Haskell 偏单体变压器
我正试图将来自的Haskell 偏单体变压器,haskell,monads,Haskell,Monads,我正试图将来自的IResultmonad分解为几部分。下面是IResult data IResult t r = Fail t [String] String | Partial (t -> IResult t r) | Done t r 这让人感觉它应该是效果、“偏爱”和失败的组合。如果失败仅表示为一个或([String],String),则可能存在偏好 data Partiality t a = Now a | La
IResult
monad分解为几部分。下面是IResult
data IResult t r = Fail t [String] String
| Partial (t -> IResult t r)
| Done t r
这让人感觉它应该是效果、“偏爱”和失败的组合。如果失败仅表示为一个或([String],String)
,则可能存在偏好
data Partiality t a = Now a | Later (t -> Partiality t a)
instance Monad (Partiality t) where
return = pure
(Now a) >>= f = f a
(Later go) >>= f = Later $ \t -> go t >>= f
class MonadPartial t m where
feed :: t -> m a -> m a
final :: m a -> Bool
instance MonadPartial t (Partiality t) where
feed _ (Now a) = Now a
feed t (Later go) = go t
final (Now _) = True
final (Later _) = False
(当您使用Partiality()
时,它将从中获取其名称)
我可以使用
Partiality
作为基本单子,但是有PartialityT
monad转换器吗?当然有!您的偏好
单子是一个免费单子:
import Control.Monad.Free -- from the `free` package
type Partiality t = Free ((->) t)
。。。相应的PartialityT
是一个自由单声道转换器:
import Control.Monad.Trans.Free -- also from the `free` package
type PartialityT t = FreeT ((->) t)
下面是一个示例程序,演示如何使用它:
import Control.Monad
import Control.Monad.Trans.Class
import Control.Monad.Trans.Free
type PartialityT t = FreeT ((->) t)
await :: (Monad m) => PartialityT t m t
await = liftF id
printer :: (Show a) => PartialityT a IO r
printer = forever $ do
a <- await
lift $ print a
runPartialityT :: (Monad m) => [a] -> PartialityT a m r -> m ()
runPartialityT as p = case as of
[] -> return ()
a:as -> do
x <- runFreeT p
case x of
Pure _ -> return ()
Free k -> runPartialityT as (k a)
我建议你阅读。然而,免费monad transformer的新官方主页是free
软件包
另外,如果您正在寻找一个有效的增量解析器,我将在几天内将其作为
pipesparse
包发布。您可以查看添加到主要问题中的。您的monad实例对于偏好t
是什么。哦,当然是!我的缺陷是,我无法确定内部一元层应该位于何处并定义data PartialityT t t m a=PT{runPT::m(Partiality t a)}
。。。这只给了我一个单一的内部一元层!感谢您向正确的方向推进(以及由此产生的明显概括!)
>>> runPartialityT [1..] printer
1
2
3
...