ReaderT 设计模式：参数化环境

Question

我基于ReaderT 设计模式构建了一个项目。我选择使用简单的处理程序注入作为函数参数，而不是使用类型类方法进行依赖项注入。这部分工作得很好，因为我们能够静态构建依赖树并动态定义环境。

环境可能包含配置以及日志记录效果:: String -> IO ()、时间效果:: IO UTCDate等。考虑以下简化示例

import Control.Monad.Reader (runReaderT, liftIO, reader, MonadReader, MonadIO)

data SomeEnv 
  = SomeEnv
  { a :: Int
  , logger :: String -> IO ()
  }

class HasLogger a where 
  getLogger :: a -> (String -> IO())

instance HasLogger SomeEnv where 
  getLogger = logger 

myFun :: (MonadIO m, MonadReader e m, HasLogger e) => Int -> m Int
myFun x = do
  logger <- reader getLogger
  liftIO $ logger "I'm going to multiply a number by itself!"
  return $ x * x

doIt :: IO Int
doIt = runReaderT (myFun 1337) (SomeEnv 13 putStrLn)

是否可以概括记录器的效果？

logger :: String -> m ()

出于使用适合 monad 堆栈的记录器的动机

myFun x = do
  logger <- reader getLogger
  logger "I'm going to multiply a number by itself!"
  return $ x * x

Answer 1

我们可以尝试以下改变：

• 使用“base”monad 参数化环境记录。
• 创建HasLogger一个将环境与“基本”单子关联起来的双参数类型类。

像这样的东西：

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE StandaloneKindSignatures #-}
import Control.Monad.IO.Class
import Control.Monad.Reader
import Data.Kind (Constraint, Type)

type RT m = ReaderT (SomeEnv m) m

type SomeEnv :: (Type -> Type) -> Type
data SomeEnv m = SomeEnv
  { a :: Int,
    logger :: String -> RT m (),
    -- I'm putting the main fuction in the record,
    -- perhaps we'll want to inject it into other logic, later.
    myFun :: Int -> RT m Int
  }

type HasLogger :: Type -> (Type -> Type) -> Constraint
class HasLogger r m | r -> m where
  getLogger :: r -> String -> m ()

instance HasLogger (SomeEnv m) (RT m) where
  getLogger = logger

_myFun :: (MonadReader e m, HasLogger e m) => Int -> m Int
_myFun x = do
  logger <- reader getLogger
  logger "I'm going to multiply a number by itself!"
  return $ x * x

现在_myFun没有这个MonadIO约束了。

我们可以创建一个示例环境并运行myFun：

env =
  SomeEnv
    { a = 13,
      logger = liftIO . putStrLn,
      myFun = _myFun
    }

doIt :: IO Int
doIt = runReaderT (myFun env 1337) env

该解决方案的一个缺点是环境中的函数签名变得更加复杂，即使使用RT类型同义词也是如此。

编辑：为了简化环境中的签名，我尝试了这些替代定义：

type SomeEnv :: (Type -> Type) -> Type
data SomeEnv m = SomeEnv
  { a :: Int,
    logger :: String -> m (), -- no more annoying ReaderT here.
    myFun :: Int -> m Int
  }

instance HasLogger (SomeEnv m) m where
  getLogger = logger

-- Yeah, scary. This newtype seems necessary to avoid an "infinite type" error.
-- Only needs to be defined once. Could we avoid it completely?
type DepT :: ((Type -> Type) -> Type) -> (Type -> Type) -> Type -> Type
newtype DepT env m r = DepT { runDepT :: ReaderT (env (DepT env m)) m r } 
    deriving (Functor,Applicative,Monad,MonadIO,MonadReader (env (DepT env m)))
instance MonadTrans (DepT env) where
    lift = DepT . lift

env' :: SomeEnv (DepT SomeEnv IO) -- only the signature changes here
env' = 
    SomeEnv
    { a = 13,
      logger = liftIO . putStrLn,
      myFun = _myFun
    }

doIt :: IO Int
doIt = runReaderT (runDepT (myFun env' 1337)) env'

DepT基本上是一个ReaderT，但人们知道它的环境是由DeptT它自己参数化的。它有通常的情况。

_myFun不需要改变这个替代定义。