使用ReaderT传递IO动作而非函数参数时的类型系统阻碍问题

问题背景 #

原本的可测试IO函数采用参数传递依赖的方式，将IO动作作为参数传入，实现生产环境和测试环境的灵活切换：

numCharactersInFile' :: Monad m => (FilePath -> m String) -> FilePath -> m Int
numCharactersInFile' f filePath = do
    contents <- f filePath
    return (length contents)

使用示例：

module Main where

import MTLPrelude

main :: IO ()
main = do
  -- 生产环境使用
  n <- numCharactersInFile' readFile "/home/enrico/fibs.hs"
  print n
  -- 测试环境使用
  print $ 18 == (runIdentity . numCharactersInFile' mockFileSystem "path")

mockFileSystem :: FilePath -> Identity String
mockFileSystem _ = return "mock file contents"

为了避免到处传递参数，尝试改用ReaderT层传递readFile，修改后的函数如下：

numCharactersInFile'' :: (MonadIO m, MonadReader (FilePath -> m String) m) => FilePath -> m Int
numCharactersInFile'' filePath = do
  f <- ask
  contents <- f filePath
  return (length contents)

但调用时报类型错误：

main :: IO ()
main = do  -- 生产环境使用
  i <- numCharactersInFile'' "/home/enrico/fibs.hs" `runReaderT` readFile
  print i
  -- 测试环境使用
  j <- numCharactersInFile'' "path" `runReaderT` mockFileSystem

  print $ 18 == j

报错信息：

• Couldn't match type: IO String
                  with: ReaderT (FilePath -> IO String) IO String
     arising from a functional dependency between:
       constraint ‘MonadReader
                     (FilePath -> ReaderT (FilePath -> IO String) IO String)
                     (ReaderT (FilePath -> IO String) IO)’
         arising from a use of ‘numCharactersInFile''’
       instance ‘MonadReader r (ReaderT r m)’ at <no location info>
 • In the first argument of ‘runReaderT’, namely
     ‘numCharactersInFile'' "/home/enrico/fibs.hs"’
   In a stmt of a 'do' block:
     i <- numCharactersInFile'' "/home/enrico/fibs.hs"
            `runReaderT` readFile
   In the expression:
     do n <- numCharactersInFile' readFile $ "/home/enrico/fibs.hs"
        print n
        print
          $ 18
              == (runIdentity . numCharactersInFile' mockFileSystem $ "path")
        i <- numCharactersInFile'' "/home/enrico/fibs.hs"
               `runReaderT` readFile
        .... [-Wdeferred-type-errors]

核心原因：类型循环依赖 #

问题出在函数的类型约束MonadReader (FilePath -> m String) m上：

• 当你用runReaderT ... readFile时，m被实例化为ReaderT (FilePath -> IO String) IO
• 此时约束要求Reader的环境是FilePath -> m String，也就是FilePath -> ReaderT (FilePath -> IO String) IO String
• 但你传入的readFile类型是FilePath -> IO String，和要求的类型完全不匹配，这就导致了类型错误

简单来说：你要求Reader存储的是能返回当前Monad上下文内值的动作，但实际传入的是返回原始IO值的动作，二者的Monad上下文不一致。

修正方案 #

方案1：适配IO动作的类型 #

调整类型约束，让Reader存储的是FilePath -> IO String类型的动作，然后在函数内部用liftIO将IO值提升到当前Monad上下文：

numCharactersInFile'' :: (MonadIO m, MonadReader (FilePath -> IO String) m) => FilePath -> m Int
numCharactersInFile'' filePath = do
  f <- ask
  contents <- liftIO $ f filePath
  return (length contents)

调用示例：

main :: IO ()
main = do
  -- 生产环境
  i <- numCharactersInFile'' "/home/enrico/fibs.hs" `runReaderT` readFile
  print i
  -- 测试环境
  let mock :: FilePath -> IO String
      mock _ = return "mock file contents"
  j <- numCharactersInFile'' "path" `runReaderT` mock
  print $ 18 == j

方案2：支持多Monad场景（需RankNTypes扩展） #

如果要同时兼容IO和Identity等Monad场景，可以使用RankNTypes扩展，让Reader存储通用的Monad动作：

{-# LANGUAGE RankNTypes #-}

numCharactersInFile''' :: (MonadReader (forall m. Monad m => FilePath -> m String) m) => FilePath -> m Int
numCharactersInFile''' filePath = do
  f <- ask
  contents <- f filePath
  return (length contents)

main :: IO ()
main = do
  -- 生产环境
  i <- numCharactersInFile''' "/home/enrico/fibs.hs" `runReaderT` readFile
  print i
  -- 测试环境
  let j = runIdentity $ numCharactersInFile''' "path" `runReaderT` mockFileSystem
  print $ 18 == j

内容的提问来源于stack exchange，提问作者Enrico