efficient (in comparison with existing logic
        programming languages).  Strong types, modes, and determinism
        provide the compiler with the information it needs to generate
        very efficient code.

The Mercury compiler is written in Mercury itself.  It was originally
bootstrapped using NU-Prolog and SICStus Prolog.  This was possible
because if you stick to an appropriate subset of Mercury, then
after stripping away the declarations of a Mercury program,
the syntax of the remaining part of the program is mostly compatible
with Prolog syntax.

The Mercury compiler compiles Mercury programs to C, which it uses as a
portable assembler.  The system can exploit some GNU C extensions to the C
language, if they are available: the ability to declare global register
variables, the ability to take the addresses of labels, and the ability to use
inline assembler.  Using these extensions, it generates code that is
significantly better than all previous Prolog systems known to us.  However,
the system does not need these extensions, and will work in their absence.

The Mercury compiler can also compile Mercury programs to Java, C# or
Erlang.  See README.Java, README.CSharp, and README.Erlang respectively
for further details.

The current Mercury system has been tested on Linux (x86, x86_64, arm),
macOS (x86_64), Windows 7 and Windows 10 (x86 and x86_64),
FreeBSD (x86_64), OpenBSD (x86_64) and AIX.

In the past Mercury has also been known to run on: Digital Unix,
OSF1, IRIX 5.x, Ultrix 4.3, BSDI 1.1, HPUX, Linux (Alpha),
Compaq Tru64 Unix (Alpha), Solaris 2.7 (SPARC), Solaris (9),
Mac OS X (PowerPC) and Windows 95, 98, ME, NT, 2000 and XP.

It should run without too many changes on other Unix variants as well.
If you do encounter any problems, please report them to us via the
Mercury bug tracking system at <http://bugs.mercurylang.org>
(see the BUGS file).

We recommend that you use gcc as the C compiler, preferably gcc version 3.4 or
later.  Do not use gcc versions 2.96, 3.0 or 4.0; those versions have bugs that
cause trouble for Mercury.  If you are using gcc, you will need at least
version 2.95 or higher, except on Mac OS X where you will need version 3.3 or
higher.  Visual C++ or clang may also be used as a C compiler.
(See README.MS-VisualC and README.clang for further details.)  
You will also need GNU make version 3.69 or higher.

The Mercury distribution contains:
     o  an autoconfiguration script
     o  the Mercury source for the compiler
     o  the Mercury source for the standard library
     o  the automatically generated C source for the compiler
        and the standard library
     o  the runtime system (written in C)
     o  Hans Boehm's conservative garbage collector for C
     o  an integrated procedural and declarative debugger
     o  some profilers
     o  some utility programs, including a make front-end for Mercury
        with automatic dependency recomputation
     o  the Mercury language reference manual
     o  the Mercury library reference manual
     o  the Mercury user's guide
     o  the Mercury frequently asked questions list
     o  the Prolog to Mercury transition guide
     o  some sample Mercury programs
     o  dynamic linking
     o  backtrackable (trailed) destructive update
     o  arithmetic
        -  arithmetic on complex and imaginary numbers
        -  arithmetic on fixed-point numbers
     o  UIs:
        -  graphics using Tk, OpenGL, GLUT, GLFW, Xlib, Allegro or Cairo.
        -  text interfaces using curses
        -  processing HTML forms using the CGI interface
     o  interfacing:
        -  XML parsing
        -  POSIX interface
        -  an ODBC database interface
     o  the Morphine trace analysis system
     o  a general purpose lexer
     o  Moose, a parser generator for Mercury.

The Mercury distribution is available via WWW from the following locations:

        Australia:
                <http://dl.mercurylang.org/index.html>

The home page of the project on the Web is:

    <http://www.mercurylang.org>.

--
The Mercury Team <http://www.mercurylang.org/development/people.html>
