calc :: String -> [Float]
 calc = foldl f [] . words
   where 
     f (x:y:zs) "+" = (y + x):zs
     f (x:y:zs) "-" = (y - x):zs
     f (x:y:zs) "*" = (y * x):zs
     f (x:y:zs) "/" = (y / x):zs
     f xs y         = read y : xs