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Date: Sat, 27 Nov 2010 12:32:54 +0100
From: Georg Bauhaus <rm-host.bauhaus@maps.futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: Callback in Ada
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Date: 2010-11-27T12:32:56+01:00
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On 11/27/10 10:47 AM, Georg Maubach wrote:
> Hi All,
>
> today I learnt what a callback function is and also how it is used in
> Python.
...
> How would I do that in Ada?

You have a number of choices.

1. Generics, since Ada 83

generic
   with function ...  -- to be applied
procedure Apply_To_Each (Els : in out Some_Seq_Type);

generic
   with function ...
function Map_Concat (Els : in out Some_Seq_Type) return Some_Seq_Type;

2. Function pointers

type To_Be_Applied is access function ( ... );
(Or the Ada 2005 anonymous variant if you really need to
avoid names.)

function Map_Concat (Els: in out Some_Seq_Type;
   f : To_Be_Applied )  return Some_Seq_Type;

3. Do what Python does when an object implements the
__call__ interface:  A callable object in Python is a
callable object in Ada if the primitive operations of
that Ada object's type includes a __call__-like subprogram.
That is, one that has a profile similar to that
of Python's __call__.

   type Callable is interface;
   function Call (Me : access Callable; Arg : T) return T is abstract;
   -- or "in out Callable" in Ada 201X

   type My_Callable is ... and Callable;
   overriding
   function Call (Me : access My_Callable; Arg : T) return T;


A difference between Ada and Python is that in 1 and 2 above,
the function thing can be passed in only, whereas Python functions
can return functions and procedures to be assigned to, for
example, global variables.  I don't think this can be
fully achieved in Ada, because then a "function object" would
have a life time longer than its (local) type.

For a more involved example, see Chris Okasaki's
http://okasaki.blogspot.com/2008/07/functional-programming-inada.html

I vaguely remember a short outline on lambda-the-ultimate.org.