Concurrency

Result(T, RegionError) r = spawn (cap1, cap2) {
    return value;
};

• The Result’s type is declared where it goes (a binding or a return); the region’s returns are checked against its T, and every path must return. A region returns values, not handles.
• The capabilities are listed explicitly, and the child’s list must not exceed the parent’s (E0214).
• The child receives a fresh capability environment: the spawn body and everything it calls may use only the listed capabilities (E0214, E0215), and the child thread runs in its own capability frame at run time.
• Execution. A region bound to a name starts when the binding runs and executes concurrently on its own thread; the first use of the name waits for its result. A spawn anywhere else (returned, matched inline) is started and waited for at once.
• Structured completion. Every region is finished before the block that bound it ends, and before any return, break or continue leaves that block, whether or not its result was used.
• Sharing. Captured values are shared from the point of capture: transient maps are frozen and unique records marked shared, so neither the parent nor the child updates them in place.
• Handles move. A handle a region captures may not be used by the parent after the spawn.
• A failure in the child (an abort of any kind) is delivered to the parent as Err(RegionError); it does not end the process.
• Spawns nest. When no thread can be created, the region runs at once in the parent, with the same result.