Re: Relational operations on discrete types / visibility

[pa.dec.com!nntpd2.cxo.dec.com!bonmot!wallace@decwrl.dec.com (Richard Wal]

Frank.Prindle@bbs.oit.unc.edu (Frank Prindle) writes:
: In the following simple example, the second if statement fails because the
: "=" is not visible, but the semantically equivalent case statement compiles
: without error (yet still implicitly requires the "=" or some combination of
: relational operations on the discrete type color_type).  
: 
: Since the compiler can generate code for the case statement, it can clearly
: generate code for the second if statement (i.e. there is no ambiguity to
: resolve).  So why the inconsistency? (personally, I don't think either
: statement should fail, but if one must, both should).  The compiler is
: VADS 6.0.3.
: 
: Sincerely,
: Frank Prindle
: Prindle@NADC.navy.mil
: 
: -------------------------------example follows--------------------------
: package color_pkg is
:         type color_type is (RED, GREEN);
: end color_pkg;
: 
: with color_pkg;
: with text_io;
: procedure color_test is
:         color:color_pkg.color_type:=color_pkg.GREEN;
: begin
:         if(color_pkg."="(color,color_pkg.GREEN))
:         then
:                 text_io.put_line("GREEN");
:         end if;
: 
:         if(color = color_pkg.GREEN)			--ERROR HERE (EXPECTED)
:         then
:                 text_io.put_line("GREEN");
:         end if;
: 
:         case color is					--WHY NO ERROR HERE?
:                 when color_pkg.GREEN=>text_io.put_line("GREEN");
:                 when others=>null;
:         end case;
: end color_test;
: --
:    The opinions expressed are not necessarily those of the University of
:      North Carolina at Chapel Hill, the Campus Office for Information
:         Technology, or the Experimental Bulletin Board Service.
:            internet:  bbs.oit.unc.edu or 152.2.22.80


Frank,

	The problem is that the function, as you have in your code above,
is equality "=" on the types.  The case statement evaluates and expression
based on the type not on a function of the type of the expression.

Hope this helps!

Richard Wallace
Digital Equipment Corporation
301 Rockrimmon Blvd. South
CXO2-1/7A
Colorado Springs, CO 80919-2398
(719)548-2792
<wallace@cookie.enet.dec.com>

	"The opinions expressed are my own, Unlce Ken or Uncle Bob
	 may, or may not, agree with me.