[mercury-users] Mercury programmer required ;-)

[Gustavo A. Ospina]

I have another version of rot13.

This version reads a line and prints the line "roted". I think it is as
declarative as Jurgen's version. Also I handle error and I used predicates
on your char library. Maybe my version is slower, but it can be discussed.

I'm in agree with these programming challenges. I hope these will be
presented more often.

All you have a nice day.

+ Gustavo.

	+---------------------------------------+
	|  Gustavo A. Ospina                    |
	|  MSc Student                          |
	|  Computing and Systems Engineering    |
	|  University of Los Andes              |
	|  Santafe de Bogota D.C., Colombia     |
	|  e-mail: g-ospina at uniandes.edu.co     |
	+---------------------------------------+

-------------------------------- Cut Here --------------------------------

:- module rot13.

:- interface.

:- import_module io.

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

:- implementation.

:- import_module char,int,list.

:- pred rot13(char::in,char::out) is det.

rot13(Char,RotChar) :-
    char__is_upper(Char) ->
        rot13(Char,0'A,RotChar)
    ;
    char__is_lower(Char) ->
        rot13(Char,0'a,RotChar)
    ;
    RotChar = Char.

:- pred rot13(char::in,int::in,char::out) is det.

rot13(Char,CodeLetterA,RotChar) :-
    char__to_int(Char,CodeChar),
    RotCode = (CodeChar - CodeLetterA + 13) mod 26 + CodeLetterA,
    char__to_int(RChar,RotCode) ->
        RotChar = RChar
    ;
        RotChar = '\a'.   
        /* Alert character (Error case. To satisfy mode check) */

:- pred printRotChars(list(char)::in,io__state::di,io__state::uo) is det.

printRotChars([]) -->
    io__nl.

printRotChars([Ch|Chs]) -->
    {rot13(Ch,RotCh)},
    io__write_char(RotCh),
    printRotChars(Chs).

% Main Program

main -->
    io__read_line(Result),
    (
        {Result = ok(Line)},
        printRotChars(Line)
    ;
        {Result = eof,
        true}
    ;
        {Result = error(Error),
        io__error_message(Error,Message)},
        io__stderr_stream(Stderr),
        io__write_string(Stderr,Message)
    ).