Use of left- and right-padding with a variable padding char

[Zach Riggle <zachriggle@gmail.com>]

As always, thank you all so much for your time.  It's genuinely appreciated.

I want to define a function "center", which takes $1="Some String" and
$2="-" or some other default padding value.

The Zsh manual on expansion says that I can do something like this,
which works well and as intended:

    function center() {
        local text="${1:-hellworld}"
        local -i columns=${COLUMNS:-$(tput cols)}
        columns=$(( columns/2 ))
        echo ${(l:${columns}::=:::r:${columns}::=:::)text}
    }

This works very well, and correctly centers "${text}" across the width
of the terminal, with "=" padding on both sides.  Perfect.

However, I also want to make the padding character ("=" in the above
example) be variable.

The following does NOT work as intended, and I'm not sure why.  My
understanding is that variable expansion is inside-out, so it SHOULD
work.

    function center() {
        local text="${1:-hellworld}"
        local pad="${2:-=}"
        local -i columns=${COLUMNS:-$(tput cols)}
        columns=$(( columns/2 ))
        echo ${(l:${columns}::${pad}:::r:${columns}::${pad}:::)text}
    }
    center "hello" "_"

The value in $1 is correctly centered, but it is padded with the
LITERAL value '${pad}', rather than the expansion of ${pad}.

I expect there's some documentation that I haven't read yet, any tips
would be useful.

Zach Riggle