我如何设法从 monad 转换器打印

Question

我设法获得了 monad 转换器 MaybeT 的编译代码。（我正在关注 the haskell wikibook）

但是，我似乎无法让我的代码打印任何东西

运行 askPassphrase 在 ghci 上，我得到一个非实例错误：No instance for (Data.Functor.Classes.Show1 IO) arising from a use of ‘print’

我什至不知道如何阅读错误！ no instance of Show 对我来说更有意义。

到目前为止我尝试了什么：基本上是用头撞墙。我什至不知道该尝试什么。也许我可以为 MaybeT IO String 编写专门版本的打印，但这似乎违背了目的...

类似

p :: Show a => MaybeT IO a -> IO ()
p maybeIO= do a' <- runMaybeT maybeIO
              print a'

有效，但似乎不对...我不能让它从内部打印 MaybeT IO 吗？

MaybeT IO 字符串代码如下

import Control.Monad
import Control.Applicative
import Control.Monad.Trans
import Control.Monad.Trans.Maybe


    
isValid :: String -> Bool
isValid s = length s >= 8

getPassphrase :: MaybeT IO String
getPassphrase = do s <- lift getLine
                   guard (isValid s)
                   return s

askPassphrase :: MaybeT IO ()
askPassphrase = do lift $ putStrLn "Insert your new passphrase:"
                   value <- getPassphrase
                   lift $ putStrLn "Storing in database..."

Answer 1

使用runMaybeT:

> runMaybeT askPassphrase
Insert your new passphrase:
asdf
Nothing
> runMaybeT askPassphrase
Insert your new passphrase:
asdfghjk
Storing in database...
Just ()

我如何设法从 monad 转换器打印

How can I manage to print from a monad transformer

haskell

monad-transformers