Re: Ada.Real_Time.Time_Last

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-11-15 16:28 , Simon Wright wrote:
> The only use case I can see for this is to allow us to write
>
>    delay until Ada.Real_Time.Time_Last;
>
> at the end of our (Ravenscar) main program. Any other offers?

Suppose your program has to consider, say, three possible future events, 
at the future times A, B, C : Ada.Real_Time.Time, and must delay until 
the first of those events happens, so you compute a variable Next_Event 
: Ada.Real_Time.Time as the minimum of A, B, and C, and then do "delay 
until Next_Time".

However, if some or all of the events A, B, C might never happen, the 
value Time_Last is a useful placeholder to indicate "this event will 
never happen", because Time_Last is a "neutral element" for the 
operation "minimum of two Time values": minimum (X, Time_Last) = X for 
any Time X.

Many algorithms working with ordered types need a minimal and a maximal 
value of the type, for such uses.

(By the way, my Ravenscar programs always end in an eternal loop, never 
in a delay until Time_Last.)

(By the further way, it is a pity that Ada does not let Ada.Real_Time 
provide the attribute functions Time'Min and Time'Max.)

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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