Freer-Simple Freer Monads 如何统一 IO 异常处理和错误效果

Question

我正在使用 freer-simple 编写一个超级简单的 DSL。它所做的只是读取一个文件。我有一个关于文件名的规则，它们不能包含字母 x。任何尝试打开其中包含字母 x 的文件的结果都会是：Left (AppError "No Xs allowed in file name")。

在 fileSystemIOInterpreter 中读取文件时如何捕获 IO 错误并将其作为应用程序错误抛出？ IE。我正在尝试将选定的 IO 异常转换为 AppErrors（请参阅 ??????）。

{- File System Lang -}

data FileSystem r where
  ReadFile :: Path a File -> FileSystem StrictReadResult

readFile :: Members '[FileSystem, Error AppError] effs => Path a File -> Eff effs StrictReadResult
readFile path = let
                  pthStr = toStr $ toFilePath path
                in
                  F.elem 'x' pthStr
                        ? throwError (AppError "No Xs allowed in file name")
                        $ send $ ReadFile path

{- Errors -}

newtype AppError = AppError String deriving Show

runAppError :: Eff (Error AppError ': r) a -> Eff r (Either AppError a)
runAppError = runError

{- File System IO Interpreter -}

fileSystemIOInterpreter :: forall effs a. (Members '[Error AppError] effs, LastMember IO effs) => Eff (FileSystem ': effs) a -> Eff effs a
fileSystemIOInterpreter = interpretM $ \case
                                          ReadFile path -> F.readFileUTF8 path
                                          -- ??????

-- this compiles: fileSystemIOInterpreter effs = throwError $ AppError "BLahh"

application :: Members '[FileSystem, Error AppError] effs => Path a File -> Eff effs StrictReadResult
application = readFile

ioApp :: Path a File -> IO (Either AppError StrictReadResult)
ioApp path = runM
              $ runAppError
              $ fileSystemIOInterpreter
              $ application path

-- running the app

demoPassApp = ioApp [absfile|C:\Vids\SystemDesign\VidList.md|]
>> Right (Right "Text content of VidList.md")

demoFailApp = ioApp [absfile|C:\Vids\SystemDesign\VidList.txt|]
>> Left (AppError "No Xs allowed in file name")

demoFailIOApp = ioApp [absfile|C:\Vids\SystemDesign\MissingFile.md|]
>> *** Exception: C:\Vids\SystemDesign\MissingFile.md: openBinaryFile: does not exist (No such file or directory)
-- I want to turn this into an AppError

Answer 1

interpretM 在 IO 中有一个解释器（its first argument has type eff ~> m 和 m ~ IO 在这里），所以这不允许你通过 AppError 抛出Members '[Error AppError] effs 约束。

您可以使用 interpret，完全访问 effs。大致看起来像：

fileSystemIOInterpreter
  :: forall effs a
  .  (Members '[Error AppError] effs, LastMember IO effs)
  => Eff (FileSystem ': effs) a -> Eff effs a
fileSystemIOInterpreter = interpret $ \case
    ReadFile path -> do
        r <- sendM (try (F.readFileUTF8 path))
        case r of
            Left (e :: IOException) -> throwError (ioToAppErr e)
            Right f -> pure f

-- for some value of
ioToAppErr :: IOException -> AppError

Answer 2

这是一个完整的工作示例：

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MonoLocalBinds #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import Data.String
import qualified Data.ByteString.Char8 as B
import Control.Exception
import Data.List
import Data.Text (Text, pack, unpack)
import Data.Text.IO
import Data.Text.Encoding (decodeUtf8)
import Control.Natural (type (~>))
import qualified  Control.Monad.Freer.Error as ER
import Control.Monad.Freer
  (
    Eff
  , LastMember
  , Member
  , Members
  , interpret
  , send
  , sendM
  , runM
  )

readFileUTF8 :: String -> IO Text
readFileUTF8 path = decodeUtf8 <$> B.readFile path

ioToAppErr :: IOException -> AppError
ioToAppErr ioe = AppError (displayException ioe)

newtype AppError = AppError String deriving Show

data FileSystem r where
  ReadFile :: FilePath -> FileSystem Text

readFile :: Members '[FileSystem, ER.Error AppError] effs => FilePath -> Eff effs Text
readFile fpath = if (elem 'x' fpath)
                     then (ER.throwError  (AppError "No Xs allowed in file name"))
                 else(send $ ReadFile fpath)

runAppError :: Eff (ER.Error AppError ': r) a -> Eff r (Either AppError a)
runAppError = ER.runError

fileSystemIOInterpreter
  :: (Members '[ER.Error AppError] effs, LastMember IO effs)
  => Eff (FileSystem ': effs) a -> Eff effs a
fileSystemIOInterpreter = interpret $ \case
    ReadFile path -> do
        r <- sendM (try (readFileUTF8 path))
        case r of
            Left (e :: IOException) -> ER.throwError (ioToAppErr e)
            Right f -> pure f

application :: Members '[FileSystem, ER.Error AppError] effs => FilePath -> Eff effs Text
application = Main.readFile

ioApp :: FilePath -> IO (Either AppError Text)
ioApp path = runM
              $ runAppError
              $ fileSystemIOInterpreter
              $ application path

main :: IO ()
main = do
  let pathX = "C:\text.info"
  let pathNoX = "C:\simple.t"
  let pathNoSuchAFile = "C:\habrahabr.bib"
  result <- ioApp pathX
  Data.Text.IO.putStrLn $ pack (show result)

依赖项：

基础 >= 4.7 && < 5
文字
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自然变换
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Freer-Simple Freer Monads 如何统一 IO 异常处理和错误效果

Freer-Simple Freer Monads How do I Unify IO Exception Handling with Error Effect

haskell

free-monad