GHC可以为monad变压器派生Functor和Applicative实例吗？ [英] Can GHC derive Functor and Applicative instances for a monad transformer?

问题描述

我试图以 mtl 库的精神实现 MaybeT 。使用这个非编译解决方案：

  { - ＃LANGUAGE FlexibleInstances，MultiParamTypeClasses，UndecidableInstances＃ - } 
 
 import Control.Monad 
 import Control.Monad.Trans $ b $ import Control.Monad.State 
 $ b $ newtype MaybeT ma = MaybeT {runMaybeT :: m（也许a）} 
 
 instance（Monad m）=> Monad（MaybeT m）其中
x>> = f = MaybeT $ runMaybeT x>> = maybe（return Nothing）（runMaybeT。f）
 return a = MaybeT $ return（Just a） 
失败_ = MaybeT $返回Nothing 
 
实例MonadTrans MaybeT其中
 lift m = MaybeT（liftM只是m）
 
实例（MonadIO m）= > MonadIO（MaybeT m）其中
 liftIO m =电梯（liftIO m）
 
实例（MonadState s m）=> MonadState s（MaybeT m）其中
 get = lift得到
 put = lift。把
 
 ... 
  

我得到错误：

无法从上下文（Monad m）的实例声明的
超类中推导出（Applicative（MaybeT m））

如果我执行以下步骤，它会编译：

  instance（Monad m）=>应用（MaybeT m）其中
 pure =返回
（*）= ap 
 
实例（Monad m）=> Functor（MaybeT m）其中
 fmap = liftM 
  

GHC可以帮我吗？ / p>

解决方案

不，GHC目前无法做到这一点。也许在未来它会。

添加应用实例的需求是一个相当新的实例，引入GHC 7.10和烧尽所有网桥提案。这固定了以前类层次结构的一些瑕疵，最终要求monad是函子的子类的应用程序的子类。不幸的是，这破坏了后向兼容性，并且引起一些不便，因为没有自动的方式来推断应用实例。

可能在未来GHC将允许像

  class Applicative m => Monad m其中
 return :: a  - > m a 
（>> =）:: m a  - > （a  - > m b） - > mb 
 default pure = return 
 default（*）= ap 
  

<这样就不需要明确超类的实例。甚至是基于Haskell模板的东西，这样图书馆作者就可以向GHC解释如何自动导出实例（这在某种程度上现在是可行的）。我们将看到什么来自GHC开发商。

I'm trying to implement MaybeT in the spirit of the mtl library. With this non-compiling solution:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-}

import Control.Monad
import Control.Monad.Trans
import Control.Monad.State

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

instance (Monad m) => Monad (MaybeT m) where
    x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
    return a = MaybeT $ return (Just a)
    fail _ = MaybeT $ return Nothing

instance MonadTrans MaybeT where
     lift m = MaybeT (liftM Just m)

instance (MonadIO m) => MonadIO (MaybeT m) where
    liftIO m = lift (liftIO m)

instance (MonadState s m) => MonadState s (MaybeT m) where
    get = lift get
    put = lift . put

...

I get the error:

Could not deduce (Applicative (MaybeT m)) arising from the superclasses of an instance declaration from the context (Monad m)

If I implement the following, it compiles:

instance (Monad m) => Applicative (MaybeT m) where
    pure = return
    (<*>) = ap 

instance (Monad m) => Functor (MaybeT m) where
    fmap = liftM

Can GHC do this for me?

解决方案

No, GHC can not currently do that. Maybe in the future it will.

The need to add applicative instances is a fairly new one, introduced with GHC 7.10 and the "burn all bridges" proposal. This fixed some warts of the previous class hierarchy, by finally requiring that monads are subclasses of applicatives which are subclasses of functors. Unfortunately, this breaks backward compatibility, and causes some inconveniences since there's no automatic way to infer the applicative instances.

Perhaps in the future GHC will allow something like

class Applicative m => Monad m where
   return :: a -> m a
   (>>=) :: m a -> (a -> m b) -> m b
   default pure = return
   default (<*>) = ap

so that one does not need to be explicit about the superclass instances. Or even something based on Template Haskell, so that a library writer can explain to GHC how to automatically derive instances (which, to some extent, is feasible right now). We shall see what comes from the GHC developers.

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