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Date: 17 Apr 93 19:54:14 GMT
From: seas.gwu.edu!mfeldman@uunet.uu.net  (Michael Feldman)
Subject: Re: Constants of type Calendar.Time (?)
Message-ID: <1993Apr17.195414.7527@seas.gwu.edu>
List-Id: <comp.lang.ada>

In article <WOODRUFF.93Apr15124008@EC2226.addvax.llnl.gov> WOODRUFF@EC2226.addv
ax.llnl.gov writes:
>Suppose I am using calendar.time as a field in a database, and I wish 
>to provide a procedure for retrieving some elements from the database
>keyed by time.  I might provide a function
>
>FUNCTION Get (Starting_At  : IN Calendar.Time := Prehistory)  
>              RETURN Array_Of_Results ;
>
>Or, I might want to make a function that returns the MOST RECENT data:
>
>FUNCTION Get (Up_Until  : IN Calendar.Time  := Now)
>              RETURN Array_Of_Results ;
>
>Can someone suggest how I can construct a constant of the type Time which
>I could recognize in the BODY of Get, and resolve my data search at run time 
>with a value that I obtain from Calendar.Clock when the search begins?
>
>>>  Just to clarify my wish, I would do the following except for the blatant
>    illegality of the type mismatch:  
>    FUNCTION Get (Up_Until  : IN Calendar.Time  := 0)  RETURN Array_Of_Results
 ;
>
All your examples look fine to me; they are all default parameter values.
The only problem is, as you pointed out, the type mismatch in the last one.

I suggest defining a named constant whose value is the beginning of Time, then
using it wherever needed:

   BeginningOfTime: CONSTANT Calendar.Time :=
     Calendar.Time_Of(Month => 1, Day => 1, Year => 1901, Seconds => 0.0);

Since Seconds will default to 0.0, its presence here is just for clarity, as
is the named association, which you can omit if you choose.

I think the point you were after was how to make a constant of Time value;
becauswe Time is private you must use the constructor Time_Of or the
current-time function Clock, or another Time-returning Calendar operation.

Cheers -

Mike Feldman
------------------------------------------------------------------------
Michael B. Feldman
co-chair, SIGAda Education Committee

Professor, Dept. of Electrical Engineering and Computer Science
School of Engineering and Applied Science
The George Washington University
Washington, DC 20052 USA
(202) 994-5253 (voice)
(202) 994-5296 (fax)
mfeldman@seas.gwu.edu (Internet)

"The most important thing is to be sincere, 
and once you've learned how to fake that, you've got it made." 
-- old show-business adage
------------------------------------------------------------------------