Notes on the first of [Miguel Sofer]'s talks about NRE.  (Please add/enhance/correct ...)

State of computation in 8.5
   * C-stack (1 per thread)
   * Nesting of C-function calls

   * CallFrame stack (1 per interp)
   * Nesting of [proc], [apply], [namespace eval]; this is what [upvar] and [uplevel] navigate

   * Tcl Evaluation Stack (1 per execEnv/interp)
   * Bytecode engine stack and workspace

   * Special allocator supported (TclStacAlloc)
   * CmdFrameStack (1 per interpereter)

Walked through a simplified stack trace of:

 proc foo {} {
      set x 1
      moo
     set x 2
 }
 proc moo {} {
     set x 1
     puts $x; # WE ARE HERE
     set x 2
 }


Above will have 3 instances of TclExecuteByteCode; full stack 16 levels deep

Stack overflow and recursion limits: Some platforms (BSD and other Unixes) have small default stack; Stack-hungry platforms (i.e., Windows debug builds); Severely stack constrained platforms (Cisco, phones?).  mod-8-3-branch created a few years ago by ActiveState for a customer.

The C stack in 8.6: above example only has 1 instance of TclExecuteByteCode(!)

Barring any undiscovered bugs, full compatability.  Only additions to the API, no changes.  Modification of struct Command, changes to Tcl_GetCommandInfo/TclSetCommandInfo designed not to break extensions that call*objectProc directly.  Changed structs (extended): Interp, Command, ExecEnv.  Extensions that include internal private headers might break, but that is what they deserve.

New C API (TIP #322):
   * Tcl_NRCreateCommand
   * Tcl_NREvalObj
   * Tcl_NREvalObjv
   * Tcl_NRCmdSwap
   * Tcl_NRAddCallback

An NRE-enabled command requires two implementations: the regular objProc and a new nreProc.  NRE provides a utility function Tcl_NRCallObjProc to make this task essentially trivial.  If he could have prohibited people from calling the objProc implementation directly a separate nreProc would not be necessary, but it is to maintain maximum compatability with existing extensions.

 int MyCmdObjProc( ... )
 {
     <preparation>
     result = ...
     <cleanup>
     ...
 }

After adaptation looks like:

 int MyCmdNreProc( ... )
 {
     <preparation>
     Tcl_NRAddCallback(interp, MyPostProc, data0, data1, NULL, NULL);
     return Tcl_NREvalObj(interp, objPtr, flags);
 }

 int MyPostProc( ... )
 {
    Tcl_Obj *fooPtr = data[0]; /* data0 retrieved */
    MyStruct *mooPtr = data[1]; /* data1 retrieved */
  
    <postprocessing>
    <cleanup>
    return = result;
    ...
 }





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