Recognize that "id" is the K combinator in disguise. Rename it as "K" and extend its semantics accordingly.

generic/tclBasic.c

@@ -224,6 +224,7 @@ static const CmdInfo builtInCmds[] = {
     {"if",		Tcl_IfObjCmd,		TclCompileIfCmd,	TclNRIfObjCmd,	CMD_IS_SAFE},
     {"incr",		Tcl_IncrObjCmd,		TclCompileIncrCmd,	NULL,	CMD_IS_SAFE},
     {"join",		Tcl_JoinObjCmd,		NULL,			NULL,	CMD_IS_SAFE},
+    {"K",		Tcl_KObjCmd,		TclCompileKCmd,	NULL,	CMD_IS_SAFE},
     {"lappend",		Tcl_LappendObjCmd,	TclCompileLappendCmd,	NULL,	CMD_IS_SAFE},
     {"lassign",		Tcl_LassignObjCmd,	TclCompileLassignCmd,	NULL,	CMD_IS_SAFE},
     {"lindex",		Tcl_LindexObjCmd,	TclCompileLindexCmd,	NULL,	CMD_IS_SAFE},

generic/tclCmdIL.c

@@ -188,6 +188,44 @@ static const EnsembleImplMap defaultInfoMap[] = {
     {NULL, NULL, NULL, NULL, NULL, 0}
 };
 
+/*
+ *----------------------------------------------------------------------
+ *
+ * Tcl_KObjCmd --
+ *
+ *	This procedure is invoked to process the "K" Tcl command. See the
+ *	user documentation for details on what it does.
+ *
+ *	With the bytecode compiler, this procedure is only called when the
+ *	#args is not known at compile time (expand operator).
+ *
+ * Results:
+ *	A standard Tcl result.
+ *
+ * Side effects:
+ *	See the user documentation.
+ *
+ *----------------------------------------------------------------------
+ */
+
+int
+Tcl_KObjCmd(
+    ClientData dummy,		/* Not used. */
+    Tcl_Interp *interp,		/* Current interpreter. */
+    int objc,			/* Number of arguments. */
+    Tcl_Obj *const objv[])	/* Argument objects. */
+{
+    if (objc>=2) {
+	Tcl_SetObjResult(interp, objv[1]);
+    } else {
+	Tcl_Obj *empty;
+
+	TclNewObj(empty);
+	Tcl_SetObjResult(interp, empty);
+    }
+    return TCL_OK;
+}
+
 /*
  *----------------------------------------------------------------------
  *

generic/tclCompCmdsGR.c

@@ -160,6 +160,57 @@ TclCompileGlobalCmd(
     return TCL_OK;
 }
 
+/*
+ *----------------------------------------------------------------------
+ *
+ * TclCompileKCmd --
+ *
+ *	Procedure called to compile the "K" combinator command.
+ *
+ * Results:
+ *	Returns TCL_OK for a successful compile. Returns TCL_ERROR to defer
+ *	evaluation to runtime.
+ *
+ * Side effects:
+ *	Instructions are added to envPtr to execute the "K" command at
+ *	runtime.
+ *
+ *----------------------------------------------------------------------
+ */
+
+int
+TclCompileKCmd(
+    Tcl_Interp *interp,		/* Used for error reporting. */
+    Tcl_Parse *parsePtr,	/* Points to a parse structure for the command
+				 * created by Tcl_ParseCommand. */
+    Command *cmdPtr,		/* Points to defintion of command being
+				 * compiled. */
+    CompileEnv *envPtr)		/* Holds resulting instructions. */
+{
+    /*
+     * General syntax: [K ?value ...?]
+     */
+    int numWords = parsePtr->numWords;
+    Tcl_Token *wordTokenPtr = TokenAfter(parsePtr->tokenPtr);
+    DefineLineInformation;	/* TIP #280 */
+
+    if (numWords>=2) {
+	int i;
+
+	for (i = 1; i < numWords; i++) {
+	    CompileWord(envPtr, wordTokenPtr, interp, i);
+	    wordTokenPtr = TokenAfter(wordTokenPtr);
+	    if (i>1) {
+		TclEmitOpcode(INST_POP,	envPtr);
+	    }
+	}
+    } else {
+	PushStringLiteral(envPtr, "");
+    }
+
+    return TCL_OK;
+}
+
 /*
  *----------------------------------------------------------------------
  *

generic/tclInt.h

@@ -3307,6 +3307,9 @@ MODULE_SCOPE int	Tcl_InterpObjCmd(ClientData clientData,
 MODULE_SCOPE int	Tcl_JoinObjCmd(ClientData clientData,
 			    Tcl_Interp *interp, int objc,
 			    Tcl_Obj *const objv[]);
+MODULE_SCOPE int	Tcl_KObjCmd(ClientData clientData,
+			    Tcl_Interp *interp, int objc,
+			    Tcl_Obj *const objv[]);
 MODULE_SCOPE int	Tcl_LappendObjCmd(ClientData clientData,
 			    Tcl_Interp *interp, int objc,
 			    Tcl_Obj *const objv[]);
@@ -3571,6 +3574,9 @@ MODULE_SCOPE int	TclCompileInfoObjectNamespaceCmd(Tcl_Interp *interp,
 MODULE_SCOPE int	TclCompileIncrCmd(Tcl_Interp *interp,
 			    Tcl_Parse *parsePtr, Command *cmdPtr,
 			    struct CompileEnv *envPtr);
+MODULE_SCOPE int	TclCompileKCmd(Tcl_Interp *interp,
+			    Tcl_Parse *parsePtr, Command *cmdPtr,
+			    struct CompileEnv *envPtr);
 MODULE_SCOPE int	TclCompileLappendCmd(Tcl_Interp *interp,
 			    Tcl_Parse *parsePtr, Command *cmdPtr,
 			    struct CompileEnv *envPtr);