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From: "Alex Mentis" <foo@invalid.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Callback in Ada
Date: Sat, 27 Nov 2010 10:26:42 +0000 (UTC)
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Date: 2010-11-27T10:26:42+00:00
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Georg Maubach wrote:

> Hi All,
> 
> today I learnt what a callback function is and also how it is used in 
> Python. I have tested the following code which runs in Python:
> 
> def apply_to (function, valueList):
>    resultList = []
> 
>    if __debug__: print "Calling function ", function
>    
>    for value in valueList:
>       if __debug__: print "value: ", value
>       r = function (value)
>       if __debug__: print "result: ", r
>       resultList.append(r)
> 
>    return resultList
> 
> def dublicate (value):
>    return (value * 2)
> 
> def power (value):
>    return (value * value)
> 
> valueList = [1, 2, 3, 4, 5]
> 
> double_values = apply_to(dublicate, valueList)
> power_values = apply_to(power, valueList)
> 
> print "Values doubled: ", double_values
> print "Values powered: ", power_values
> 
> Reference: http://en.wikipedia.org/wiki/Callback_(computer_science)
> 
> If put into a file called "callback-test-in-python.py" and called
> python -O callback-test-in-python.py it will deliver the output
> 
> Values doubled: 2, 4, 6, 8, 10
> Values powered: 1, 4, 9, 16, 25
> 
> How would I do that in Ada?
> 
> Regards
> 
> George

Basically the same idea as Dimitry, except I use functions so the code
works more similarly to your Python:

with Ada.Text_IO, Ada.Integer_Text_IO;
use  Ada.Text_IO, Ada.Integer_Text_IO;

procedure Callback is
   
   type Int_Array is array (1..5) of Integer;
   
   function Apply_To 
     (Func       : not null access function (Value : Integer) return
Integer;
      Value_List : Int_Array) return Int_Array is            
      
      Result : Int_Array;
      
   begin -- Apply_To
      for Value in Value_List'Range loop
         Result (Value) := Func (Value_List(Value));
      end loop;
      return Result;
   end Apply_To;       
   
   function  Dublicate (Value : Integer) return Integer is
   begin -- Dublicate
      return Value * 2;
   end Dublicate;
   
   Initial : Int_Array := (1, 2, 3, 4, 5);
   Result : Int_Array;
   
begin
   
   Result := Apply_To (Dublicate'Access, Initial);
   
   Put("Values doubled: ");
   for Value in Result'Range loop
      Put (Result (Value), 0);
      if Value /= Result'Last then
         Put (", ");
      end if;
   end loop;
   
end Callback;