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Newsgroups: comp.lang.ada
Date: Mon, 12 Aug 2019 06:44:41 -0700 (PDT)
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Subject: Re: What is the difference?
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:57039
Date: 2019-08-12T06:44:41-07:00
List-Id: <comp.lang.ada>

> What is here the meaning of: is <>?

That is a default; if there is a matching subprogram (name, parameters) then it can use that without being specified. Example:

Generic
  Type Element(<>) is private;
  Unity : Element;
  with Function "*"(Left, Right : Element) return Element is <>;
Function Generic_Exponent( Left : Element; Right : Natural ) return Element;
--...
Function Generic_Exponent( Left : Element; Right : Natural ) return Element is
(case Right is
  when 0 => Unity,
  when 1 => Left,
  when 2 => Left * Left,
  when others =>
   (if Right mod 2 = 0 
    then  (Left ** (Right/2) ** 2
    else ((Left ** (Right/2) ** 2) * Left
   )
);

Now you can say something like
Procedure Test is
  Type X is 1..8;
  Function Exp is new Generic_Exponent(X, 1, others => <>);
  Function "**"(Left : X; Right : Natural) Return X renames Exp;
Begin
  --Testing procedures
End Test;