Re: newbie can't get exceptions to work!

["Jean-Pierre Rosen" <rosen@adalog.fr>]

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"Mats Karlssohn" <mats@mida.se> a �crit dans le message news:
3ADBEAE0.7D69463E@mida.se...
> [...], I've always (almost anyway)
> wondered why the attributes were designed to have the syntax that they
> have today. Not that I find them that annoying, but in half of their
> use the attributes behaves just like functions, the other half (when
> setting object'size := 14; for example) is another matter.
>
> Do someone remember the history behind this ?
>
Here is how I explain it to my students.
When you declare a type, the compiler defines some constants and functions
that can be helpful. Now the problem is that the compiler must find names
for these constants and functions, and those names must not conflict with
names defined by the user. The trick is that the compiler allows itself to
put an apostrophe in the name, something that is not allowed to regular
identifiers. Therefore, there can be no conflict. In short, when you define:
   type My_Int is range 0..100;
the compiler adds the declarations:
   My_Int'First : constant My_Int := 0;
   My_Int'Last : constant My_Int := 100;
   function My_Int'Image (Item : My_Int) return String;
   etc.

So, if you see attributes not as special constructs, but simply as
identifiers with a special character in them, they are not different from
other language constructs, and allowing the user to define them would defeat
their purpose. Actually, I find it quite nice that their is no "magic"
identifier recognized in a special way by the compiler, like sizeof() in C
for example.
With attributes, you know without ambiguity what is defined by the compiler
and what is defined by the user.

(David: if you want to put this in the FAQ, you are welcome)

--
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           J-P. Rosen (rosen@adalog.fr)
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