有没有用于获取monad的变压器版本的库或类型类？ [英] Is there a library or typeclass for getting the transformer version of a monad?

问题描述

在我目前的项目中，我遇到了需要将各种monad转换为它们的变形对象，例如

  stateT :: Monad m =>国家a  - > StateT s $ a 
 stateT stf = StateT $ return。 runState stf 
  

为我需要的monads编写这些实用函数是很简单的，但是我想知道是否有已经存在一个库，它包含标准monad的这个功能，或者是一个抽象这种转换的类型类。类似于

  class（Monad f，MonadTrans t）=> LiftTrans f t | f  - > t其中
 liftT :: Monad m => f a  - > tma 
  

（lift可能是这里使用的错误术语，但我不确定是什么）

解决方案

检出函数 hoist mmorph 包。

其签名是

  hoist :: Monad m => （全部a  - > a a） - > t m b  - > t b 
  

这意味着它可以改变变压器底层的monad。

现在，在 trasformers 包中，许多基本monad实现为应用于 Identity monad，像这样：

  type State s = StateT s Identity 
  

因此，我们可以定义以下函数（采用将 mmorph <
$ b

  import Data.Functor.Identity 
 
 generalize ::（（code> documentation）： Monad m）=>身份a  - > ma 
 generalize m = return（runIdentity m）
  

并结合提升： p>



  hoist generalize ::（Monad m，MFunctor t）=> t身份b  - > tmb 
  

这种方法不适用于没有定义为应用于<$ c的转换器的简单monad $ c> Identity ，就像 Maybe 和任一 monad。您被 hoistMaybe < a>和 hoistEither 为这些。

In my current project I've run into the need to turn various monads into their transformer counterparts e.g.

stateT :: Monad m => State s a -> StateT s m a
stateT stf = StateT $ return . runState stf

It's trivial to write these utility functions for the monads I need, but I was wondering if there already exists a library that contains this functionality for the standard monads and maybe a typeclass that abstracts this sort of transformation. Something like

class (Monad f, MonadTrans t) => LiftTrans f t | f -> t where
    liftT :: Monad m => f a -> t m a

("lift" is probably the wrong term to use here, but I wasn't sure what else to call it.)

解决方案

Check out function hoist from the mmorph package.

Its signature is

hoist :: Monad m => (forall a. m a -> n a) -> t m b -> t n b

Meaning that it can change the base monad underlying a transformer.

Now, in the trasformers package, many "basic" monads are implemented as transformers applied to the Identity monad, like this:

type State s = StateT s Identity

Therefore, we can define the following function (taken form the Generalizing base monads section of the mmorph documentation):

import Data.Functor.Identity

generalize :: (Monad m) => Identity a -> m a
generalize m = return (runIdentity m)

and combine it with hoist:

hoist generalize :: (Monad m, MFunctor t) => t Identity b -> t m b

This method won't work for simple monads which are not defined as transformers applied to Identity, like the Maybe and Either monads. You are stuck with hoistMaybe and hoistEither for these.

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