custom command completion for a zshdb, a REPL

[Rocky Bernstein <rocky.bernstein@gmail.com>]

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I would like to add debugger command completion to the zsh debugger, zshdb.
 I have looked over the manual pages zshzle and zshcompsys and am still a
bit confused.

I would be grateful if someone could give a simple script as an example.
Suppose the list of REPL (read, print, loop) commands is in  a
blank-delimited string variable "cmds", and that one of the commands is
"foo" which has the list of subcommands for it in string variable "subcmd".

For comparison here is the corresponding bash code:

    typeset cmds; cmds='foo food woot'  # My RLE commands
    level0_complete_fn() {
        if (( COMP_POINT >  0)) ; then
            COMPREPLY=( $(compgen -W  "$cmds" "$COMP_LINE") )
        else
            COMPREPLY=( $cmds )
        fi
    }

    typeset subcmds; foo_subcmds='bar baz'
    foo_subcmd_complete() {
        COMPREPLY=( $foo_subcmds )
    }

    # Use a special version of bash read that handle's readline completion
    enable -f /src/external-vcs/bashdb/builtin/readc readc

    complete -D -F level0_complete_fn
    complete -F foo_subcmd_complete foo

    # Shell initialization
    shopt -s progcomp
    set -o emacs
    bind 'set show-all-if-ambiguous on'
    bind 'TAB:menu-complete'

    readc -e -p 'huh? ' line
    echo "you sez $line"

I would not like to have completion files the available commands and
sub-commands can be dynamic. For example, on the stack frame completion
might be a positive integer up to the number of stack frames. And this
changes in the course of execution.

Thanks.