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From: Ruvim <ruvim.pinka@gmail.com>
Newsgroups: comp.lang.forth
Subject: Re: Back & Forth - Co-routines
Date: Mon, 3 Feb 2025 13:07:36 +0400
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On 2025-02-02 15:13, albert@spenarnc.xs4all.nl wrote:
> In article <vnkvvq$2a5o$1@dont-email.me>, Ruvim  <ruvim.pinka@gmail.com> wrote:
>> On 2025-01-31 11:33, dxf wrote:
>>>
>>> Not sure if previously mentioned but here's another version of LOCAL
>>>
>>> : ;:  >r ;
>>>
>>> : LOCAL ( x adr -- )
>>>     r> -rot dup @ over 2>r ! ;: 2r> ! ;
>>>
>>> variable A  variable B  8 a !  7 b !
>>>
>>> : divide ( a b -- )  b local  a local
>>>     a @  b @  /  . cr  ;
>>>
>>> 15 3 divide  a ? b ?
>>>
>>
>>
>> This approach does not work well with catch/throw. Because `throw` must
>> restore the values of all "local" variables that are used in the
>> definitions whose execution is being terminated. And this is difficult
>> to implement.
>>
>>
>> See also 13.3.3.1, item c,
>> <https://forth-standard.org/standard/locals#subsubsection.13.3.3.1>
>>
>> | ABORT shall release all local storage resources,
>> | and CATCH / THROW (if implemented) shall release
>> | such resources for all definitions whose execution
>> | is being terminated.
> 
> Nice catch!
> 
> However, this is highly artificial. You have to have a recursive routine in this
> vein:
> 
> RECURSIVE
> : fun .. fun ... fun . 'fun CATCH .. fun  ... ;
> 
> otherwise the global VARIABLE's can be ignored.
> 
> I would be interested to see a remotely plausible example of this.
> Mixing recursion and exception is ill advised by iq<160.


Recursion is not necessary. It is enough to use the same-name "local" 
variables in different functions, some of which throw exceptions, and 
other catch exceptions.

An artificial example:


: local ( x2 addr1 -- ; R: nest-sys1 -- x1 addr1 nest-sys.xt nest-sys1 )
   \ This definition assumes that nest-sys size is 1 cell,
   \ and xt is a subtype of nest-sys
   r>  ( x2 addr nest-sys1 )
   over dup @ >r >r  [: 2r> ! ;] >r
   ( x2 addr1  nest-sys1 ) >r  !
;


: idiv ( n1 n2\0 -- n3  |  n1 0 -- never )
   dup if / exit then -10 throw
;

variable a
variable b

: foo ( n1 n2 -- )
   b local  a local
   a @  b @  idiv
   ."  idiv result is " . cr
;
: bar ( u1 -- u1 )
   a local
   100  a @  ['] foo catch if 2drop then
   a @
;

0 bar .
\ this must print 0, but will print 10



--
Ruvim