Re: EBNF

VBDis wrote:
  > I would like to hear opinions about the differences
  > between formal lexer and parser grammars.


Usually, the difference is in the expressiveness of the grammars: the
lexical syntax is a set of regular expressions and the 'parser grammar'
is a context-free grammar over the kind of tokens defined by the lexical
syntax. This separation is inspired by performance concerns: by
separating the input in tokens, the number of elements the parser has to
consider is reduced.


In scannerless parsing there is no separate lexical analysis phase:
every input character is a token. I think that this is conceptually even
*cleaner* then using a separate scanner. The problem of using a separate
scanner is that the context of the token in the input cannot be considered.


VBDis wrote:
> IMO it's not a good idea to
> use the same meta language for both kinds of grammars


Why isn't this a good idea? Scanners


> even if it were
> possible to construct such a super language?


It is possible: SDF ( http://syntax-definition.org ) integrates the
definition of the 'lexer grammar' and 'parser grammar'. SDF is
implemented by scannerless generalized LR parsing.


Cheers,
Martin Bravenboer

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