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• Peaker 15y ago
  This is an example from this parsing framework: text = lexeme[+(char_ - '<') [_val += _1]]; node = (xml | text) [_val = _1]; start_tag = '<' >> !lit('/') >> lexeme[+(char_ - '>') [_val += _1]] >> '>' ; end_tag = "</" >> string(_r1) >> '>' ; xml = start_tag [at_c<0>(_val) = _1] >> *node [push_back(at_c<1>(_val), _1)] >> end_tag(at_c<0>(_val)) ; The (>>) operator here is defined in the same as the Applicative/Monad bind: (>>). For the effect of full monadic bind (>>=), the explicit assignment into "val" is used. Here is the same example encoded using Monadic Parsec: text = many (notChar '<') node = xml <|> text start_tag = do char '<' notChar '/' val <- many (notChar '>') char '>' return val end_tag name = do string "</" string name char ">" xml = do name <- start_tag val <- node end_tag name return (name, val) Now, one interesting difference between Parsec and the spirit framework, is that Parsec instantiated the Monad class to allow composition of Parsers, whereas spirit defines its own ad-hoc combinators. This means that with parsec, I can use all of the existing monadic functions. For example, I can use the "replicateM" function, which replicates a monadic action N times to say I want 3 XML elements in sequence: replicateM 3 xml Or I can use one of the hundreds of others of available combinators. Of course boost spirit can also encode these combinators on top of their own ad-hoc operators, but they would be: * Duplicate code with every other monadic framework * Another vocabulary to learn in order to compose parsers * Cannot re-use any third party code that works with every monad in existence