bug#12689: 24.2; Eshell ${cmd} substitution

[Jim Porter]

On 2/9/2022 1:12 AM, Lars Ingebrigtsen wrote:
> Aidan Gauland <aidalgol@no8wireless.co.nz> writes:
>
> > In Eshell, ${cmd} expands to nil.
> > e.g.
> > $ echo ${/bin/echo -e "foo\nbar"}
> > $
> > Note the absence of any output from `echo'.
> >
> > $ echo ${/bin/echo -e "foo\nbar"}-foo
> > nil-foo
> > n$
>
> (I'm going through old bug reports that unfortunately weren't resolved
> at the time.)
>
> Looks like this has been fixed in the decade since it was reported:
>
> / $ echo ${/bin/echo -e "foo\nbar"}
> ("foo" "bar")
> / $ echo ${/bin/echo -e "foo\nbar"}-foo
> ("foo" "bar")-foo
>
> So I'm therefore closing this bug report.  If there's more to be done
> here, please respond to the debbugs address and we'll reopen.

I think the fix is from bug#30725. I'd meant to post an update to this 
bug as well, but I haven't had a chance to investigate the other half of 
this bug yet. Specifically, I think there's at least an argument that 
the result *should* look like this:

  / $ echo ${/bin/echo -e "foo\nbar"}
  foo
  bar
  / $ echo ${/bin/echo -e "foo\nbar"}-foo
  foo
  bar-foo

I'm still not 100% sure what's going on here, but I think the ${} 
subcommand evaluation splits the output on "\n" to make a list (which 
seems reasonable to me). However, when it gets converted back to a 
string (either to print directly, as in the first case, or to 
concatenate, as in the second case), it ends up looking like '("foo" 
"bar")'. You can see a little more clearly how these cases are different 
by calling `message' instead of `echo':

  ~ $ message %S ${*echo -e "foo\nbar"}
  ("foo" "bar")
  ~ $ message %S ${*echo -e "foo\nbar"}-foo
  "(\"foo\" \"bar\")-foo"

So the result of "${...}" is a list, and the result of "${...}-foo" is a 
string.

I think it would make sense to do something to improve how "foo\nbar" 
gets round-tripped here so that the result is more consistent with 
regular shells. I have a couple of ideas, but I'll need a bit of time to 
tinker with things to see how they work, and to find a solution that 
keeps incompatible changes to a minimum.