Re: [Haskell-beginners] Consuming Rule Based Parsing

On 11/16/2012 06:04 PM, Karl Voelker wrote:

On Fri, Nov 16, 2012 at 5:13 PM, Christopher Howard mailto:christopher.howard@frigidcode.com> wrote:

Thank you for your help. Can you elaborate a little more on your explanation? So, would "a" be some data type representing the atoms of the grammar? Or some kind of recursive data type that could represent any kind of valid structure? Or...?

The type parameter "a" is the output of that particular parser. So, a parser for an integer might have type String -> Integer, while a parser for some complicated data type Foo would have type String -> Foo.

I'll wait until I'm clear on that point before asking about how I would combine parsers. (Presumably, two combined parsers would both have to have the same "a" type.)

Since the parsers in this scheme are just functions, there are endless ways they could be combined, and the input and output types may or may not match.

Some combinators you will probably want:

andThen :: Parser a -> (a -> Parser b) -> Parser b orElse :: Parser a -> Parser a -> Parser a

And you might also want:

succeedWith :: a -> Parser a

-Karl

Maybe I'm thinking about this all wrong... But it isn't quite clear to me how I make a generic function or operator that combines two Parsers of one (or two) types to make another Parser of a separate type. Say, for instance, I have a grammar which consists of atomic Nouns, atomic Verbs, and Sentences which are a combination of the two. So naturally I'd expect to have something like: data Noun = Noun String data Verb = Verb String data Sentence = Sentence Noun Verb nounParser :: Parser Noun nounParser = ... verbParser :: Parser Verb verbParser = ... sentenceParser :: Parser Sentence sentenceParser = nounParser <+> verbParser (<+>) :: ? (<+>) f g = ? -- frigidcode.com