[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.  It requires [cmdSplit%|%scriptSplit] and [cmdSplit%|%nocomments].  These commands are also available as `[ycl%|%ycl proc tcl step]`.

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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}] {
                set status [
                    ::catch [::list ::uplevel 1 [
                        ::cmdhandler $command]] cres copts]
                ::if {$status} {
                    ::tailcall ::errorhandler $status $cres $copts
                }
            }
            lindex $cres
        }
    ]] [uplevel {namespace current}]]]
}

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

::tcl::proc errorhandler {code cres copts}  {
    ::puts stderr [list {got an error} $cres]
    #puts [uplevel info locals]
    tailcall return -code $code -options $copts $cres
}
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Here is a variant that improves runtime performance quite a bit by rewriting
each command of the body when the procedure is created.  This leaves no private
space for `[catch]` to create its output variables in, so it stuffs them into
`$::tcl::cres` and `::tcl::copts` instead instead:


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::tcl::proc proc {name args body} {
    set newbody {}
    ::foreach command [nocomments [scriptSplit $body]] {
        ::append newbody [string map [list {${command}} [list $command]] {
            set ::tcl::ccode [
                ::catch {{*}[
                    ::cmdhandler ${command}]} ::tcl::cres ::tcl::copts]
              ::if {$::tcl::ccode} {
                  ::errorhandler $::tcl::ccode $::tcl::cres $::tcl::copts
              } else {
                  ::lindex $::tcl::cres
              }
        }]
    }
    ::uplevel [::list ::tcl::proc $name $args $newbody]
}

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

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


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