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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Expressive Case Statements (was: Software landmines)
Date: 1998/09/01
Message-ID: <EACHUS.98Sep1193718@spectre.mitre.org>#1/1
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Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-09-01T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <6shit4$eaj@dfw-ixnews5.ix.netcom.com> Richard D Riehle <laoXhai@ix.netcom.com> writes:

 > It is certainly easy to represent any set of conditions with a sequence
 > of if ... elsif statements but I am seeking something else.  What I have
 > been trying to accomplish is some generalized algorithm in Ada that 
 > allows me to design a package that approximates the power of the EVALUATE
 > statement in COBOL.  Some of the new features of Ada 95 such as generic
 > formal package parameters, access to subprogram, etc. have helped a
 > little, but ...

     type Truth_Table_3 is (False_False_False, False_False_True,...
                                 True_True_True); 
     function To_TT3(Cond_1,Cond_2,Cond_3 : Boolean := False)
                                            return Truth_Table_3;

     case To_TT3(Condition_1, Condition_2, Condition_3) is
       when True_True_False  => some_action_stub_statements;
       when True_False_False => some_action_stub statements2;
       when False_False_True => some_action_stub statements3;
       when others => null;
     end case;

     Obviously you want the various truth table types in a package,
and you have several ways of writing the bodies of the conversion
functions.  I prefer:

     function To_TT3(Cond_1,Cond_2,Cond_3 : Boolean := False) 
                                        return Truth_Table_3 is
     begin
       return Truth_Table_3'Val(Cond_1'Pos * 4 + Cond_2'Pos * 2 + Cond_3);
     end To_TT3;

     I think I'd write a program to crank out the type declarations
though.
--

					Robert I. Eachus

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