In zsh, how do I pass anonymous arrays into functions?

Question

In zsh, how do I pass anonymous arrays into functions? e.g. looking for something like:

foo() {
  echo ${1[2]} '\n';
}

a=(abc def ghi)
foo $a

--> def

Or ideally:

foo (abc def ghi)

Answer 1

You can pass the name of the array to the function and then the function can read the array by interpreting the name as a variable name. It is a technique that is also useful for things like associative arrays in bash. ZSH makes it very easy to do, as the (P) operator will interpret the variable in the desired way.

An example will make this clear. If you define this function:

function i_want_array () {
    local array_name=$1

    echo "first array element is: " ${(P)${array_name}[1]}
}

Then the following code will execute it:

% a=(one two three)
% i_want_array "a"
first array element is:  one

And just to clarify, this works by operating on the real array, so any whitespace is handled correctly:

% a=("one two" three)
% i_want_array "a"
first array element is:  one two

Answer 2

You can't. Functions take positional parameters like any other command.

Note also that your workaround doesn't allow any of the "array" elements to contain whitespace.

The cleanest thing I can think of is to require that the caller create a local array, then read it from the function:

$ foo() {
   for element in $FOO_ARRAY
   do
       echo "[$element]"
   done
}
$ local FOO_ARRAY; FOO_ARRAY=(foo bar baz quux)
$ foo
[foo]
[bar]
[baz]
[quux]

I know bash does similar acrobatics for its completion system, and I think zsh might, too. It's not too unusual.