Haskell 如何包装?
我有Haskell手册中的以下代码片段,逐步展示monad transformer将如何展开:Haskell 如何包装?,haskell,Haskell,我有Haskell手册中的以下代码片段,逐步展示monad transformer将如何展开: module OuterInner where import Control.Monad.Trans.Except import Control.Monad.Trans.Maybe import Control.Monad.Trans.Reader -- We only need to use return once -- because it's one big Monad
module OuterInner where
import Control.Monad.Trans.Except
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader
-- We only need to use return once
-- because it's one big Monad
embedded :: MaybeT (ExceptT String (ReaderT () IO)) Int
embedded = return 1
maybeUnwrap :: ExceptT String (ReaderT () IO) (Maybe Int)
maybeUnwrap = runMaybeT embedded
-- Next
eitherUnwrap :: ReaderT () IO (Either String (Maybe Int))
eitherUnwrap = runExceptT maybeUnwrap
-- Lastly
readerUnwrap :: () -> IO (Either String (Maybe Int))
readerUnwrap = runReaderT eitherUnwrap
有一个练习,我必须再次包装所有东西:
embedded :: MaybeT (ExceptT String (ReaderT () IO)) Int
embedded = ??? (const (Right (Just 1)))
我做了如下尝试:
embedded' :: MaybeT (ExceptT String (ReaderT () IO)) Int
embedded' = MaybeT (ExceptT (ReaderT (const (Right (Just 1)))))
但编译器抱怨:
D:\haskell\chapter26\src\OuterInner.hs:24:15: error:
* Couldn't match type `Either a0' with `IO'
Expected type: MaybeT (ExceptT String (ReaderT () IO)) Int
Actual type: MaybeT (ExceptT String (ReaderT () (Either a0))) Int
* In the expression:
MaybeT (ExceptT (ReaderT (const (Right (Just 1)))))
In an equation for embedded':
embedded' = MaybeT (ExceptT (ReaderT (const (Right (Just 1)))))
D:\haskell\chapter26\src\OuterInner.hs:24:32: error:
* Couldn't match type `Maybe Integer'
with `Either String (Maybe Int)'
Expected type: ReaderT () (Either a0) (Either String (Maybe Int))
Actual type: ReaderT () (Either a0) (Maybe Integer)
* In the first argument of `ExceptT', namely
`(ReaderT (const (Right (Just 1))))'
In the first argument of `MaybeT', namely
`(ExceptT (ReaderT (const (Right (Just 1)))))'
In the expression:
MaybeT (ExceptT (ReaderT (const (Right (Just 1)))))
Failed, modules loaded: none.
如何解决此练习?您已定义了除字符串读取器Int之外的MaybeT。您错过了最内层IO。与其他类型不同,IO没有具体的表示形式,但您当然可以使用return或pure duh将纯值注入其中
您已经定义了除字符串读取器Int之外的MaybeT。您错过了最内层IO。与其他类型不同,IO没有具体的表示形式,但您当然可以使用return或pure duh将纯值注入其中
GHC型孔扩展到救援!它可以让你在表达式中的某个地方找到a u,并让GHC告诉你需要的类型。那么,让我们从
:t MaybeT (ExceptT (ReaderT (const _)))
<interactive>:1:33: error:
* Found hole: _ :: m (Either e (Maybe a))
GHC型孔扩展到救援!它可以让你在表达式中的某个地方找到a u,并让GHC告诉你需要的类型。那么,让我们从
:t MaybeT (ExceptT (ReaderT (const _)))
<interactive>:1:33: error:
* Found hole: _ :: m (Either e (Maybe a))
这对救援意味着什么?嗯。就像奇普戴尔。来帮你的。在表达式中的任意位置放置一个u。根据规范,它是一个无效的标识符,因此无论如何它都是一个错误。GHC可以自由地推断它的类型,并在导致该错误之前打印该类型。你建立了一个表达式,把uu放在你不确定的地方,然后你在某种程度上,让类型写代码,在一个模糊的意义上。这意味着什么GHC类型的孔扩展到救援?嗯。就像奇普戴尔。来帮你的。在表达式中的任意位置放置一个u。根据规范,它是一个无效的标识符,因此无论如何它都是一个错误。GHC可以自由地推断它的类型,并在导致该错误之前打印该类型。你建立了一个表达式,把uu放在你不确定的地方,然后你在某种程度上,让类型在模糊的意义上写代码。
embedded = MaybeT (ExceptT (ReaderT (const (return (Right (Just 1))))))