Am Sonntag 08 November 2009 18:53:48 schrieb John Moore:
Hi, I'm trying to find a way to parseRPN (Reverse Polish Numbers) to expressions rather than to just numbers. e.g. I want the answer to be in the form
(Multiply (Val 2) (Val 3)) rather than just the answer.
I'd suggest using something like type Stack = [Expression] parseRPN :: Parser Expression parseRPN = rpn [] parseVal :: Parser Expression parseVal = do num <- parseNumber return (Val num) rpn :: Stack -> Parser Expression rpn stack = (do char '+' case stack of (x:y:ts) -> rpn (Add x y:ts) _ -> parsecFail "BinOp requires two values on Stack") <|> (do char '*' case stack of (x:y:ts) -> rpn (Multiply x y:ts) _ -> parsecFail "BinOp requires two values on Stack") <|> ... <|> (do v <- parseVal rpn (v:stack)) <|> (do eof case stack of [x] -> return x _ -> parsecFail "No parse") -- You could also use the userstate of Parsec for the stack
Are these anyway near the steps parseRPN :: String->Expression
This is a lot more complicated then I thought.!!!
First do we have to read in a string is this (IsString)
fromString :: String -> a
Then this goes on a stack
pushStack :: a -> Stack -> Stack (Takes a value and puts in on a stack)
Later we pop it off
popStack :: Stack -> (a,Stack) -- takes the value of the stack and leaves the stack
Do we also have to define taking off the stack such as head(popstack) or fst(popstack) if we do we would probably have one for putting it onto a stack.
Do we then turn the value into an Expression.?