Re: Eiffel and Java + Ada dispatching

[donh@syd.csa.com.au (Don Harrison)]

Vincent WEBER writes:

:In article <E00t53.CG2@syd.csa.com.au>, donh@syd.csa.com.au says...
:
:>
:>Don't mistake Ada's symmetrical syntax for multiple dispatching (binding
:>driven by more than one parameter). Ada is still single dispatched so for
:>dispatching operations, the symmetry is syntactic rather than semantic.  
:>
:
:Hum... I'm lost, two ada specialists saying two different things :) 

Sorry, I don't qualify as an Ada specialist (and comp.lang.ada will back me
up here). :)  However, ...

:Let me cote 
:an article from the french magazine "L'Objet" (Vol 1, no 1, Spring 95), by 
:Antoine Bertier (Tomson Software Products) and Pascal Plisson (Alsys). I tried 
:to translate from french to english, please apologize the mistakes if any :)

Don't know how the French was, but the English is pretty good. :)

:"It is important to note than tags of all the parameters have to be identical 
:at run-time to have the dispatching done to only one effective operation. 

Notice he's (you're) saying "only one effective effective operation" 
(single dispatching). What I didn't mention is that in Ada, control over
dispatching can be *shared* by multiple parameters, but for a call to be valid
the actual dispatching parameters must have the same type ensuring that
binding to a unique procedure occurs. In the example below, type TILABLE_WINDOW 
is presumably tagged so both parameters A and B jointly control dispatching. 
However, MAIN_WINDOW and MESSAGE_WINDOW must have the same type; otherwise, an exception is raised).

It almost appears that having B as a dispatching parameter is redundant 
but it conveys the extra information that it must have the same type as A.

:Therefore there is no priviliged parameter for dispatching as it is the case in 
:other languages. This is an important advantage of Ada 95 's model compared to 
:other languages such as C++, that break the natural symetry of some operations 
:in privileging the parameter used for the dispatching ( et need a way to 
:reference this parameter : "this" in C++). For instance :
:  
:  type TILABLE_WINDOW is new WINDOW;
:  TILE(A,B : TILABLE_WINDOW);
:    -- display the two window one under the other
:    -- in covering all the screen
:  MAIN_WINDOW, MESSAGE_WINDOW : TILABLE_WINDOW;
:  TILE(MAIN_WINDOW, MESSAGE_WINDOW);
:    -- the call is perfectly symetric. the parameters can be swapped
:    -- in C++ we would have had a dissymetric call like:
:    -- MAIN_WINDOW.TILE(MESSAGE_WINDOW);
:
:  " (end of quotation).

A symmetrical solution in Eiffel which also conveys the fact that the windows
must have the same type might be:

class TILABLE_WINDOW is ...
class TILABLE_WINDOW_OPS is
  ...
  tile (a, b: TILABLE_WINDOW) is 
  require same_type (a, b)
  do ... end
  ...
end

and somewhere:

  main_window, message_window : TILABLE_WINDOW
  window_ops: TILABLE_WINDOW_OPS
  ...
  window_ops.tile (main_window, message_window)

We've had to invent another class to get symmetry. Oh, well at least it's 
obvious we are using single-dispatching.


Don.
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Don Harrison             donh@syd.csa.com.au