jchludzinski at grci.com wrote:
> 
> I attempted to derive a class from Semaphore but died sending the message #forMutualExclusion (to the derived class).  The error occurs when the derived method #signal sends "self primitiveFailed".  Why?
> 

The primitive for Semaphore>>signal in Squeak looks like this:

	primitiveSignal

		| sema |
		sema _ self stackTop.  "rcvr"
		self assertClassOf: sema is: (self splObj: ClassSemaphore).
		successFlag ifTrue: [ self synchronousSignal: sema ].


Note the second-to-last line which explicitly checks the class of the
receiver--the receiver is expected to be *exactly* class Semaphore, and
not a subclass.

I guess it would be nice to be able to subclass Semaphore.  Perhaps the
VM should check whether the receiver is a subclass of Semaphore, and
allow that as well.  It doesn't seem like *that* big a deal that such a
low level class can't be subclassed, but then again, it would make for
awesome bragging rights for Squeak.  :)


-Lex