GNU bug report logs - #26073
How should cl-symbol-macrolet interact with rebindings?

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From: Paul Pogonyshev <pogonyshev <at> gmail.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 26073 <at> debbugs.gnu.org
Subject: bug#26073: workaround
Date: Mon, 20 Mar 2017 10:04:16 +0100

> I'm wondering why generator.el uses cl-symbol-macrolet like that

Because that's how it works. It needs to store variables in an outer
scope so that their values are preserved between calls to `iter-next'
on the same iterator. Therefore it "rebinds" local variables to
different symbol and declares those symbols in outer scope, so that
they become non-local variables captured by resulting closures.

It's not a problem within generator function builder, rather it's a
bug in in `cl-symbol-macrolet' that incorrectly applies `bindings' to
forms with lambdas. The bug is that it rebinds variables even inside
`lambda', not realizing that they (at least in lexical scope) are
shadowed by the lambda's arguments. In comparison, it does handle
shadowing with `let' properly.

Paul

On 20 March 2017 at 01:49, Stefan Monnier <monnier <at> iro.umontreal.ca> wrote:
>> Attached patch is a workaround for the bug. It adds special handling
>> of lambdas to `cl--sm-macroexpand', similar to the way it handles
>> `let'. However, there is an unhandled case of "binding" symbols to
>> forms. I'm also not sure what would be correct behavior in old
>> non-lexical-scope code (do we still care?).
>
> Hmm... I'm wondering why generator.el uses cl-symbol-macrolet like that,
> but indeed the precise behavior of cl-symbol-macrolet is currently
> a bit fishy.
>
>
>         Stefan

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