--- gforth/except.fs	2003/02/17 22:42:09	1.6
+++ gforth/except.fs	2007/02/19 00:08:20	1.18
@@ -1,6 +1,6 @@
 \ catch, throw, etc.
 
-\ Copyright (C) 1999,2000 Free Software Foundation, Inc.
+\ Copyright (C) 1999,2000,2003,2006 Free Software Foundation, Inc.
 
 \ This file is part of Gforth.
 
@@ -20,77 +20,146 @@
 
 \ !! use a separate exception stack?           anton
 
-\ user-definable rollback actions
-
-Defer 'catch
-Defer 'throw
-
-' noop IS 'catch
-' noop IS 'throw
-
 \ has? backtrace [IF]
 Defer store-backtrace
 ' noop IS store-backtrace
 \ [THEN]
 
+\ Ok, here's the story about how we get to the native code for the
+\ recovery code in case of a THROW, and why there is all this funny
+\ stuff being compiled by TRY and RECOVER:
+
+\ Upon a THROW, we cannot just return through the ordinary return
+\ address, but have to use a different one, for code after the
+\ RECOVER.  How do we do that, in a way portable between the various
+\ threaded and native code engines?  In particular, how does the
+\ native code engine learn about the address of the native recovery
+\ code?
+
+\ On the Forth level, we can compile only references to threaded code.
+\ The only thing that translates a threaded code address to a native
+\ code address is docol, which is only called with EXECUTE and
+\ friends.  So we start the recovery code with a docol, and invoke it
+\ with PERFORM; the recovery code then rdrops the superfluously
+\ generated return address and continues with the proper recovery
+\ code.
+
+\ At compile time, since we cannot compile a forward reference (to the
+\ recovery code) as a literal (backpatching does not work for
+\ native-code literals), we produce a data cell (wrapped in AHEAD
+\ ... THEN) that we can backpatch, and compile the address of that as
+\ literal.
+
+\ Overall, this leads to the following resulting code:
+
+\   ahead
+\ +><recovery address>-+
+\ | then               |
+\ +-lit                |
+\   (try)              |
+\   ...                |
+\   (recover)          |
+\   ahead              |
+\   docol: <-----------+
+\   rdrop
+\   ...
+\   then
+\   ...
+
+\ !! explain handler on-stack structure
+
+Variable first-throw
+: nothrow ( -- ) \ gforth
+    \G Use this (or the standard sequence @code{['] false catch drop})
+    \G after a @code{catch} or @code{endtry} that does not rethrow;
+    \G this ensures that the next @code{throw} will record a
+    \G backtrace.
+    first-throw on ;
+
 : (try) ( ahandler -- )
+    first-throw on
     r>
     swap >r \ recovery address
-    rp@ 'catch >r
     sp@ >r
     fp@ >r
     lp@ >r
     handler @ >r
     rp@ handler !
-    backtrace-empty on
     >r ;
 
-: try ( compilation  -- orig ; run-time  -- ) \ gforth
-    \ !! does not work correctly for gforth-native
-    POSTPONE lit >mark POSTPONE (try) ; immediate compile-only
+: try ( compilation  -- orig ; run-time  -- R:sys1 ) \ gforth
+    \G Start an exception-catching region.
+    POSTPONE ahead here >r >mark 1 cs-roll POSTPONE then
+    r> POSTPONE literal POSTPONE (try) ; immediate compile-only
 
-: (recover) ( -- )
+: (endtry) ( -- )
     \ normal end of try block: restore handler, forget rest
     r>
     r> handler !
     rdrop \ lp
     rdrop \ fp
     rdrop \ sp
-    r> rp!
     rdrop \ recovery address
     >r ;
 
-: recover ( compilation  orig1 -- orig2 ; run-time  -- ) \ gforth
+: handler-intro, ( -- )
+    docol: here 0 , 0 , code-address! \ start a colon def 
+    postpone rdrop                    \ drop the return address
+;
+
+: iferror ( compilation  orig1 -- orig2 ; run-time  -- ) \ gforth
+    \G Starts the exception handling code (executed if there is an
+    \G exception between @code{try} and @code{endtry}).  This part has
+    \G to be finished with @code{then}.
     \ !! check using a special tag
-    POSTPONE (recover)
-    POSTPONE else ; immediate compile-only
+    POSTPONE else handler-intro,
+; immediate compile-only
 
-: endtry ( compilation  orig -- ; run-time  -- ) \ gforth
-    POSTPONE then ; immediate compile-only
+: restore ( compilation  orig1 -- ; run-time  -- ) \ gforth
+    \G Starts restoring code, that is executed if there is an
+    \G exception, and if there is no exception.
+    POSTPONE iferror POSTPONE then
+; immediate compile-only
+
+: endtry ( compilation  -- ; run-time  R:sys1 -- ) \ gforth
+    \G End an exception-catching region.
+    POSTPONE (endtry)
+; immediate compile-only
+
+: endtry-iferror ( compilation  orig1 -- orig2 ; run-time  R:sys1 -- ) \ gforth
+    \G End an exception-catching region while starting
+    \G exception-handling code outside that region (executed if there
+    \G is an exception between @code{try} and @code{endtry-iferror}).
+    \G This part has to be finished with @code{then} (or
+    \G @code{else}...@code{then}).
+    POSTPONE (endtry) POSTPONE iferror POSTPONE (endtry)
+; immediate compile-only
 
 :noname ( x1 .. xn xt -- y1 .. ym 0 / z1 .. zn error ) \ exception
     try
 	execute 0
-    recover
-        nip
-    endtry ;
+    iferror
+	nip
+    then endtry ;
 is catch
 
 :noname ( y1 .. ym error/0 -- y1 .. ym / z1 .. zn error ) \ exception
     ?DUP IF
 	[ here forthstart 9 cells + ! ]
-	store-backtrace
+	first-throw @ IF
+	    store-backtrace error-stack off
+	    first-throw off
+	THEN
 	handler @ ?dup-0=-IF
 	    >stderr cr ." uncaught exception: " .error cr
 	    2 (bye)
 \	    quit
 	THEN
-	rp!
-	r> handler !
+	dup rp!
+	rdrop
 	r> lp!
 	r> fp!
-	r> swap >r sp! drop r>
-	rdrop 'throw
+	r> -rot 2>r sp! drop 2r>
+	r@ swap rp! perform
     THEN ;
 is throw
-