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Re: Parsing and reporting errors?

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On 2016-09-30 18:03, Kaz Kylheku wrote:
> On 2016-09-30, Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>> On 2016-09-30 15:51, Stefan Ram wrote:
>>>   A parser (syntactical analyser) gets tokens from the scanner
>>>   (lexical analyser), which gets characters from a character source.
>>>   (In terms of OOP, each of these, the parser, the scanner and
>>>   the character source might be an individual object.)
>>>
>>>   It might happen that the parser detects a syntax error.
>>>   How can it then report this?
>>>
>>>   Ok, it might print the token and a diagnostic message.
>>>
>>>   But the parser does not know the line and the offset in the
>>>   line, so it can't print the line number and source context.
>>>   This has already been abstracted away by the lower entities
>>>   (the scanner and the character source). Usually, the parser
>>>   does not need to know these lower-level details.
>>>
>>>   How does one usually deal with this? (Printing the line number,
>>>   the line offset and the region of the source code where the
>>>   error was detected when the parser detects an error)
>>
>> By not using lexers, of course. The parser scans the source, the current
>> location in the source describes everything necessary to generate
>> message, file name, line number, start character, end character. There
>> are two source location pointers one to end of the parsed part and
>> another to the beginning of the unparsed one. The space in-between in
>> the lexeme being analyzed.
>>
>>>   However, due to "lookahead" or some "buffering",
>>>   the current position of the scanner might already
>>>   be ahead of that current token of the parser.
>>
>> There is no need to do lookahead etc. When the recursive descent parser
>> goes down it can pass the location where it started, so that you can
>> report errors like "Closing bracket expected at xxx:xx for the opening
>> one at xxx:xx"
>
> If your recursive descent parser has a function like
>
>    if ((kw = maybe_keyword(context)) {
>      switch (kw) {
>      case KW_IF: ...
>      case KW_DO: ...
>      }
>    } else ...
>
> then you *effectively* have lexer: it consists of the
> recursive descent functions that recognize /de facto/
> tokens.

Simply because the language has lexemes? There is no Lexer in the sense 
of an operation that takes the input and produces an output (a sequence 
of lexemes), so that the next stage does not start until the output of 
lexer is complete.

> For instance, the above maybe_keyword function has to scan
> multiple characters. If they turn out not to be a keyword,
> they all get pushed back into the stream, so that the
> maybe_ident(context) call can do its job.

Not really, normally the parser simply matches the source against the 
current table of alternatives expected:

    switch (Statements.Match (Source))
    {
       case If_Statement :
          ...
       case When_Statement :
          ...
          ...
       default :
          Error
            << "One of keywords "
            << Image (Statements)
            << " is expected at "
            <<  Image (Source.Location);

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

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Re: Parsing and reporting errors? "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2016-09-30 17:17 +0200
  Re: Parsing and reporting errors? Kaz Kylheku <221-501-9011@kylheku.com> - 2016-09-30 16:03 +0000
    Re: Parsing and reporting errors? "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2016-09-30 18:45 +0200
      Re: Parsing and reporting errors? Kaz Kylheku <221-501-9011@kylheku.com> - 2016-09-30 17:46 +0000
        Re: Parsing and reporting errors? "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2016-09-30 22:07 +0200
          Re: Parsing and reporting errors? Kaz Kylheku <221-501-9011@kylheku.com> - 2016-09-30 22:50 +0000

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