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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Announce: Grace project site operational
References: <3CE15D0A.3050100@mail.com>
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Date: Wed, 15 May 2002 23:33:43 GMT
Xref: archiver1.google.com comp.lang.ada:24135
Date: 2002-05-15T23:33:43+00:00
List-Id: <comp.lang.ada>

> template<typename RepType, int Mass, int Distance, int Time>
> ...
  The attempt below is is not as nice as "type Velocity is new Float
dimension Distance/Time;" and it needs parenthesization, but it works in
Ada 95.  Each new unit requires a "type unit is <real type>" and each
legal pair needs two lines:  a generic instantiation and a "use".

generic
  type left is digits <>;
  type right is digits <>;
  type result is digits <>;
package multiply is
  function "*"(a : left; b : right) return result;
  function "*"(a : right; b : left) return result;
end multiply;
package body multiply is
  function "*"(a : left; b : right) return result is
  begin return result(float(a)*float(b));end "*";
  function "*"(a : right; b : left) return result is
  begin return result(float(a)*float(b));end "*";
end multiply;

generic
  type left is digits <>;
  type right is digits <>;
  type result is digits <>;
package ratio is
  function "/"(a : left; b : right) return result;
end ratio;
package body ratio is
  function "/"(a : left; b : right) return result is
  begin return result(float(a)/float(b));end "/";
end ratio;

with Multiply,Ratio;
procedure try_d is

  type Seconds is new Float;
  type Centimeters is new Float;
  type Grams is digits 4;

  type Velocity is new Float;
  package Centimeters_per_Second is new Ratio(Centimeters, Seconds, Velocity);
  use Centimeters_per_Second;

  type Acceleration is new Float;
  package Velocity_per_Second is new Ratio(Velocity, Seconds, Acceleration);
  use Velocity_per_Second;

  type Dynes is new Float;
  package Grams_Acceleration is new Multiply(Grams, Acceleration, Dynes);
  use Grams_Acceleration;

  t : Seconds;
  d : Centimeters range 0.0 .. 100.0;
  v : Velocity;
  m : Grams;
  a : Acceleration;
  f : Dynes;
  x,y,z : Float;

begin
  v := d/t;
  a := v/t;
  f := m*a;
  f := m*(v/t);
  x := float(d)*float(t);
  y := d*t;                    -- line 35, erroneous
  z := float(d*t);             -- line 36, erroneous
end try_d;

gnatchop/gnatmake gives:
gcc -c try_d.adb
try_d.adb:35:09: invalid operand types for operator "*"
try_d.adb:35:09: left operand has subtype of "centimeters" defined at line 22
try_d.adb:35:09: right operand has type "Seconds" defined at line 5
try_d.adb:36:15: invalid operand types for operator "*"
try_d.adb:36:15: left operand has subtype of "centimeters" defined at line 22
try_d.adb:36:15: right operand has type "Seconds" defined at line 5
gnatmake: "try_d.adb" compilation error