Re: Beware: Rep spec on an enumeration type causes code explosion

[Ken Garlington <kennieg@nospam.flash.net>]

John G. Volan wrote:
> 
> Ken Garlington wrote:
> >
> > One of the nice things about "holey" representations in Ada, when used
> > for
> > interfacing to external systems, is the use of 'Valid to check for valid
> > input
> > data in a very readable manner. However, I would convert the raw input
> > to a non-holey type after the check, using either a map or case
> > statement
> > (depending upon the compiler).
> 
> There is no need for anything so elaborate.  Just derive a holey
> enumeration type from a contiguous enumeration type. Then you can simply
> use ordinary Ada type-conversions, and the compiler will implicitly do
> the mapping for you:
> 
>     with Ada.Text_IO;
>     procedure Holey_Enum_Demo is
> 
>       type Contiguous_Type is (A, B, C, D, E);
>       -- *no* rep spec!
> 
>       type Holey_Type is new Contiguous_Type;
>       for Holey_Type use (A => 1, B => 2, C => 4, D => 8, E => 16);
> 
>       Contiguous : Contiguous_Type := A;
>       Holey      : Holey_Type      := B;
> 
>     begin
> 
>       Contiguous := Contiguous_Type (Holey);
>       -- implicitly does equivalent of 'Pos
> 
>       Holey      := Holey_Type (Contiguous);
>       -- implicitly does equivalent of 'Val
> 
>     end Holey_Enum_Demo;

This assumes Holey will always be one of A, B, C, D, E. What happens
when the underlying
I/O device returns a value of 6 for Holey?

> 
> My feeling is that "holey" enumeration types (like other Chapter 13
> features) should really only be used at the "edges" of a program,
> wherever you're immediately reading from or writing to some external
> system that is expecting a certain data format.  Contiguous enumeration
> types should be used internally everywhere else in your program, for all
> the efficiency reasons that have been ... enumerated :-) ... in this
> thread. Luckily, Ada provides this convenient way of converting between
> holey and contiguous representations.
> 
> --
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