Re: Changing the way bash expands associative array subscripts

[Chet Ramey]

On 4/13/21 5:18 AM, Koichi Murase wrote:

> I actually doubt whether there are real use cases for the behavior
> that «iref=a[@]; ${!iref}» or «declare -n nref=a[@]; $nref» expands to
> all the elements of *associative* arrays.  Maybe it is useful to
> expand the indexed arrays in this way because indexed arrays contain a
> sequence of values.  But associative arrays usually contain a set of
> unordered key-value pairs, so I guess there are not so many chances to
> expand only the values without specifying keys.

I'd dispute that. There are multiple use cases for the set of values in an
associative array.

> It's just a naive idea and may be imperfect, but how about changing
> the behavior of assoc[@] (except for "${assoc[@]}") only for
> associative arrays while keeping the current behavior for indexed
> arrays?

Some of this is reasonable.

 In the following list, the items marked with (*) are the
> behavior different from the current one. I listed examples for key=@,
> but the same for key=*.
>
>    unset 'indexed' # remove the indexed array
>    unset 'assoc' # remove the associative array
>    unset 'indexed[@]' # empty the array (*)
>    unset 'assoc[@]' # remove the element of key=@ (*)

We've already discussed these. I'm ok with the change to associative
arrays, since there's already a way to do it and it seems more
straightforward to remove the element with key `@'. I'd have to think about
the changed to indexed arrays, since you can already do it with
`indexed=()'. It's a novel new use case.


>    iref=indexed[@]; ${!iref} # all the elements
>    iref=assoc[@]; ${!iref} # the element of key=@ (*)

No, there's no reason to have ${!iref} and ${assoc[@]} behave differently.

>    declare -n nref=indexed[@] # reference all the elements
>    declare -n nref=assoc[@] # reference the element of key=@ (*)

Same reason.

>    test -v 'indexed[@]'
>    [ -v 'indexed[@]' ]
>    [[ -v indexed[@] ]] # test if the indexed array is non-empty
>    test -v 'assoc[@]'
>    [ -v 'assoc[@]' ]
>    [[ -v assoc[@] ]] # test if the associative array has the key '@' (*)

I'm ok with this.

>    printf -v 'indexed[@]' xxx # error
>    printf -v 'assoc[@]' xxx # assign to the element of key=@ (*)

I think this is ok, too, for the same reason as:

> We don't change the following behavior of associative arrays where the
> syntactic information may distinguish a[$key] from a[@]:
>
>    indexed[@]=xxx, assoc[@]=xxx # error

I think it's ok to allow

assoc[@]=value

since

indexed[@]=xxx is going to be an error no matter which behavior you choose
because it's an invalid arithmetic expression.


--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/