Haskell lexer help

[ckwolfe]

Hello everyone, I've been doing a project for class in Haskell. Basically I've completed my program except some parts in my lexer function. I have functions marked undefined where I haven't completed it.

lexer :: String -> Pos -> [Token]
lexer str pos = 
     case str of
       x:xs | isSpace x -> lexer xs (updatePosChar pos x)
       x:xs | isIdChar x -> identifierOrKeyword [x] xs pos
       x:xs | isDigit x ->  identifierOrKeyword [x] xs pos
       x:xs | x == '\'' ->  result1 : result2 where
			result1 = TokPos pos (reserved [x])
			result2 = lexer xs newPos
			newPos = updatePosChar pos x
       x:xs | isSep1 [x] -> TokPos pos (reserved [x]) : 
                            lexer xs (incSourceCol pos 1)
       x:y:xs | isComment [x, y] -> undefined -- FIXME
       x:y:xs | isOp2 [x, y] -> undefined -- FIXME
       x:xs | isOp1 [x] -> TokPos pos (reserved [x]) :
			lexer xs (incSourceCol pos 1)
       x:_ -> error (show pos ++ ": Lexer error: unknown character " ++ [x]) 
       [] -> []
     where
       identifierOrKeyword :: String -> String -> Pos -> [Token]
       identifierOrKeyword ident str pos = 
           case str of 
             x:xs | isIdChar x || isDigit x -> identifierOrKeyword (ident ++ [x]) xs pos
             xs -> let newp = updatePosString pos ident
                       newtok = if isKeyword ident
				then reserved ident
				else TokIdent ident 
                   in TokPos pos newtok : lexer xs newp

The line I need help on is:

x:y:xs | isComment [x, y] -> undefined -- FIXME

I have tried numerous things, but reverted to calling it undefined so I could work on the rest of the function.
You can see how I do the other cases such as isOp1 [x], etc.. I know how these work when I am only expecing a single character or predictable string (like "Bool", "True", etc...) but when I need to recognize a comment such as "//here is a comment" I need to be able to make the program understand if // then all characters before /n is part of that comment. That is what y is for. But i can't figure out the syntax to make it work.

If you need additional explanation to help me, post it. Also, please ignore any possible formatting error, the copy&paste didn't go over very well :D
Thanks

[Rashakil Fol]

You need to make a helper function, analogous to identifierOrKeyword, that scans past the first newline in the string. The span or break functions might come in handy.