Re: Word splitting/joining inside [[ ]]

[Peter Stephenson <p.w.stephenson@xxxxxxxxxxxx>]

On Fri, 11 Dec 2015 20:31:47 -0800
Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx> wrote:
> [[ $words = ${(on)=words} ]]
> 
> Doesn't work.  It's always true.  Confused, I setopt xtrace and discover:
>>>
> By accident I discovered that adding a supposedly-meaningless extra level
> of nested expansion makes it work:
> 
> torch% [[ $words = ${${(on)=words}} ]]
> +zsh:12> [[ 'I have a string of words' == a\ have\ I\ of\ string\ words ]]
> 
> What's going on here?

I guess this is something to do with the effect of singsub() and the
PREFORK_SINGLE flag which propagates into paramsubst() but not further
into multsub() which handles nested expansion.  There are likely to be
lots of combinations of effects like this that have never been thought
through.

In particular, forcing single substitution at the top level inhibits
various things happening (sometimes even if you've explicitly indicated
an affect that would later undo them) whereas the combination of multsub
and a paramsubst on top of it is smarter about trying to work out the
end effect.  So in your second case multsub produces array behaviour which
the top level paramsubst can then rationalise back down to a single
expression, while in your first case there's no leeway for array
behaviour at any point.

I don't have any ideas about easy ways to improve singsub.  It looks
a potential minefield.

pws