Here's what I've got so far. -- Text is considered everything up to //. However, the problem -- is that this consumes the //. parseText = manyTill anyChar (try (string "//")) -- Because the // is already consumed, parseKeyword just grabs -- the available letters. parseKeyword :: Parser String parseKeyword = many1 letter -- Test function. parseSome = do t1 <- parseText k1 <- parseKeyword t2 <- parseText return (t1,k1,t2) On "some text//keyword more text//" this gives ("some text","keyword"," more text") On "some text//keyword more text" this gives the error "expecting //" I wonder how I can get the manyTill to be happy with eof before finding the //? I tried parseText = manyTill anyChar (try (string "//") <|> eof) but got a type error. minh thu wrote:

2009/4/17 Michael P Mossey :

...

I want to write a parser that can read a file with this format: the file has sections which are demarcated by keywords. Keywords always begin with two forward slashes and consist of letters, digits, and underscore. The text can be anything, including special characters. For instance:

//keyword some text and more text //another_keyword and) some { more text //ya_keyword $$ -- text

I'm not sure how to write a parser that considers anything but a double slash to be a valid part of the text.

Maybe you can use a combination of 'many', 'noneOf' or 'manyTill' ?

Cheers, Thu

My confusion is that text is by definition followed by // or eof. minh thu wrote:

You can use 'notFollowedBy' (probably with 'many1' and 'try'). Something like (untested):

notFollowedBy (try $ string "//")

Thu

2009/4/17 Michael Mossey :

...

Here's what I've got so far.

-- Text is considered everything up to //. However, the problem -- is that this consumes the //. parseText = manyTill anyChar (try (string "//"))

-- Because the // is already consumed, parseKeyword just grabs -- the available letters. parseKeyword :: Parser String parseKeyword = many1 letter

-- Test function. parseSome = do t1 <- parseText k1 <- parseKeyword t2 <- parseText return (t1,k1,t2)

On "some text//keyword more text//" this gives

("some text","keyword"," more text")

On "some text//keyword more text"

this gives the error "expecting //"

I wonder how I can get the manyTill to be happy with eof before finding the //? I tried

parseText = manyTill anyChar (try (string "//") <|> eof)

but got a type error.

minh thu wrote:

...

2009/4/17 Michael P Mossey :

...

I want to write a parser that can read a file with this format: the file has sections which are demarcated by keywords. Keywords always begin with two forward slashes and consist of letters, digits, and underscore. The text can be anything, including special characters. For instance:

//keyword some text and more text //another_keyword and) some { more text //ya_keyword $$ -- text

I'm not sure how to write a parser that considers anything but a double slash to be a valid part of the text. Maybe you can use a combination of 'many', 'noneOf' or 'manyTill' ?

Cheers, Thu

---

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Jason Dusek wrote:

2009/04/17 minh thu :

...

2009/04/17 Michael Mossey :

...

I wonder how I can get the manyTill to be happy with eof before finding the //? I tried

parseText = manyTill anyChar (try (string "//") <|> eof)

but got a type error. You can use 'notFollowedBy' [...]

You get a type error because `string "//"` parses to a `String` while `eof` parses to a `()`. Instead you might use:

parseText = manyTill anyChar (try (string "//" >> return ()) <|> eof)

-- Jason Dusek

Ah.. I think I get it... in the function manyTill, the second argument type doesn't matter.. doesn't have to match the first argument type. Here's what I have so far. It works, but it's a bit weird to consume the // as part of the text rather than the keyword. That happens because the try( string "//" ), which is part of the end arg to manyTill, consumes the // when it succeeds. But maybe it is the most natural way to express the problem. parseKeyword :: Parser String parseKeyword = many1 (alphaNum <|> char '_') parseText :: Parser String parseText = manyTill anyChar ((try (string "//") >> return ()) <|> eof) parsePair :: Parser (String,String) parsePair = do k <- parseKeyword t <- parseText return (k,t) parseFile :: Parser [(String,String)] parseFile = do _ <- parseText -- to skip any text at beginning and 'sync up' p <- many parsePair return p

I've just about got this parser working, but wondering about something. Turns out I need "try" inside the "lookahead" here. parseText :: Parser String parseText = manyTill anyChar $ lookAhead (try (string "//")) Without try, if I give it an input with a single slash, like "some/text" It stops with the error "unexpected t; expecting //" I'm curious why that happens when lookAhead is used with manyTill like this. I was under the impression that if the end parser given to manyTill failed, then manyTill would just continue with the main parser. Apparently there are two ways to fail: in some contexts, failing means that manyTill will just continue. In other contexts, such as the one above, there is some sense in which 'string' demands the entire string to be present. Can anyone explain? Thanks, Mike