Scoped values: API design

[Andrew Haley]

I've been experimenting with alternatives to try to design a
reasonable API, with a view to making it as versatile as necessary and
as small as possible. After a lot of failed attempts I've come up with
this.


A class ScopedBinder, which is a tuple of (Scoped<T> name, T value).

A class ScopedCollection, which is an immutable collection of
ScopedBinder. ScopedCollection implements run(Runnable) and
call(Callable).

A static Scoped method is(T value), which produces an (immutable)
instance of ScopedBinder.

A static scoped method collect(ScopedBinder<?> ... values) which
produces an (immutable) instance of ScopedCollection.


Putting all this together, if you want to bind some scoped values
and call some code, you'll:

        Scoped.collect(myVar.is(999), myInt.is(-1)).run(
            () -> System.out.println(myVar.get() + ", " + myInt.get())
        );

A framework which needs some scoped values to be set can do something
like this:

        ScopedCollection getBindings() {
            return Scoped.collect(myVar.is(999), myInt.is(-1));
        }

and the caller would do:

        myFramework.getBindings().run(
            () -> myFramework.start(someArgs)
        );

It makes sense also to have a shorthand for the single-argument form:

        myVar.is(999).run(
            () -> System.out.println(myVar.get())
        );


We could keep the bind() methods from my earlier proposal, but I'm not
sure that they offer anything much.

With regard to the earlier AutoCloseable variants of the API we could
also do something like

        try (var x = Scoped.collect(myVar.is(999), myInt.is(-1)).bind()) {
            System.out.println(myVar.get() + ", " + myInt.get());
        }

but again I'm not sure it offers very much over the Runnable and
Callable versions and it's far more open to abuse because programmers
aren't forced to use it in a well-structured way.

Comments needed!

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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