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From: =?iso-8859-15?Q?Yannick_Duch=EAne_=28Hibou57=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Sat, 14 Aug 2010 15:33:07 +0200
Organization: Ada At Home
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Date: 2010-08-14T15:33:07+02:00
List-Id: <comp.lang.ada>

Le Sat, 14 Aug 2010 15:13:51 +0200, Ludovic Brenta  =

<ludovic@ludovic-brenta.org> a =E9crit:
>    type Access_T is access T;
>    type Internal_T (Atom : Boolean :=3D False) is record
>       case Atom is
>          when True  =3D> Value : Ada.Strings.Unbounded.Unbounded_Strin=
g;
>          when False =3D>
>             Car : Access_T;
>             Cdr : Access_T;
>       end case;

Just a detail : what about an enumerated type instead of a boolean ?


    type Item_Kinds is (An_Atom, A_List);

    type Internal_T (Kind : Item_KInds :=3D A_List) is record
       case Kind is
          when An_Atom  =3D> Value : Ada.Strings.Unbounded.Unbounded_Str=
ing;
          when A_List =3D>
             Car : Access_T;
             Cdr : Access_T;
       end case;

Perhaps more expressive (refer to things using their name instead of  =

predicates) and more resilient (just imagine you want to later have a  =

special handing of nil).

Personal note: I do not agree with the use of a Boolean every where ther=
e  =

is a two element enumeration. Boolean are made for Boolean arithmetic an=
d  =

to represent items of the Boolean set.

-- =

There is even better than a pragma Assert: a SPARK --# check.
--# check C and WhoKnowWhat and YouKnowWho;
--# assert Ada;
--  i.e. forget about previous premises which leads to conclusion
--  and start with new conclusion as premise.