parse :: Bool -> Bool -> String -> Double
parse b1 b2 xs = case span (`notElem` "+-") xs of
    (y, x:ys  )     -> parse b1 b2 y + parse (x == '+') True ys
    (y, _     )     -> case span (`notElem` "*/") y of
        (y, x:xs  ) -> parse b1 b2 y * parse True (x == '*') xs
        ([],_     ) -> 0
        (x, _     )
            | b1 && b2     ->        read x
            | b2           -> (-1) * read x
            | b1 && not b2 ->   1  / read x
            | otherwise    ->  -1  / read x
 
main = interact (show . (parse True True))
 
 

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MyArIDEwMC8xMC8yLzUgLSA0IC0gNC8yCg==

3 + 100/10/2/5 - 4 - 4/2