Re: Running a preprocessor from GPS?

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

On 15-08-01 13:47 , EGarrulo wrote:
> On Saturday, August 1, 2015 at 10:32:47 AM UTC+2, Simon Wright wrote:
>> EGarrulo  writes:
>>
>>>     if Some_Condition then
>>>        declare
>>>           with Ada; use Ada;
>>>        begin
>>>           Singleton := Singleton_Package.Singleton_Instance; -- Read variable.
>>>           -- Use singleton.
>>>        end;
>>>     end if;
>>>
>>> But the compiler complains that "aspect specifications not allowed
>>> here".  Or does "with Package" put a package in scope without
>>> elaborating it until "use Package", and therefore I should have left
>>> only the "use" directive?
>>
>> "with Pkg" can only occur in a compilation unit's context clauses.
>>
>> "use Pkg" can appear there, and also in a declarative region (e.g. your
>> "declare" section).
>>
>> "use" has no effect on elaboration, only on visibility. See ARM 10.2,
>> Program Execution.
>>
>> http://www.adaic.org/resources/add_content/standards/12rm/html/RM-10-2.html
>
> Thanks.  Therefore there is no way to postpone the creation of a singleton
> until client code needs it, by relying on package elaboration alone, right?
> Therefore Ada -- too -- needs the Singleton pattern.

Discussion of design patterns easily degenerates into a debate about 
terminology.

To second Simon's post, an Ada (library-level) package, as a collection 
of state and operations, with necessarily exactly once instance of the 
state, matches the concept of a singleton object in all respects except 
as regards the "lazy initialization" thing, which to me is a very 
secondary aspect of singletons.

If an Ada "singleton" package is a significant consumer of memory and/or 
initialization time, it is easy to arrange for most of the memory 
allocation and initialization to occur lazily, when the package (or that 
part of the package) is used for the first time. One way to do that is 
to insert a check for initialization at the start of every operation; 
another way is to have some kind of Get_Instance operation that 
initializes the package (i.e. "creates the singleton") and returns a 
handle that is a required parameter for all other operations.

While elaboration order is defined by the "need" relationship between 
modules, this is the static relationship defined by the "with" clauses, 
and is satisfied before the main subprogram starts. Whether this means 
that Ada "needs" the singleton pattern is a moot point.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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