TIP #356: NR-ENABLED SUBSTITUTIONS FOR EXTENSIONS 
===================================================
 Version:      $Revision: 1.1 $
 Author:       Don Porter <dgp_at_users.sf.net> 
 State:        Draft
 Type:         Project
 Tcl-Version:  8.6
 Vote:         Pending
 Created:      Thursday, 17 September 2009
 URL:          http://purl.org/tcl/tip/356.html
 WebEdit:      http://purl.org/tcl/tip/edit/356
 Post-History: 

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 ABSTRACT 
==========

 This TIP proposes the new public routine *Tcl_NRSubstObj* to provide 
 extension commands that evaluate Tcl substitutions the ability to do so 
 in a non-recursive manner. 

 BACKGROUND 
============

 Continuing in the path of [TIP #322] and [TIP #353], we want extensions 
 to be able to create NR-enabled commands, and any command procedures 
 currently calling the *Tcl_SubstObj* routine are not NR-enabled. The 
 solution is to provide the NR-enabled counterpart. 

 PROPOSAL 
==========

 Add the following routine to Tcl's public interface: 

       int *Tcl_NRSubstObj*(Tcl_Interp */interp/, Tcl_Obj */objPtr/, int 
       /flags/) 

 This routine places on the NR stack a request that the Tcl 
 non-recursive trampoline evaluate the /objPtr/ value as a Tcl 
 substitution in interpreter /interp/, as controlled by the value of 
 /flags/. The /flags/ value is the same combination of 
 *TCL_SUBST_BACKSLASHES*, *TCL_SUBST_COMMANDS*, and 
 *TCL_SUBST_VARIABLES* that control *Tcl_SubstObj*. This routine returns 
 the value *TCL_OK*, since there is (currently) no way this request 
 operation can fail. The proposed interface still provides for an int 
 return value so that future revisions to Tcl's internals have the 
 freedom to change that without need to change the public interface. 

 After the trampoline completes the requested substitution, it will pass 
 the return code, either *TCL_OK* or *TCL_ERROR*, to the next callback 
 on the NR-stack, and either the result of the substitution or the error 
 message will be stored in the result of /interp/. The caller of 
 *Tcl_NRSubstObj* may also call *Tcl_NRAddCallback* to request a 
 *Tcl_NRPostProc* callback routine be placed on the NR stack to receive 
 these results, if needed to achieve the task the caller is performing. 

 IMPLEMENTATION 
================

 The proposed routine is already present in the HEAD of Tcl as the 
 internal routine *TclNRSubstObj*. This proposal would simply promote it 
 to Tcl's public interface. 

 COMPATIBILITY 
===============

 There should be no compatibility issues, since at the interface level 
 this is just the addition of a new routine. 

 MIGRATION 
===========

 As an example for extensions to follow, consider this template for a 
 *Tcl_ObjCmdProc* currently calling *Tcl_SubstObj*. 

   int ObjCmd(ClientData cd, Tcl_Interp *interp, int objc, Tcl_Obj *const objv[])
       Tcl_Obj *resultPtr;
       /* determine text to be substituted, objPtr */
       /* determine flags value to control substitution */
       resultPtr = Tcl_SubstObj(interp, objPtr, flags);
       if (resultPtr == NULL) {return TCL_ERROR}
       /* resultPtr holds substitution result; continue */
   }
   
   Tcl_CreateObjCommand(interp, name, ObjCmd, /* ... */);

 To use *Tcl_NRSubstObj* to NR-enable this command, rewrite along these 
 lines: 

   int ObjCmd(ClientData cd, Tcl_Interp *interp, int objc, Tcl_Obj *const objv[])
   {   return Tcl_NRCallObjProc(interp, NRObjCmd, cd, objc, objv);  }
   
   int NRObjCmd(ClientData cd, Tcl_Interp *interp, int objc, Tcl_Obj *const objv[])
   {   
       /* determine text to be substituted, objPtr */
       /* determine flags value to control substitution */
       Tcl_NRAddCallback(interp, Callback, /*...*/);
       return Tcl_NRSubstObj(interp, objPtr, flags);
   }
   
   int Callback(ClientData data[], Tcl_Interp *interp, int code)
   {   Tcl_Obj *resultPtr;
       if (code == TCL_ERROR) {return TCL_ERROR;}
       resultPtr = Tcl_GetObjResult(interp);
       /* resultPtr holds expression result; continue */
   }
   
   Tcl_NRCreateCommand(interp, name, ObjCmd, NRObjCmd, /*...*/);

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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