...implemented as coroutines which are expected to yield the lvalue.

Not really. I may appear that way, but coroutines flip back and forth. This doesn't.

In actuality, there would be very little difference between this and using a tie/STORE method to validate.

• The method is called to obtain the lvalue which is returned to the calling code.
• At some point the calling code makes an assignment to the lvalue and (because that lvalue is tied) the STORE method is called.

Except in this case the "STORE method" is the code inline from the LVALUE statement to the end of method.

One distinct difference between this and a tie/STORE method, is that if the lvalue in question is an array or a slice of an array, the "callback code" would only be called once. Not once for every value in the list being assigned.

I am still unsure what happens in the calling code if the STORE method (disguised or not) rejects the attempt to store a value?

I also thought about whether the PRE/POST or ENTER/LEAVE blocks associable with a method could be used to provide for a single place in which all validation is done, but my reading of the Apos & Syns leaves me very confused about when these get called, if ever, with an lvalue sub.

Of course, this whole discussion is a waste of time. Who are we to question whether there is a better way? Explain to plebians. Pah!