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import Data.List
import Control.Monad
import Text.Read
foldingFunction :: [Double] -> String -> Maybe [Double]
foldingFunction (x:y:ys) "*" = return ((y * x):ys)
foldingFunction (x:y:ys) "/" = return ((y / x):ys)
foldingFunction (x:y:ys) "+" = return ((y + x):ys)
foldingFunction (x:y:ys) "-" = return ((y - x):ys)
foldingFunction xs numberString = liftM (:xs) (readMaybe numberString)
solveRPN :: String -> Maybe Double
solveRPN st = do
[result] <- foldM foldingFunction [] (words st)
return result
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