[mercury-users] Mercury programmer required ;-)

[Warwick HARVEY]

Tyson writes:
> The people who have the "99 bottles of beer on the wall" program
> in many different languages have asked for a version of the
> famous Rot-13 algorithm in Mercury.

Hmmm, does it have to be clean?  And in true Mercury style?  Or anything
like that?

Given that it's for all the world to see, I'd hate to have a poor example of
Mercury programming get posted.  So in the spirit of mercury-devel, I'm
posting my program here and requesting that somebody review it.  :-)

Warwick

/*
** rot13:
**
** Program to read its input, apply the rot13 algorithm, and write it out
** again.
*/

:- module rot13.

:- interface.

:- import_module io.

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

:- implementation.

:- import_module char, int, require.

	% rot13a/2
	% A table to map the alphabetic characters to their rot13 equivalents
	% (fails if the input is not alphabetic).
:- pred rot13a(char, char).
:- mode rot13a(in, out) is semidet.

rot13a('a', 'n').
rot13a('b', 'o').
rot13a('c', 'p').
rot13a('d', 'q').
rot13a('e', 'r').
rot13a('f', 's').
rot13a('g', 't').
rot13a('h', 'u').
rot13a('i', 'v').
rot13a('j', 'w').
rot13a('k', 'x').
rot13a('l', 'y').
rot13a('m', 'z').
rot13a('n', 'a').
rot13a('o', 'b').
rot13a('p', 'c').
rot13a('q', 'd').
rot13a('r', 'e').
rot13a('s', 'f').
rot13a('t', 'g').
rot13a('u', 'h').
rot13a('v', 'i').
rot13a('w', 'j').
rot13a('x', 'k').
rot13a('y', 'l').
rot13a('z', 'm').
rot13a('A', 'N').
rot13a('B', 'O').
rot13a('C', 'P').
rot13a('D', 'Q').
rot13a('E', 'R').
rot13a('F', 'S').
rot13a('G', 'T').
rot13a('H', 'U').
rot13a('I', 'V').
rot13a('J', 'W').
rot13a('K', 'X').
rot13a('L', 'Y').
rot13a('M', 'Z').
rot13a('N', 'A').
rot13a('O', 'B').
rot13a('P', 'C').
rot13a('Q', 'D').
rot13a('R', 'E').
rot13a('S', 'F').
rot13a('T', 'G').
rot13a('U', 'H').
rot13a('V', 'I').
rot13a('W', 'J').
rot13a('X', 'K').
rot13a('Y', 'L').
rot13a('Z', 'M').

	% rot13/2
	% Applies the rot13 algorithm to a character.
:- pred rot13(char, char).
:- mode rot13(in, out) is det.

rot13(Char, RotChar) :-
	( if rot13a(Char, TmpChar) then
		RotChar = TmpChar
	else
		RotChar = Char
	).

main -->
	io__read_char(Res),
	( { Res = ok(Char) },
		{ rot13(Char, RotChar) },
		io__write_char(RotChar),
		main
	; { Res = eof }
	; { Res = error(ErrorCode) },
		{ io__error_message(ErrorCode, ErrorMessage) },
		io__write_string("rot13: error reading input: "),
		io__write_string(ErrorMessage),
		nl
	).