Solving expression in Reverse Polish Notation using Haskell pattern match:

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solveRPN :: (Read a, Num a) => String -> a
solveRPN line = head $ foldl push [] $ words line
  where
    push (y:x:res) "*" = (x*y) : res
    push (y:x:res) "+" = (x+y) : res
    push (y:x:res) "-" = (x-y) : res
    push res n = read n : res

main = do
  print $ solveRPN "10 4 3 + 2 * -" -- => -4