Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.java.programmer > #37808

Re: Two exceptions

Show all headers | View raw

Responding to Eric Sosman:
> On 12/9/2017 11:15 AM, Stefan Ram wrote:
>>    The type of a result of a parse is
>>
>> class ParseResult { ...
>>
>>    which can have subclasses
>>
>> class ProperParseResult extends ParseResult { ...
>> class ErrorParseResult extends ParseResult { ...
>>
>>    which in turn can have several subclasses each of which
>>    might implement several interfaces.
>>
>>    A parseResult might contain other parseResults (AST).
>
>      This would work, yes, but I think it would lead to clumsy
> code at the call site, something like
>
>       ParseResult result = parse();
>       if (result instanceof ProperParseResult) {
>          ProperParseResult ppr = (ProperParseResult) result;
>          ...
>       } else if (result instanceof ErrorParseResult) {
>           ErrorParseResult epr = (ErrorParseResult) result;
>           ...
>       } else {
>           throw new IllegalStateException("bad result", result);
>       }

Not necessarily, or rather, not at all. Ignoring the AST suggestion...

ParseResult could declare a #matched() method, equivalent to the
#matched field in your single-class example. Handling would be identical
to your suggestion, but the types would be clearer, IMHO. Members that are
"only relevant if" another member carries a certain value feels like a code
smell to me, too.

There could also be a ParseResult#resultValue() method that throws in the
ErrorParseResult case. This would give the top level caller the same experience
 as in the exception-signaling approach, but based on a cleaner implementation.

Furthermore there could be a ParseResult#fold[T]() method that takes two
lambdas, Object -> T and Throwable -> T, and invokes the corresponding lambda
for the success resp. error case.

Finally, the instanceof cascade could be replaced with a proper Visitor
implementation over ParseResult. Stefan mentioned the Visitor pattern, but
constrained it to the tree traversal context. It is primarily about double
dispatch, though, and would apply nicely to the ParseResult hierarchy, where
there's no tree in sight.

Actually, all the approaches listed above could be supported in parallel,
leaving the result resolution style to the discretion of the caller - which is
what the author(s) of parsecj[1] opted for (excluding the Visitor option).

The next step from this approach might be the introduction of combinators that
merge multiple results into one.

Best regards,
Patrick

[1] https://github.com/jon-hanson/parsecj#types

Back to comp.lang.java.programmer | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Two exceptions Victor Porton <porton@narod.ru> (Victor Porton) - 2017-12-09 00:53 +1200
  Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> (Eric Sosman) - 2017-12-08 18:21 +1200
    Re: Two exceptions Victor Porton <porton@narod.ru> (Victor Porton) - 2017-12-09 01:55 +1200
      Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> (Eric Sosman) - 2017-12-09 07:39 +1200
        Re: Two exceptions Victor Porton <porton@narod.ru> (Victor Porton) - 2017-12-10 02:39 +1200
          Re: Two exceptions Victor Porton <porton@narod.ru> (Victor Porton) - 2017-12-10 02:46 +1200
          Re: Two exceptions Kiwi User <martin@mydomain.invalid> (Kiwi User) - 2017-12-10 00:52 +1200
            Re: Two exceptions Kiwi User <martin@mydomain.invalid> - 2017-12-10 21:50 +0000
      Re: Two exceptions Patrick Roemer <sangamon@netcologne.de> (Patrick Roemer) - 2017-12-09 13:47 +1200
  Re: Two exceptions Arne Vajhøj <arne@vajhoej.dk> (Arne Vajhøj) - 2017-12-08 18:49 +1200
    Re: Two exceptions v_borchert@despammed.com (Volker Borchert) (Volker Borchert) - 2017-12-09 07:53 +1200
  Re: Two exceptions ram@zedat.fu-berlin.de (Stefan Ram) (Stefan Ram) - 2017-12-09 16:15 +1200
    Re: Two exceptions Eric Sosman <esosman@comcast-dot-net.invalid> (Eric Sosman) - 2017-12-09 14:55 +1200
      Re: Two exceptions Patrick Roemer <sangamon@netcologne.de> (Patrick Roemer) - 2017-12-11 15:09 +1200
        Re: Two exceptions Victor Porton <porton@narod.ru> - 2017-12-12 11:14 +0200

csiph-web