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From: manis@UBC.CSNET (Vincent Manis)
Newsgroups: net.lang.mod2
Subject: Exceptions
Message-ID: <913:manis@cs.ubc.cdn>
Date: Mon, 3-Feb-86 17:26:24 EST
Article-I.D.: cs.913:manis
Posted: Mon Feb  3 17:26:24 1986
Date-Received: Wed, 5-Feb-86 05:06:08 EST
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I'm personally strongly opposed to hardwiring exception handling into 
programming languages. In part, this is because one wants to discourage
routine use of exceptions for other than error handling; in part, because
one of the major virtues of Modula-2 is its small size.

Exceptions, by their very nature, are global. One signals an exception, and
any module in the system (or at least in the dynamic execution history of
the program) can catch them. As a result, two modules may have different
ideas on what to do with a given exception. Thus, there are very good 
program structuring arguments against extensive use of exceptions. 

What many people have missed is that Modula-2 gives the programmer ''local
exceptions'', in the form of procedure variables. Rather than exporting an
exception, a module may export a procedure variable (or a procedure which
accepts a procedure parameter). The client module can assign this variable a
value; when an exceptional condition occurs, the procedure is invoked, and
does whatever the client deems appropriate.

It should be noted that this possibility is not present in Ada(TM), as
procedure variables and procedural parameters are not provided (the Steelman
specification explicitly ruled them out; I never understood why).

There are, however, a number of situations in which exception handling (or
more precisely non-local jumps) are necessary. Jeffrey McArthur's proposals
are quite reasonable (though I'd prefer to represent exception names by
integers rather than character strings, for efficiency's sake). The C
setjmp/longjmp facility, which McArthur references, is quite good enough by
itself, though, and very easy to implement.