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https://github.com/briot/Ada-promises

Implementing promises in Ada. Type-safe, efficient, thread-safe, ...
https://github.com/briot/Ada-promises

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Implementing promises in Ada. Type-safe, efficient, thread-safe, ...

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README 

        

Ada Promises

============



The promise pattern is now ubiquitous in programming languages.

The Wikipedia page on promises lists a lot of implementations.

But there are none for Ada yet !



This repository is a starting point to evaluate how we could

implement such promises in Ada.



To compile:



    gprbuild -p -Pdefault.gpr



To run:



    ./obj/test_promises



Start looking at the code from the test case (`test_promises.adb`)

and then look at the specs.



Ada improvements that would be useful

-------------------------------------



This code does not require any change in the language, and works with

Ada 2012 compilers.

However, there are a number of improvements to the language that would

make it more convenient to use this package:



   - lambda or anonymous functions

     Rather than have to create objects for the callbacks, it would often

     be useful to use anonymous expression functions declared directly

     in the call to When_Done.



   - Dot notation with generic class wide operations

     It would be nice to extend the cases in which the dot notation can be

     used when calling subprograms. For instance, it would be nice, after

     instantiating a `Chains` package, to use "P.When_Done (A).When_Done (B)"

     with explicit function calls rather than the trick of overloading

     operators as we have done.

     This is minor at this point.



   - Implicit instantiations

     Even though it looks like it would be nice if we did not have to

     instantiate `Chains` and `Promises` explictly, such implicit

     instantiations like C++ has make the code much harder to following

     in practice. So no real benefit here !



Todo

----



Various improvement ideas for this library:



   - Look at reactive programming, observables and filter on them

     Link these observables to containers or tasks



   - Avoid memory alloc when there is a single callback set for a

     promise. This is the general case and should be optimized.



   - Should we have a wait() operation on a promise to wait until it is

     resolved ? This would only work when the routines are implemented

     with tasks, not with asynchronous I/O for instance.



   - Should we have an unsubscribe() operation ?

     Not so useful on promises, but more useful on observables



   - Can we get rid of `Unrestricted_Access` ?

     These are used so that one can write "new A" directly in the call

     to `When_Done`, but this unsafe.

     Also, we should have a way of preventing the reuse of the same access

     for multiple callbacks (`Chains` already does this via its

     `Is_Registered` function, but this is not the case for `Promises`)



     One way to limit this is by taking subprogram access instead of

     tagged types. The latter are still useful when additional data needs

     to be passed though (simulates lambda functions for instance).



   - Add more use cases

     Show how to use tasks along with promises.

     Show how to use promises in a gtk+ context



   - Should we raise an exception when a promise is freed but was never

     resolved or failed ?

     Or when Set_Value is called multiple times ?