[Vm-dev] musing on thread immutability

[Ben Coman]

Just a random idea that been floating in my head for a few weeks...

As I understand it, with Spur we can now have immutable objects.  I guess
this will help when a functional style programming may be useful (such as
background tasks).   Newly designed applications might make good use of
this, but I am wondering about where you might have a large existing code
base (e.g. Squeak/Pharo) and you want to background some small part of the
existing code.  A great effort would be needed to dig deep into object
composition to understand how disparate parts of the system may hold
references to object internals several layers down, that may get updated
unexpectedly.

So I am wondering if instead / or as well as object-immutability it is
feasible to get thread-immutability, meaning:
* writing to any object state is prevented - which prevents the background
thread changing the foreground (UI) state unexpectedly; and/or
* reading or writing any object automatically sets its immutability bit -
which prevents other unknown threads changing the state the background
thread has come to rely on.

No doubt pushing any existing single-threaded code into such a background
immutable-thread will cause immutablity-write-errors - but the advantage is
that such conflicts are brought to light rather than biting you from the
shadows.

cheers -ben
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