Main.hs

module Main where

import Text.Parsec
import Text.Parsec.String



main :: IO ()
main = do
  input <- getLine
  case runP parseTree () "stdin" input of
    Right n -> print (reduce n)
    Left err -> print err

data Tree
  = Cons Float
  | Minus Tree Tree
  | Plus Tree Tree
  | Div Tree Tree
  | Mult Tree Tree
  deriving Show

reduce :: Tree -> Float
reduce (Cons x) = x
reduce (Plus x y) = (reduce x) + (reduce y)
reduce (Minus x y) = (reduce x) - (reduce y)
reduce (Mult x y) = (reduce x) * (reduce y)
reduce (Div x y) = (reduce x) / (reduce y)

parseTree :: Parser Tree
parseTree =
  many space *> (
  try (Cons <$> float) <|>
  try (op "+" (Plus <$> parseTree <*> parseTree)) <|>
  try (op "-" (Minus <$> parseTree <*> parseTree)) <|>
  try (op "*" (Mult <$> parseTree <*> parseTree)) <|>
  try (op "/" (Div <$> parseTree <*> parseTree))
  )
  where
    float = read <$> many1 (digit <|> char '.') -- hacky af
    op :: String -> Parser Tree -> Parser Tree
    op o p = string o *> p