Editing Mercury (programming language) (section)

== Examples ==
[[Hello world program|Hello World]]:
<syntaxhighlight lang="prolog">
 :- module hello.
 :- interface.
 :- import_module io.
 :- pred main(io::di, io::uo) is det.

 :- implementation.
 main(!IO) :-
 	io.write_string("Hello, World!\n", !IO).
</syntaxhighlight>

Calculating the 10th [[Fibonacci number]] (in the most obvious way):<ref name="tutorial">Adapted from [http://www.mercurylang.org/documentation/papers/book.pdf Ralph Becket's Mercury tutorial]</ref>
<syntaxhighlight lang="prolog">
 :- 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).
</syntaxhighlight>
<code>!IO</code> is a "state variable", which is [[syntactic sugar]] for a pair of variables which are assigned concrete names at compilation; for example, the above is desugared to something like:
<syntaxhighlight lang="prolog">
 main(IO0, IO) :-
        io.write_string("fib(10) = ", IO0, IO1),
        io.write_int(fib(10), IO1, IO2),
        io.nl(IO2, IO).
</syntaxhighlight>