我不明白如何使用ask函数，我知道如何使用asks函数，但我不知道它们是否相关。
我正在阅读斯蒂芬的"我希望我在学习哈斯克尔时知道什么"，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
所以，我的问题是：名单是从哪里拿来的？
ask是asks id。在...
do
x <- asks f
-- etc.
。x将是将f应用于MonadReader计算环境的结果，在...
do
x <- ask
-- etc.
。x将是环境本身。