On Mon, Nov 23, 2020, 4:32 PM Eric V. Smith 

I just commented on Steve's post over on Discourse. The problem with this is that the called function (m.case, here) needs to have access to the caller's namespace in order to resolve the expressions, such as StringNode and PairNone. This is one of the reasons f-strings weren't implemented as a function, and is also the source of many headaches with string type annotations.

Is this really true though? Yes, it would require magic of the sort zero-argument `super()` does. But can't the Matcher constructor capture the locals one step up the stack on instance creation? That's largely why my strawman version is slightly different from Steve's strawman.

I'd put the question this way: assuming Matcher can be written (with a bit of stack magic), and assuming that the strings inside m.case() calls are exactly the same mini-languague PEP 634 proposes, would you want a syntax change?

That's not rhetorical, I'm of mixed feeling myself.