On 23 February 2010 20:15, Amit Deshwar wrote:

Hi Haskell-cafe My problem:  I'm trying to obtain the current position of the lexer once it reaches the end of the file (line and row number). I'm trying to do this in a function: getEndPosition = do   (a,b,c) <- alexGetInput   return a

Unfortunately, the type of a is 'Alex AlexPosn' instead of just 'AlexPosn' How do I strip the Alex so I'm left with just a AlexPosn object?

Hi Amit, Are you sure about the type of a? It looks like you are using the "monad" wrapper, described here: http://www.haskell.org/alex/doc/html/wrappers.html If that is true, then: alexGetInput :: Alex AlexInput and type AlexInput = (AlexPosn, Char, String)

From that we can infer from your code: getEndPosition :: Alex AlexPosn and thus: a :: AlexPosn

If you want to manipulate the value bound to a, you can simply apply a function to it in the body of getEndPosition, and return the result of that application (still inside the Alex type). Or you can use the function: runAlex :: String -> Alex a -> Either String a Cheers, Bernie.