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bug#27674: 26.0.50; cl-progv: strange scoping due to implementation

From: npostavs
Subject: bug#27674: 26.0.50; cl-progv: strange scoping due to implementation
Date: Wed, 12 Jul 2017 21:54:26 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.2.50 (gnu/linux)

Michael Heerdegen <address@hidden> writes:

> Eh - no.  Maybe I have a wrong mental model.  I thought that the free
> variable `x' in the lambda is (also) in the scope of the dynamical
> binding created by `progv', and because that binding is established
> inside the `let' establishing the lexical binding of `x', it would
> shadow the lexical binding.

Oh, you want lexical and dynamic binding on the same variable?  I think
the answer is "don't do that".

> Why does the lambda still refer to the lexical binding?

Maybe it would be more obvious if we wrote it like this:

    (let ((x 0))
      (cl-progv (list (intern (read-string "Enter var: "))) (list 1)
        (funcall (lambda () x))))

Clearly the inner x must refer to the lexical let-binding, right?  Even
if the user happens to enter `x' at the prompt this remains true.

> Does a lexical binding always beat a dynamical one?

Yes, lexical analysis is performed first and then the names are thrown
away, so you can't even tell when the "same" variable has been
dynamically bound as well.

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