Re: "${assoc[@]@k}" doesn't get expanded to separate words within compound assignment syntax

[Dennis Williamson]

On Sun, Mar 24, 2024 at 1:56 PM Greg Wooledge <greg@wooledge.org> wrote:

> It would be pretty reasonable to have a builtin that could take an array
> name plus any number of additional argument pairs, and load those pairs
> as keys/values into said array.  Then you could do something like this:
>
>     declare -A hash=(...)
>     kvlist=( "${hash[@]@k}" )
>
>     declare -A newhash
>     addtoarray newhash "${kvlist[@]}"
>
> Some readers may observe that this looks a bit like Tcl's "array set"
> command.  That's not a coincidence.  Whenever I see "a list of alternating
> keys and values", that's where my mind goes.
>
> I'm debating mentally whether this hypothetical new builtin would only
> work with associative arrays, or also double as an "lappend" (append new
> elements to the end of a list/array) if given an indexed array name.
> I'm leaning toward having it be both.  It wouldn't be any *more* confusing
> than the current situation already is.  The main difference would be that
> with an indexed array, every argument after the array name becomes an
> array element, instead of half of them being keys and the other half
> being values.
>
> Then again, I'm not likely to attempt to implement it, so anyone who
> actually writes the code gets to make all the decisions.
>
>
The @K transform outputs key value pairs for indexed arrays as well as
associative arrays (you used the @k transform which does word splitting and
loses the k-v sequence). Thus the @K allows preserving indices in a sparse
indexed array. So your hypothetical builtin would only depend on the type
of the receiving array. So it wouldn't be every argument as an element - it
would still be keys (indices) and values alternating.

    declare -A hash=(...)
    kvlist=( "${hash[@]@K}" )

    declare -A newhash
    addtoarray newhash "${kvlist[@]}"

    declare -a indexed=(...)    # or initialized / built up in another way
- with sequential or sparse indices
    kvlist=( "${indexed[@]@K}" )

    declare -a newindexed # or omitted
    addtoarray newindexed "${kvlist[@]}"

addtoarray should have options for dealing with index collisions (last wins
(default), first wins, error, warning) and a clear before adding would be
handy but trivial to do in a script. Another handy but not needed feature
would be an option to declare the receiving array and add to it in one go.

I'm also unlikely to implement it.

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