Speed/Time Example

:- module main.
:- interface.
:- use_module io.

:- pred main(io.io::di, io.io::uo) is det.

:- implementation.
:- use_module string.
:- use_module exception.
:- import_module int.
:- import_module list.

:- pred get_int(string::in, int::out, io.io::di, io.io::uo) is det.

get_int(Name, Out, !IO) :- 
    io.write_string("Please enter ", !IO),
    io.write_string(Name, !IO),
    io.nl(!IO),
    io.read_line_as_string(LineResult, !IO),
    (
        ( LineResult = io.error(_) ; LineResult = io.eof ),
        exception.throw(exception.software_error("IO Error"))
    ;
        LineResult = io.ok(Line),
        ( if
            string.to_int(string.strip(Line), Int)
        then
            Out = Int
        else
            io.write_string("Not an integer.\n", !IO),
            get_int(Name, Out, !IO)
        )
    ).

:- pred print_speed(int::in, int::in, io.io::di, io.io::uo) is det.

print_speed(Speed, Hour) -->
    io.write_string("Hours: "),
    io.write_int(Hour), io.write_char(('\t')),
    io.write_string("Distance: "),
    io.write_int(Distance), io.nl,
    { Distance = Hour * Speed }.

main(!IO) :-
    get_int("speed", Speed, !IO),
    get_int("hours", Hours, !IO),
    list.foldl(print_speed(Speed), 0 .. Hours, !IO).