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Re: namref dicussion continued: [PATCH] export -r reference


From: Dan Douglas
Subject: Re: namref dicussion continued: [PATCH] export -r reference
Date: Thu, 5 May 2016 17:02:41 -0500

On Thu, May 5, 2016 at 2:46 AM, Piotr Grzybowski <address@hidden> wrote:
>  ok, so Dan wants this patch.

Yes I think your bash patch is probably an improvement at least in
this particular case.

> Maybe you can comment on wether the patches are valid.

The posix export and readonly don't produce locally scoped variables
at all, so it's expected that e.g. `typeset -x` and `export` will
differ.

This is yet another problem that's better solved by immutable C++-like
references. At least, the problem goes away when variables are passed
explicitly.

The exact mechanism isn't really documented. When ksh determines that
a nameref assignment is to a positional parameter during evaluation,
you get a completely different type of reference. That is, if `$1`
expands to `x`, then `typeset -n ref=$1` and `typeset -n ref=x` aren't
equivalent like they are in bash.  In the former case, any use of
`ref` except for `typeset +n` and `unset -n` applies to the exact
object that `$1` referred to at the time the reference was created,
even if it's another nameref in which case that reference is followed.
The latter gives you a bash-like dynamic reference that points to
somewhere within either the current scope or the global scope (due to
static scoping) unless it happens to point to another reference
parameter of course.

When you changed that `typeset` line to `export -r _x`, things become
less clear. Ksh sees that _x is a reference parameter and because of
the rules it does the only thing that makes sense which is apply the
export attribute to whatever was passed to the function for both
`export` or `typeset -x`, though there may be some other differences
between those.

Bash on the other hand never does what you'd want for either `export`
or `typeset -x`. For typeset, it tears down the reference and
redefines it as a new local with the export attribute. It does that
indiscriminately for any usage of typeset, which means you can't
correctly modify an attribute by reference in bash. If you actually
wanted that result, the correct way would be to first use `typeset +n
_x` followed by `typeset -x _x`. If you choose `export`, the correct
behavior is even less clear thanks to dynamic scope.

Your patch is a partial solution that makes `export` modify the
attribute in the way you would expect in the absence of the reference
parameter feature. Ignoring namerefs, the only other way for bash to
modify the attribute of the currently visible dynamically scoped local
without declaring a new local is to use export, which is why bash
behaves as it did. This solution doesn't solve the same problem for
attributes other than export and readonly. Bash can't use e.g.
`typeset -u` by reference. It also doesn't solve the many other
problems caused by this like name conflicts.



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