Re: Nofork ${{var}...} edge cases

[Oliver Kiddle <opk@xxxxxxx>]

Bart Schaefer wrote:
> Just seeking opinions:
>
> Should ${{} true} (empty variable name) result in "bad substitution"?

Given that ${} just outputs nothing, the current behaviour of also
outputting nothing is entirely consistent.

That is just a zshism, however:
  ksh: syntax error: `}' unexpected
  bash: ${}: bad substitution
  /bin/sh: ${}: Bad substitution

I prefer zsh's behaviour here - ignoring the error - but would guess the
reason for it may be related to the support for nested substitutions. And
while uses of ${} alone are rare, variants such as ${:-xx} are useful.
Your patch doesn't allow a substitution inside the {}, however so nested
substitutions are not a reason here.

> Otherwise it's all side-effects, because nothing will be substituted.
> The prior ${|| true} form was a parse error.
>
> Should ${{var}} be a "bad substitution", or print a warning about an
> empty command?  Otherwise it just substitutes $var.

Again, I think I prefer the approach of doing as the user says even if
it doesn't appear to make much sense - so just substitude $var. If that
produces an error, what about ${{var}$=CMD} where CMD is unset.

> What about ${{var};} or ${{var}{}} etc.?

And there's probably a further half-dozen ways we've not thought of.

> Or we could declare ${{REPLY}...} as NOT synonymous with ${|...} and
> localize REPLY only in the latter of those.  That might actually make
> more sense.

That definitely makes most sense to me.

> Among the reasons I listed for not doing this, I forgot to mention
> that subscripts are allowed and you can't localize a subscripted
> parameter.

That's a fair argument for not making the parameter local.

Subscripts could be useful for something like:

  print ${{a[i]} i=2}

If we want automatic local for the subscript then maybe ${{a[]} REPLY=2}

While ${{arr} ... } does return arrays, it doesn't appear to be possible
to force array output from ${| ... }
In mksh:
  print ${|REPLY=(one two)}
does just print "one".

But various forms like ${${|REPLY=(one two)}[1]} return just "n"

I have built zsh with your latest patch and have not found any issues
with any tests I've thrown at it.

Perhaps worth including in a test case is the following which does
break after running the echo.
  while :; do; echo ${|REPLY=x;break}; done

Oliver