Typestates
==========

Simple typestates
-----------------

(Copied from syntaxtest.txt)

    type Lock = struct { ... }
    # only one type state in a typestate group can be active at a time.
    typestate Lock { noforget Locked, Available }
    # it is still a void return type
    func var threaded Lock.lock() [[Available]] -> [[Locked]]
    func var threaded Lock.release() [[Locked]] -> [[Available]]

Or maybe this syntax:

    type Lock = struct { ... }
    # only one type state in a typestate group can be active at a time.
    typestate Lock { noforget Locked, Available }
    func var threaded Lock.lock()
        from Available
        to Locked
    func var threaded Lock.release()
        from Locked
        to Available


"Coupled-object" typestates
---------------------------

E.g. for allocations, if de-allocation has to be done with the
same allocator as allocation.


Typestate-preserving functions
------------------------------

    typestate string { ValidUtf8 }

    func arena string.reverse() -> string
        ?[[ValidUtf8]] -> [[ValidUtf8]]

    func arena string.reverse() -> string
        preserves ValidUtf8