Mercury is a new, purely declarative logic programming language. Like
Prolog and other existing logic programming languages, it is a very
high-level language that allows programmers to concentrate on the
problem rather than the low-level details such as memory management.
Unlike Prolog, which is oriented towards exploratory programming,
Mercury is designed for the construction of large, reliable, efficient
software systems by teams of programmers. As a consequence, programming
in Mercury has a different flavour than programming in Prolog.
The main features of Mercury are:
- 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.
- 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 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.
- 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
- 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
:- 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
- 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.
- Mercury supports higher-order programming,
with closures, currying, and lambda expressions.
- 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 bootstrapped
using NU-Prolog and SICStus Prolog. This was possible because 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.