data Console a
= PutStrLn String a
| GetLine (String -> a)
deriving (Functor)
type ConsoleM = Free Console
runConsole :: Console (IO a) -> IO a
runConsole cmd =
case cmd of
(PutStrLn s next) -> do
putStrLn s
next
(GetLine nextF) -> do
s <- getLine
nextF s
runConsoleM :: ConsoleM a -> IO a
runConsoleM = iterM runConsole
consolePutStrLn :: String -> ConsoleM ()
consolePutStrLn str = liftF $ PutStrLn str ()
consoleGetLine :: ConsoleM String
consoleGetLine = liftF $ GetLine id
data File a
= ReadFile FilePath (String -> a)
| WriteFile FilePath String a
deriving (Functor)
type FileM = Free File
runFile :: File (MaybeT IO a) -> MaybeT IO a
runFile cmd = case cmd of
ReadFile path next -> do
fileData <- safeReadFile path
next fileData
WriteFile path fileData next -> do
safeWriteFile path fileData
next
runFileM :: FileM a -> MaybeT IO a
runFileM = iterM runFile
rightToMaybe :: Either a b -> Maybe b
rightToMaybe = either (const Nothing) Just
safeReadFile :: FilePath -> MaybeT IO String
safeReadFile path =
MaybeT $ rightToMaybe <$> (try $ readFile path :: IO (Either IOException String))
safeWriteFile :: FilePath -> String -> MaybeT IO ()
safeWriteFile path fileData =
MaybeT $ rightToMaybe <$> (try $ writeFile path fileData :: IO (Either IOException ()))
fileReadFile :: FilePath -> FileM String
fileReadFile path = liftF $ ReadFile path id
fileWriteFile :: FilePath -> String -> FileM ()
fileWriteFile path fileData = liftF $ WriteFile path fileData ()
data Program a = File (File a) | Console (Console a)
deriving (Functor)
type ProgramM = Free Program
runProgram :: Program (MaybeT IO a) -> MaybeT IO a
runProgram cmd = case cmd of
File cmd' ->
runFile cmd'
Console cmd' ->
-- ????
runProgramM :: ProgramM a -> MaybeT IO a
runProgramM = iterM runProgram
我想撰寫兩個免費的 monadConsoleM和FileM. 所以，我做了組合函子Program。然后我開始撰寫解釋器函式runProgram，但我無法定義函式。因為runConsole和MaybeT IO a型別不匹配。如何將 runConsole 函式提升runConsole :: Console (IO a) -> IO a為具有型別Console (MaybeT IO a) -> MaybeT IO a？
（我想用 Free monads 來實作這個程式，而不是 Eff monad。）
uj5u.com熱心網友回復：
現在你有cmd'type Console (MaybeT IO a)，并且想把它傳遞給一個函式Console (IO a)。您可以做的第一件事是在MaybeT里面運行 monadConsole并獲取Console (IO (Maybe a)). 您可以通過fmapping來執行此操作runMaybeT。
一旦你得到Console (IO (Maybe a))，你可以將它傳遞給runConsole并得到IO (Maybe a)。現在，您可以將其提升到MaybeT IO a使用MaybeT.
所以它會是這樣的。
runProgram :: Program (MaybeT IO a) -> MaybeT IO a
runProgram cmd = case cmd of
File cmd' ->
runFile cmd'
Console cmd' ->
MaybeT $ runConsole $ fmap runMaybeT cmd'
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