[Haskell-beginners] newbie seeking code review

[Thomas Davie]

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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