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Annotation of gforth/except.fs, revision 1.9

1.1       anton       1: \ catch, throw, etc.
                      2: 
1.7       anton       3: \ Copyright (C) 1999,2000,2003 Free Software Foundation, Inc.
1.1       anton       4: 
                      5: \ This file is part of Gforth.
                      6: 
                      7: \ Gforth is free software; you can redistribute it and/or
                      8: \ modify it under the terms of the GNU General Public License
                      9: \ as published by the Free Software Foundation; either version 2
                     10: \ of the License, or (at your option) any later version.
                     11: 
                     12: \ This program is distributed in the hope that it will be useful,
                     13: \ but WITHOUT ANY WARRANTY; without even the implied warranty of
                     14: \ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
                     15: \ GNU General Public License for more details.
                     16: 
                     17: \ You should have received a copy of the GNU General Public License
                     18: \ along with this program; if not, write to the Free Software
1.3       anton      19: \ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
1.1       anton      20: 
                     21: \ !! use a separate exception stack?           anton
                     22: 
                     23: \ user-definable rollback actions
                     24: 
                     25: Defer 'catch
                     26: Defer 'throw
                     27: 
                     28: ' noop IS 'catch
                     29: ' noop IS 'throw
                     30: 
                     31: \ has? backtrace [IF]
                     32: Defer store-backtrace
                     33: ' noop IS store-backtrace
                     34: \ [THEN]
                     35: 
1.9     ! anton      36: \ Ok, here's the story about how we get to the native code for the
        !            37: \ recovery code in case of a THROW, and why there is all this funny
        !            38: \ stuff being compiled by TRY and RECOVER:
        !            39: 
        !            40: \ Upon a THROW, we cannot just return through the ordinary return
        !            41: \ address, but have to use a different one, for code after the
        !            42: \ RECOVER.  How do we do that, in a way portable between the various
        !            43: \ threaded and native code engines?  In particular, how does the
        !            44: \ native code engine learn about the address of the native recovery
        !            45: \ code?
        !            46: 
        !            47: \ On the Forth level, we can compile only references to threaded code.
        !            48: \ The only thing that translates a threaded code address to a native
        !            49: \ code address is docol, which is only called with EXECUTE and
        !            50: \ friends.  So we start the recovery code with a docol, and invoke it
        !            51: \ with PERFORM; the recovery code then rdrops the superfluously
        !            52: \ generated return address and continues with the proper recovery
        !            53: \ code.
        !            54: 
        !            55: \ At compile time, since we cannot compile a forward reference (to the
        !            56: \ recovery code) as a literal (backpatching does not work for
        !            57: \ native-code literals), we produce a data cell (wrapped in AHEAD
        !            58: \ ... THEN) that we can backpatch, and compile the address of that as
        !            59: \ literal.
        !            60: 
        !            61: \ Overall, this leads to the following resulting code:
        !            62: 
        !            63: \   ahead
        !            64: \ +><recovery address>-+
        !            65: \ | then               |
        !            66: \ +-lit                |
        !            67: \   (try)              |
        !            68: \   ...                |
        !            69: \   (recover)          |
        !            70: \   ahead              |
        !            71: \   docol: <-----------+
        !            72: \   rdrop
        !            73: \   ...
        !            74: \   then
        !            75: \   ...
        !            76: 
        !            77: \ !! explain handler on-stack structure
        !            78: 
1.5       anton      79: : (try) ( ahandler -- )
1.1       anton      80:     r>
1.5       anton      81:     swap >r \ recovery address
1.1       anton      82:     rp@ 'catch >r
                     83:     sp@ >r
                     84:     fp@ >r
                     85:     lp@ >r
                     86:     handler @ >r
                     87:     rp@ handler !
                     88:     backtrace-empty on
1.5       anton      89:     >r ;
1.1       anton      90: 
                     91: : try ( compilation  -- orig ; run-time  -- ) \ gforth
1.5       anton      92:     \ !! does not work correctly for gforth-native
1.8       anton      93:     POSTPONE ahead here >r >mark 1 cs-roll POSTPONE then
1.9     ! anton      94:     r> POSTPONE literal POSTPONE (try) ; immediate compile-only
1.1       anton      95: 
                     96: : (recover) ( -- )
                     97:     \ normal end of try block: restore handler, forget rest
                     98:     r>
                     99:     r> handler !
                    100:     rdrop \ lp
                    101:     rdrop \ fp
                    102:     rdrop \ sp
                    103:     r> rp!
                    104:     rdrop \ recovery address
                    105:     >r ;
                    106: 
                    107: : recover ( compilation  orig1 -- orig2 ; run-time  -- ) \ gforth
                    108:     \ !! check using a special tag
                    109:     POSTPONE (recover)
1.9     ! anton     110:     POSTPONE else
        !           111:     docol: here 0 , 0 , code-address! \ start a colon def 
        !           112:     postpone rdrop                    \ drop the return address
        !           113: ; immediate compile-only
1.1       anton     114: 
                    115: : endtry ( compilation  orig -- ; run-time  -- ) \ gforth
                    116:     POSTPONE then ; immediate compile-only
                    117: 
                    118: :noname ( x1 .. xn xt -- y1 .. ym 0 / z1 .. zn error ) \ exception
                    119:     try
                    120:        execute 0
                    121:     recover
                    122:         nip
                    123:     endtry ;
                    124: is catch
                    125: 
                    126: :noname ( y1 .. ym error/0 -- y1 .. ym / z1 .. zn error ) \ exception
                    127:     ?DUP IF
                    128:        [ here forthstart 9 cells + ! ]
                    129:        store-backtrace
                    130:        handler @ ?dup-0=-IF
1.6       anton     131:            >stderr cr ." uncaught exception: " .error cr
1.1       anton     132:            2 (bye)
1.6       anton     133: \          quit
1.1       anton     134:        THEN
                    135:        rp!
                    136:        r> handler !
                    137:        r> lp!
                    138:        r> fp!
                    139:        r> swap >r sp! drop r>
1.9     ! anton     140:        rdrop 'throw r> perform
1.1       anton     141:     THEN ;
                    142: is throw
                    143: