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