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Date: 13 Sep 93 14:24:24 GMT
From: agate!howland.reston.ans.net!wupost!cs.utexas.edu!csc.ti.com!tilde.csc.ti
.com!mksol!strohm@ucbvax.Berkeley.EDU  (john r strohm)
Subject: Re: thoughts on "holey" enumerated types
Message-ID: <1993Sep13.142424.18799@mksol.dseg.ti.com>
List-Id: <comp.lang.ada>

In article <1993Sep12.214524.13370@seas.gwu.edu> mfeldman@seas.gwu.edu (Michael
 Feldman) writes:
>The classic example is a device that requires
>exactly one "1" bit out of, say, 4, to represent a command, say,
>(Up, Down, Left, Right). One would then define
>
>  TYPE Commands IS (Up, Down, Left, Right);
>  FOR Commands USE (Up=>1, Down=>2, Left=>4, Right=>8);
>
>The device byte would somehow be "read" (memory-mapped IO, whatever).
>and no conversion would be necessary at all.
>
>More interesting is to allow _two_ mappings:
>
>  TYPE Commands IS (Up, Down, Left, Right); -- default used within program
>  --                 0     1     2      3   (probably, but we don't care)
>
>  TYPE ExtCommands IS NEW Commands;         -- type derivation
>  FOR  ExtCommands USE (Up=>1, Down=>2, Left=>4, Right=>8);
>
>Now: how do we get from one to the other? This is simpler than many realize:
>suppose we have
>
>  C: Commands;
>  E: ExtCommands;
>  . . .
>  C := Commands(E):
>  E := ExtCommands(C);
>
>in other words, a straightforward explicit conversion, does the trick.
>This is suggested in the Ada Rationale. I have checked it on perhaps
>a dozen compilers and found that it works as I have just suggested.

The catch is that the conversion from ExtCommands (1-of-4) to Commands
involves a table search operation, and there is no general way around it.
(This particular case COULD be handled by a really smart compiler
recognizing the 1-of-4 encoding.  Consider a spec that used 2,3,5,7.)