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From: wpp@marie.physik.tu-berlin.de (Kai Petzke)
Newsgroups: comp.lang.beta
Subject: nested patterns and leave/restart
Date: 12 Aug 95 16:34:27 GMT
Lines: 69
Message-ID: <wpp.808245267@marie>
NNTP-Posting-Host: marie.physik.tu-berlin.de (130.149.40.96)
NNTP-Posting-User: wpp
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Summary: How to handle cases, where a leave/restart is executed out of place?
Keywords: bug, leave, restart, pattern, nesting

BETA implements the LEAVE and RESTART imperatives, which leave a
block respective restart a block.  It is possible, to use LEAVE
and RESTART from blocks, that are nested within the block to
leave respective restart.  This yields an interesting problem,
though, which is best demonstrated with a code example:

	ORIGIN '~beta/basiclib/v1.4/betaenv'
	---program: Descriptor---
	(#
	   m: (# i: @integer enter i do 'p'->put; i->putint; newline #);
	   f1:
	     (# o: ^Object
	     do l1: &(# do 1->m; leave l1; 2->m #)[]->o[]
	     exit o[]
	     #);
	   f2: (# todo: ^Object enter todo[] do todo #);
	do
	   f1->f2; 3->m
	#)

This code compiles correctly with the Mjolner BETA system, but
when run on linux, it just prints "p1", then crashes with a bus
error.

The problem is, that the object descriptor following the l1 label
is not executed in place.  Rather, an object is dynamically
generated from that object descriptor, and executed later in a
different environment.  When the "leave l1" imperative is reached
there, it is quite understandable, that this causes confusion to
the run-time system.

My question is to how expand the BETA language defintion to
handle this particular case.  I can right now think of the
following two possibilities:

1) Forbid LEAVE and/or RESTART of enclosing blocks inside of
   patterns, that are not inserted items.
2) Allow LEAVE and/or RESTART also in non-safe places, like
   dynamically generated items or general attributes.  When such
   an unsafe LEAVE/RESTART is encountered at run-time, the call
   stack is traversed to check, if an apopriate label can be
   found.  If that is not the case, a runtime error occurs.

Note, that concept 1) is a restriction of the current semantics,
while concept 2) is a generalisation, a bit similiar to the
throw/try/catch mechanism of C++ or the setjmp()/longjmp() of C.
It would allow the following usage, which has been illegal so
far:

	patt:
	  (#
	     part1: (# do (if (* some error condition *) then leave patt if) #);
	     part2: (# ... #);
	  do part1; part2
	  #)

This example requests, that part1 is called directly or
indirectly from the do-part of patt.  In many cases, this is
exactly, what you want: abort a set of routines, if some error or
event occurs.



Kai
-- 
Kai Petzke                      | How fast can computers get?
Technical University of Berlin  |
Berlin, Germany                 | Warp 9, of course, on Star Trek.
wpp@marie.physik.tu-berlin.de   |