Re: Char type verification

[Simon Wright <simon@pushface.org>]

Yves Bailly <kafka.fr@laposte.net> writes:

> First create a generic package:
> --8<-----8<-----8<-----8<-----8<-----8<-----8<---
> generic
>    type T is private;
>    type H is range <>;
>    with function Hash(val: in T) return H;
> package P is
>    function Is_In_Range(lower: in T;
>                         upper: in T;
>                         value: in T)
>       return Boolean;
> end P;
> --8<-----8<-----8<-----8<-----8<-----8<-----8<---
>
> The body performs the actual check of validity:
> --8<-----8<-----8<-----8<-----8<-----8<-----8<---
> package body P is
>
>    function Is_In_Range(lower: in T;
>                         upper: in T;
>                         value: in T)
>       return Boolean is
>    begin
>       return Hash(value) in Hash(lower)..Hash(upper);
>    end Is_In_Range;
>
> end P;
> --8<-----8<-----8<-----8<-----8<-----8<-----8<---

I would have thought it would be a lot clearer if you required "<" say
instead of Hash. To me a hash is typically a pseudo-random conversion
of a private value to an integral type; if you want ordering, use
functions with ordering-related names!

Then you could say

   return not (upper < value) and then not (value < lower);

(I think! that's the third time I've rewritten that ...)

By the way, did you know you can have (library-level, even) generic
subprograms?

generic
   type T is private;
   type H is range <>;
   with function Hash(val: in T) return H;
function Is_In_Range(lower: in T;
                     upper: in T;
                     value: in T)
   return Boolean;