"ask"在Haskell中是什么意思，它和"asks"函数有什么区别？

Question

我不懂ask函数怎么用，我知道asks函数怎么用，但不知道有没有关系

我正在阅读 Stephen 的 "What I wish I knew when learning Haskell"，我发现了这个例子：

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.Reader
import Control.Monad.Writer
import Control.Monad.State

type Stack = [Int]
type Output = [Int]
type Program = [Instr]

type VM a = ReaderT Program (WriterT Output (State Stack)) a

newtype Comp a = Comp { unComp :: VM a }
    deriving (Functor, Applicative, Monad, 
                MonadReader Program, MonadWriter Output, 
                  MonadState Stack)

data Instr = Push Int
            | Pop
            | Puts

evalInstr :: Instr -> Comp ()
evalInstr instr = case instr of
                    Pop -> modify tail
                    Push n -> modify (n:)
                    Puts -> do
                        tos <- gets head
                        tell [tos]

eval :: Comp ()
eval = do
    instr <- ask
    case instr of
      [] -> return ()
      (i:is) -> evalInstr i >> local (const is) eval

execVM :: Program -> Output
execVM = flip evalState [] . execWriterT . runReaderT (unComp eval)

program :: Program
program = [
        Push 42,
        Push 27,
        Puts,
        Pop,
        Puts,
        Pop
    ]

main :: IO ()
main = mapM_ print $ execVM program

那么，我的问题是：该列表是从哪里获取的？

Answer 1

ask 是 asks id。在...

do
    x <- asks f
    -- etc.

...x 将是将 f 应用于您的 MonadReader 计算环境的结果，在...

do
    x <- ask
    -- etc.

...x 将是环境本身。