Re: feature request: new builtin `defer`, scope delayed eval

[Lawrence Velázquez]

On Sat, Oct 8, 2022, at 2:11 AM, Koichi Murase wrote:
> 2022年10月8日(土) 12:04 Cynthia Coan <cynthia@coan.dev>:
>> [...]
>>
>> Otherwise, I think we can perhaps reword this into two smaller
>> features: "function local trap signals",
>
> The existing RETURN trap is exactly the trap that can be used to clean
> up resources local to functions and is already ``function-local''
> unless the user changes its behavior by setting `declare -ft', `set
> -T', or `shopt -s extdebug'. One thing to note is that the RETURN trap
> should be cleaned up inside the RETURN trap itself by running `trap -
> RETURN', or otherwise the RETURN trap is erroneously considered by
> Bash to be set for the caller function.

The latter behavior is why I didn't describe RETURN as function-local
in my earlier message, but I forgot the workaround of unsetting
the trap from within itself.  Nice.

>> and "options to safely
>> append/prepend to a trap" (for some definition of 'safe', perhaps this
>> just means separating by ';'?
>
> Currently, there is no way to register multiple handlers to a trap in
> a safe way, but if you can accept the approach by `;' or newline, you
> can just do it by getting the current trap string with `trap -p
> RETURN' and then overwrite the trap with the adjusted trap string.

Another strategy is to construct the complete trap string before
setting the trap, using whichever concatenation methods are preferred:

    f() {
        local trapcmds=cleanup
        if something; then
            trapcmds+='; more_cleanup'
        else
            trapcmds+='; so_fresh_and_so_clean'
        fi
        trap "$trapcmds; trap - RETURN" RETURN

        rest_of_function
    }

-- 
vq