TIP 227: Interface to Get and Set the Return Options of an Interpreter

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by Aug 21.
Author:         Don Porter <dgp@users.sf.net>
State:          Final
Type:           Project
Vote:           Done
Created:        30-Oct-2004
Keywords:       Tcl
Tcl-Version:    8.5


This TIP proposes new public C routines to allow the same access to interpreter return options [90] as are provided at the script level by the catch and return commands.


In Tcl 8.5, the return command has aready been extended to accept all option value pairs passed to it as arguments, to permit extensions to augment any custom return code values with whatever additional data values are appropriate. The errorInfo and errorCode values associated with the TCL_ERROR return code are already converted to this mechanism.

The ability to set custom return options in the interp has been limited to the script level though. For extension commands implemented entirely in C, it is inconvenient and somewhat unreliable to perform a Tcl_Eval("return ...") to be able to access this capability.

Likewise, the catch command is able to capture the current set of return options in the interp, but doing so requires both a script level command, and use of a variable. Extension commands implemented in C are better served with a direct interface to fetch the dictionary value.


Two new routines will be added to Tcl's public stub table:

int Tcl_SetReturnOptions(Tcl_Interp *interp, Tcl_Obj *options)

Tcl_Obj *Tcl_GetReturnOptions(Tcl_Interp *interp, int result)

These routines already exist in the HEAD of Tcl's development sources, but as private routines. The Tcl source code itself already makes calls to these routines where appropriate, and their functioning is tested by the test suite. This TIP merely proposes making these routines available as part of the public interface.

The Tcl_SetReturnOptions routine is essentially equivalent to the return -options command. The int value returned is the return code that the return command would return given the same options. In fact, a valid implementation for the return command would be:

 int Tcl_ReturnObjCmd(ClientData cd, Tcl_Interp *interp,
			int objc, Tcl_Obj *const objv[])
     int explicitResult = (0 == (objc %2));
     int numOptions = objc - 1 - explicitResult;
     if (explicitResult) Tcl_SetObjResult(interp, objv[objc-1]);
     return Tcl_SetReturnOptions(interp, Tcl_NewListObj(numOptions, objv+1));

Note that Tcl_SetReturnOptions is like Tcl_SetObjResult and Tcl_SetVar2Ex in that you can pass it a newly created Tcl_Obj confident that it will be freed later. No refCount manipulation is required.

The Tcl_GetReturnOptions routine is used by catch to fetch the value to be stored in the optionsVarName variable, if any. The result argument should be whatever return code was returned by the script evaluation, or other interp activity whose return options you wish to retrieve.

The (Tcl_Obj *) returned by Tcl_GetReturnOptions points to a newly created, unshared Tcl_Obj. It can be written to as returned; no need to implement copy-on-write checks. In particular, new key value pairs can be added to the dictionary, or existing ones changed or removed. As noted above, such a value is also suitable for passing right back to Tcl_SetReturnOptions.


Some extensions provide commands that offer the ability to evaluate a Tcl script in some other context, and return the complete result of that evaluation. For example, the Thread package provides the thread::send command that evaluates a script in another interp in a different thread. The thread::send command ultimately has a return code and a result that matches those produced by the script evaluation in the other thread. Furthermore, the ::errorInfo and ::errorCode variables are set according to the script evaluation outcome in the other thread as well. Extensions that accomplish such passing of full evaluation results achieve it now with copies of all portions of the full evaluation results: the return code, the interp result, and when appropriate, the values of ::errorInfo and ::errorCode.

Such mechanisms will continue to work unchanged in Tcl 8.5. However, they will no longer represent the complete evaluation results of the script. In order to continue to communicate back the full outcome of script evaluation, these extensions will want to call Tcl_GetReturnOptions after the script completes, transport that value back, and call Tcl_SetReturnOptions in the original interp. Note that use of these routines will automatically take care of ::errorInfo and ::errorCode, so the complete outcome of script evaluation will be able to be communicated by the return code, the interp result, and the dictionary of return options. Because the return options dictionary is itself extensible, this interface will not need to change again.

Reference Implementation

See Tcl Patch 1060579.


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This document has been placed in the public domain.