Question

我想在Haskell程序中有一些更高级别的函数来调用其他函数，这些函数最终会调用使用某些状态或配置的函数，而不必在所有这些函数调用中传递状态。我知道这是状态monad（或可能是Reader monad？）的经典用法。

（我也不确定是否应该使用StateT（如下面的示例所示）来启用IO，或者是否应该以某种方式分别输出结果。）

在此阶段，我对这里的所有教程，博客文章和类似问题感到非常困惑，无法选择解决方案。还是我误解了隐藏的东西？

这是一个小例子：

import Control.Monad.State

-- Here's a simple configuration type:
data Config = MkConfig {
      name :: String
    , num  :: Int
    } deriving Show

-- Here's a couple of configurations.
-- (They're hard coded and pre-defined.)
c1 = MkConfig "low" 7
c2 = MkConfig "high" 10

-- Here's a lower level function that explicitly uses the config.
-- (The String is ignored here for simplicity, but it could be used.)
fun :: Config -> Int -> Int
fun (MkConfig _ i) j = i*j

-- testA and GoA work fine as expected.
-- fun uses the different configs c1,c2 in the right way.
testA = do
    a <- get
    lift (print (fun a 2))
    put c2
    a <- get
    lift (print (fun a 4))

goA = evalStateT testA c1
-- (c1 could be put at the start of testA instead.)

-- But what I really want is to use fun2 that calls fun, 
-- and not explicitly need state.
-- But this function definition does not compile:
fun2 :: Int -> Int
fun2 j = 3 * fun cf j  
-- fun needs a config arg cf, but where from?

-- I would like a similar way of using fun2 as in testB and goB here.
testB = do
    a <- get
    lift (print (fun2 3))  -- but fun2 doesn't take the state in a 
    put c2
    a <- get
    lift (print (fun2 42))  -- but fun2 doesn't take the state in a 

goB = evalStateT testB c1

我想使配置远离程序中的fun2之类的高级功能，同时仍保留更改配置并使用新配置运行那些功能的能力。这是一个“怎么做的问题”（除非我完全弄错了主意）。

Answer 1

您当然不能在类型签名中完全“隐藏配置”：普通的旧函数Int -> Int必须是参照透明的，因此它也不能依赖或接受某些{{ 1}}值。

您可能想做的事情是这样的：

Config

然后只要有fun2 :: Int -> State Config Int -- An `Int -> Int` that depends on `Config` state. -- Compare to how `Int -> IO Int` is like an -- `Int -> Int` function that depends on IO. fun2 j = do c1 <- get return (3 * fun c1 j)，就可以通过类似

的方法获得结果

c :: Config

另请参阅Combining StateT IO with State：

let result = evalState (fun2 42) c    -- An Int.

然后您可以编写类似的内容

hoistState :: Monad m => State s a -> StateT s m a
hoistState = StateT . (return .) . runState

如何从调用其他使用该状态的函数的函数中隐藏状态

1 个答案: