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Re: Two exceptions

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Eric Sosman wrote:

> On 12/8/2017 6:55 PM, Victor Porton wrote:
>> Eric Sosman wrote:
>> 
>>> On 12/8/2017 5:53 PM, Victor Porton wrote:
>>>> [... using exceptions to control a recursive descent parser ...]
>>> [... maybe exceptions aren't the best tool ...]
>>>       Instead of announcing a "venial" mismatch by throwing an
>>>       exception,
>>> consider returning a status value from each parse method.  This might
>>> be a boolean ("It's an X1" / "It's not an X1") or a reference value
>>> ("It's an X1 and here it is" / "It's not an X1 and here's a null"),
>>> or something more structured ("Here's a status-and-result-and-info
>>> object; look at its fields to find out what happened").  [...]
>> 
>> The main reason I chose to use exceptions for this, is to allow parse()
>> method to return the result of parsing (not a boolean value). For example
>> result of parsing an integer may return "long".
>> 
>> Can you propose another alternative? It is to be easily programmable (if
>> I find handling exceptions too slow, I always can rewrite my program).
>> 
>> Oh, I see, you proposed to return "null" instead of my ParseException. It
>> however also seems to me not a quite good solution, because returning
>> null may be a non-exceptionable situation (as for a missing optional
>> object in the parsed resource).
> 
>      If `null' is a legitimate result (so you can't use a `null' return
> to mean "Not matched"), use a status-and-result-and-info object like
> 
> class ParseResult {
> final boolean matched;  // true = success, false = failure
> final Object result;  // only relevant if matched == true
> // ... other info as desired ...
> }

I haven't considered this perhaps because Ada does not allow a class to be a 
member of object (without wrapping it into something called 
Indefinite_Wrapper or something similar). So in Ada this would be 
inconvenient.

> ... and return a ParseResult instance from each parse() method.  The
> `result' field might be `null', but if `matched' is true the `null' is
> a "real" `null', not a failure indication.
> 
-- 
Victor Porton - http://portonvictor.org

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Two exceptions Victor Porton <porton@narod.ru> - 2017-12-09 00:53 +0200
  Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-12-08 18:21 -0500
    Re: Two exceptions Victor Porton <porton@narod.ru> - 2017-12-09 01:55 +0200
      Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-12-09 07:39 -0500
        Re: Two exceptions Victor Porton <porton@narod.ru> - 2017-12-10 02:39 +0200
          Re: Two exceptions Victor Porton <porton@narod.ru> - 2017-12-10 02:46 +0200
          Re: Two exceptions Kiwi User <martin@mydomain.invalid> - 2017-12-10 00:52 +0000
      Re: Two exceptions Patrick Roemer <sangamon@netcologne.de> - 2017-12-09 13:47 +0100
  Re: Two exceptions Arne Vajhøj <arne@vajhoej.dk> - 2017-12-08 18:49 -0500
    Re: Two exceptions v_borchert@despammed.com (Volker Borchert) - 2017-12-09 07:53 +0000
  Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-12-09 14:55 -0500
    Re: Two exceptions Patrick Roemer <sangamon@netcologne.de> - 2017-12-11 15:09 +0100

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