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Re: Two exceptions

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Victor Porton wrote:

> 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.

Hm, well, I should have considered this. In Ada this can be made with a 
constructor function (just like as in Java) and an accessor hiding 
Indefinite_Wrapper as a details of implementation. Weird that I have not 
considered this. Well, maybe this is because I didn't wonder about 
performance very much and considered many exceptions OK.

But thanks for the idea. I think, I will follow your advice and remove 
ParseException altogether leaving only FatalParseException.

>> ... 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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