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Date: 10 Dec 92 22:57:32 GMT
From: aio!dnsurber@eos.arc.nasa.gov  (Douglas N. Surber)
Subject: Suggestion for multivalue boolean case for 9X
Message-ID: <dnsurber.724028252@node_26400>
List-Id: <comp.lang.ada>

How many times have you seen code like the following:

    if cond_1 then
        if cond_2
	    if cond_3 then
		s_t_t_t;
	    else
		s_t_t_f;
	    end if;
	else
	    if cond_3 then
		s_t_f_t;
	    else
		s_t_f_f;
	    end if;
	end if;
    else
        if cond_2
	    if cond_3 then
		s_f_t_t;
	    else
		s_f_t_f;
	    end if;
	else
	    if cond_3 then
		s_f_f_t;
	    else
		s_f_f_f;
	    end if;
	end if;
    end if;

I find this hard to read and even harder to maintain.  Cond_3 appears
four times.  That's a maintenance nightmare.  A case statement that
worked on arrays of booleans would be a big improvement.  Something like
the following:

    case (cond_1, cond_2, cond_3) is
	when (true,  true,  true ) =>
	    s_t_t_t;
	when (true,  true,  false) =>
	    s_t_t_f;
	when (true,  false, true ) =>
	    s_t_f_t;
	when (true,  false, false) =>
	    s_t_f_f;
	when (false, true,  true ) =>
	    s_f_t_t;
	when (false, true,  false) =>
	    s_f_t_f;
	when (false, false, true ) =>
	    s_f_f_t;
	when (false, false, false) =>
	    s_f_f_f;
    end case;

I don't know if it's too late to consider this for 9X, but I think it
would be neat.  The compiler could verify that you had covered all of the
cases and not listed any case twice, etc.  I suppose it could be generalized
to work on arrays and records of discrete types.  That might be even better.

Yes I know you could implement a function to turn an array of bools into
an enumeration type or an integer, but that is less clear. Anyway, just
an idea.

Douglas Surber
Lockheed
Houston, TX