package require Thread

tsv::set app tfunc {

    if {[tsv::incr app i -1] == 0} {
        return
    }

    puts "[thread::id] [tsv::get app i]"

    # this won't work
    set t [thread::create -joinable]
    thread::send -async $t [tsv::get app tfunc]
    thread::join $t
}

tsv::set app i 10

set t [thread::create -joinable]
thread::send -async $t [tsv::get app tfunc]
thread::join $t