Personally, I'd completely separate the parsing from the evaluation here. This would create a parse function in the Either monad, and an eval function that doesn't need to be concerned with errors. Bob On 28 Apr 2011, at 08:26, Sean Perry wrote:
I just finished "Learn you a Haskell". Fine book, really enjoyed it. Early on he develops a very simple RPN evaluator. I played with it a bit and expanded it to use Either instead of Maybe. Also, '_' recalls the last value so you can do:
10 5 + 15.0 _ 3 * 45.0 _ 5 / 9.0
This is one of my first complete pieces of Haskell. I would appreciate comments on style, formatting, etc. There are comments in places I am seeking particular guidance.
If you are also reading along in LYAH, warning spoiler alert......
import Control.Monad (foldM, liftM) import Control.Monad.Error import System.Exit (exitFailure, exitSuccess) import System.IO (hFlush, stdout) import System.IO.Error (isEOFError)
readNumMaybe :: String -> Either String Double readNumMaybe st = case reads st of [(x, "")] -> Right x [(x, s)] -> Left $ "incomplete parse: '" ++ st ++ "'" [] -> Left $ "unknown value: '" ++ st ++ "'"
evalRPN :: [Double] -> String -> Either String [Double] evalRPN _ "clear" = return [] evalRPN (x:y:xs) "+" = return $ (y + x):xs evalRPN (x:y:xs) "-" = return $ (y - x):xs evalRPN (x:y:xs) "*" = return $ (y * x):xs evalRPN (x:y:xs) "/" = return $ (y / x):xs evalRPN (x:y:xs) "^" = return $ (y ** x):xs evalRPN (x:xs) "ln" = return $ (log x):xs evalRPN xs "sum" = return $ [sum xs] evalRPN xs num = liftM (:xs) (readNumMaybe num)
evalRPNTokens :: [String] -> Either ([String], String) (Maybe Double) evalRPNTokens tokens = case foldM evalRPN [] tokens of Right [] -> Right Nothing Right [x] -> Right (Just x) Right result -> Left $ (tokens, show (length tokens) ++ " items left on stack.") Left msg -> Left (tokens, msg)
-- is this used with map ok or am I missing a standard function? replaceif p new s = if s == p then new else s
showEval :: String -> Either ([String], String) (Maybe Double) -> IO String showEval _ (Right Nothing) = return "" showEval _ (Right (Just result)) = do -- show result then use this value for the next iteration print result return $ show result showEval last (Left (input, msg)) = do putStrLn $ "Error: '" ++ unwords input ++ "': " ++ msg -- do not let errors eat the last successful value return last
getLineOrQuit = catch getLine (\e -> if isEOFError e then putStr "\n" >> exitSuccess else exitFailure)
prompt = "> "
evalLoop :: String -> IO () evalLoop last = do putStr prompt hFlush stdout -- ensure the prompt is shown
-- is this too "cute"?? result <- getLineOrQuit >>= showEval last . evalRPNTokens . map (replaceif "_" last) . words evalLoop result
main = evalLoop ""
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