Zsh parameter expansion on output of command s flag doesn't work

Question

In zsh, if I run:

a=$(echo "foo, bar")
echo ${a[(ws:, :)1]}

I get foo as you would expect ( w causes the index to refer to words s:, : makes , be the word separator).

However, if I try to combine these:

echo ${$(echo "foo, bar")[(ws:, :)1]}

I get foo, . For some reason the w flag is working correctly, but the s:, : flag is completely ignored.

What am I doing wrong here?

More info: This is just a problem with $() inside ${}. If I nest ${} inside ${} there is no such problem.

$ zsh --version
zsh 5.2 (x86_64-unknown-linux-gnu)

Answer 1

Field splitting is done before the index operation kicks in, so the behaviour you see is what I would expect. You can get the expected result by

echo ${"$(echo "foo, bar")"[(ws:, :)1]}

One note aside: My first attempt was to rewrite your expression using the ?{==...} form of parameter substitution, which is supposed to inhibit field splitting, ie

echo ${${==$(echo "foo, bar")}[(ws:, :)1]}

and this did not work. I have no idea why...