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From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
Organization: jrcarter at acm dot org
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Subject: "Use" and "=" for Tags (was: Re: Instantiating private types with
 discriminants?)
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> 
>    --  problem line:
>    if Var_A'Tag = Type_A'Tag then
>       Ada.Integer_Text_IO.Put ( Type_A (Var_A.all).Data);
>    elsif Var_A'Tag = Type_B'Tag then
>       Ada.Float_Text_IO.Put ( Type_B (Var_A.all).Data);
>    end if;

You've received some specific solutions, but no general discussion of 
why this doesn't work. The function "=" you're trying to call is defined 
in Ada.Tags, and not directly visible here. You can make it directly 
visible with a "use" clause for Ada.Tags, as you've already discovered, 
or with a "use type" clause for Ada.Tags.Tag. You can use prefix 
notation to access it [Ada.Tags."=" (...)]. The subtype membership 
operation {"in"} is not a subprogram and is always visible, like 
assignment (":="). Finally, you have the Ada-83 solution to avoiding 
"use", renaming:

function "=" (Left : Ada.Tags.Tag; Right : Ada.Tags.Tag) return Boolean
renames Ada.Tags."=";
...
if This_Tag = That_Tag then

The operators for the predefined types, being in Standard, are always 
directly visible, so you don't encounter this problem with them. For the 
most part, though, well designed Ada deals with user-defined types, so 
understanding this concept is useful.

-- 
Jeff Carter
"Apart from the sanitation, the medicine, education, wine,
public order, irrigation, roads, the fresh water system,
and public health, what have the Romans ever done for us?"
Monty Python's Life of Brian
80