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From: nospam2159@cs.au.dk (Peter von der =?iso-8859-1?q?Ah=E9?=)
Newsgroups: comp.lang.beta
Subject: Re: exceptions: (atle // confused (# again #) atle)
Date: 19 Jun 2000 18:44:55 +0200
Organization: Computer Science Department of the University of Aarhus
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Xref: news.cs.au.dk comp.lang.beta:12434

Atle, this might not help you, but anyways...

Let my elaborate a bit on the topic of exceptions.

I think that the exception mechanism in BETA is very inflexible and
inconvenient.

One of the very strong features of exceptions is that it allows you to
write programs as they should have looked if no errors occurs,
consider:

"file.txt" -> file.open;  (* open file for reading *)
(* an error can occur here, eg. file.txt does not exist *)
file.getInt -> i; (* read an integer *)
(* an error can occur here, eg. the first word in the file is not an
 * integer *)

In most modern language you have a try/catch mechanism (in BETA pseudo
code):

try(#
      catch::
        (# (* handle errors *) #)
   do
      file.txt" -> file.open;
      file.getInt -> i;
   #)

But in BETA you have to say something like:

     "file.txt" -> file.open
       (# noSuchFileError::(# (* handle error *) #) #);
     file.getInt
       (# syntaxError::(# (* handle error *) #) #)
       -> i;

Consider this:

file.getInt -> i;
file.getInt -> j;
file.getInt -> k;

You can either make a specialised version of file or handle exceptions
at each line explictly.  Well of cause, you would select to make a
specialised version - that's the most convenient and resembles the
try/catch mechanism.  But then consider the task of implementing a
parser.  Assume that this parser should be able to handle all kinds of
streams, both text and file and others not even invented yet.  Then
you realize that BETA exception isn't very convenient.

This leads me to my point:  BETA does not have exceptions.  I think
the most needed language feature in BETA is exceptions.  Alternatively
closures could be implemented properly, that is allow:

p: (# closure:@(# do leave p #)
    exit closure[]
    #)
c:^object;
do
   p -> c; c;

This could be used to implement proper exceptions in BETA.

-- 
YMMV