Q. What's a simple template for building a two-module mercury program? Module_1 defines and exports a simple function or predicate. Module_2 imports the function/predicate to compute a useful result and outputs the result.
I would use the following approach, first define the module with the function(s) or predicate(s) or predicates you want to export (interface section):
% File: gcd.m
:- module gcd.
:- interface.
:- import_module integer.
:- func gcd(integer, integer) = integer.
:- implementation.
:- pragma memo(gcd/2).
gcd(A, B) = (if B = integer(0) then A else gcd(B, A mod B)).
The file using the exported function in the gcd module (gcd/2):
% File: test_gcd.m
:- module test_gcd.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module char.
:- import_module gcd.
:- import_module integer.
:- import_module list.
:- import_module std_util.
:- import_module string.
main(!IO) :-
command_line_arguments(Args, !IO),
ArgToInteger = integer.det_from_string `compose` list.det_index0(Args),
A = ArgToInteger(0),
B = ArgToInteger(1),
Fmt = (func(Integer) = s(integer.to_string(Integer))),
GCD = gcd(A, B),
io.format("gcd(%s, %s) = %s\n", list.map(Fmt, [A, B, GCD]), !IO).
To compile and run on Windows (cmd.exe): Please note that mmc is also a Windows system command, so please use the Mercury environment provided by the Mercury distribution installer:
> mmc --use-grade-subdirs -m test_gcd
> test_gcd 12 4
To compile and run on Linux/MacOS/etc (any Bash-like shell):
$ mmc --use-grade-subdirs -m test_gcd
$ ./test_gcd 12 4