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authorMiguel Sofer <miguel.sofer@gmail.com>2015-03-23 02:11:30 (GMT)
committerMiguel Sofer <miguel.sofer@gmail.com>2015-03-23 02:11:30 (GMT)
commitbc0fe5c0707bdaa3ae780c3f48b106234c623e1f (patch)
treee4c44f4a320ac9b570bec6c66f5e4982cf35c48d
parentb9c867e7674aca37280bca058d0191b6793203ef (diff)
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updated, improved tailcall comments
-rw-r--r--generic/tclBasic.c45
1 files changed, 21 insertions, 24 deletions
diff --git a/generic/tclBasic.c b/generic/tclBasic.c
index 361ed49..e92e8e9 100644
--- a/generic/tclBasic.c
+++ b/generic/tclBasic.c
@@ -4142,8 +4142,9 @@ TclNREvalObjv(
/*
* data[1] stores a marker for use by tailcalls; it will be set to 1 by
- * command redirectors (imports, alias, ensembles) so that tailcalls
- * finishes the source command and not just the target.
+ * command redirectors (imports, alias, ensembles) so that tailcall skips
+ * this callback (that marks the end of the target command) and goes back
+ * to the end of the source command.
*/
if (iPtr->deferredCallbacks) {
@@ -4406,7 +4407,7 @@ NRCommand(
iPtr->numLevels--;
/*
- * If there is a tailcall, schedule it
+ * If there is a tailcall, schedule it next
*/
if (data[1] && (data[1] != INT2PTR(1))) {
@@ -8170,27 +8171,24 @@ Tcl_NRCmdSwap(
}
/*****************************************************************************
- * Stuff for tailcalls
+ * Tailcall related code
*****************************************************************************
*
- * Just to show that IT CAN BE DONE! The precise semantics are not simple,
- * require more thought. Possibly need a new Tcl return code to do it right?
- * Questions include:
- * (1) How is the objc/objv tailcall to be run? My current thinking is that
- * it should essentially be
- * [tailcall a b c] <=> [uplevel 1 [list a b c]]
- * with two caveats
- * (a) the current frame is dropped first, after running all pending
- * cleanup tasks and saving its namespace
- * (b) 'a' is looked up in the returning frame's namespace, but the
- * command is run in the context to which we are returning
- * Current implementation does this if [tailcall] is called from within
- * a proc, errors otherwise.
- * (2) Should a tailcall bypass [catch] in the returning frame? Current
- * implementation does not (or does it? Changed, test!) - it causes an
- * error.
- *
- * FIXME NRE!
+ * The steps of the tailcall dance are as follows:
+ *
+ * 1. when [tailcall] is invoked, it stores the corresponding callback in
+ * the current CallFrame and returns TCL_RETURN
+ * 2. when the CallFrame is popped, it calls TclSetTailcall to store the
+ * callback in the proper NRCommand callback - the spot where the command
+ * that pushed the CallFrame is completely cleaned up
+ * 3. The NRCommand schedules the tailcall callback to run immediately after
+ * NRCommand returns
+ *
+ * One delicate point is to properly define the NRCommand where the tailcall
+ * will execute. There are functions whose purpose is to help define the
+ * precise spot: TclMarkTailcall ("this is the spot") and TclSkipTailcall
+ * ("skip the next command: we are redirecting to it, tailcalls should run
+ * after WE return"), TclPushTailcallPoint (special for OO).
*/
void
@@ -8282,8 +8280,7 @@ TclNRTailcallObjCmd(
/*
* Create the callback to actually evaluate the tailcalled
* command, then set it in the varFrame so that PopCallFrame can use it
- * at the proper time. Being lazy: exploit the TclNRAddCallBack macro to
- * build the callback.
+ * at the proper time.
*/
if (objc > 1) {