[PYK] 2015-11-08:   '''procstep''' is a drop-in replacement for `[proc]` that
evaluates one command of a procedure body at at time, and a hook to inspect and
manipulate the command before it is evaluated, and another to react to return
conditions of the evaluation.

======
rename proc ::tcl::proc

::tcl::proc proc {name args body} {
    ::uplevel [::list ::tcl::proc $name $args [::list apply [::list args [
        ::string map [::list {${body}} [::list $body]] {
            ::foreach command [::scriptSplit ${body}] {
                ::if {[::catch [::list ::uplevel 1 [
                    ::cmdhandler $command]] cres copts]} {
                    ::tailcall ::errorhandler $cres $copts
                }
            }
        }
    ]] [uplevel {namespace current}]]]
}

::tcl::proc cmdhandler command {
   ::puts stderr [::list {now executing} $command]
   return $command
}

::tcl::proc errorhandler {cres copts}  {
   ::puts stderr [list {got an error} $cres]
   #puts [uplevel info locals]
}
======

Here is variant that improves runtime performance quite a bit by rewriting the
body when the procedure is created.  This leaves no place to put the result
variables of the `[catch]`, so they get stuffed into `::tcl` namespace instead:


======
::tcl::proc proc {name args body} {
    set newbody {}
    ::foreach command [::scriptSplit $body] {
        ::append newbody [::string map [::list {${command}} [::list $command]] {
            ::if {[::catch {if 1 [
                ::cmdhandler ${command}]} ::tcl::cres ::tcl::copts]} {
                ::errorhandler $::tcl::cres $::tcl::copts
            }
        }]
    }
    ::uplevel [::list ::tcl::proc $name $args $newbody]
}
======


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