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Martin Hope
planarian
Asked: 2025-04-02 08:29:03 +0800 CST
  • 8

以下是《Learn You a Haskell》第 13 章中状态单子的定义

instance Monad (State s) where
    return x = State $ \s -> (x,s)
    (State h) >>= f = State $ \s -> let (a, newState) = h s
                                        (State g) = f a
                                    in  g newState

根据此定义，在我看来，虽然返回的是较新的版本，但tuple 中的状态f a似乎将被 替换。以Real World Haskell第 10 章中的具体示例为例，这显然不是作者的本意（请注意使用而不是 ，因为他们尚未正式引入 monad）newState ==>>>=

parseByte :: Parse Word8
parseByte =
    getState ==> \initState ->
    case L.uncons (string initState) of
      Nothing ->
          bail "no more input"
      Just (byte,remainder) ->
          putState newState ==> \_ ->
          identity byte
        where newState = initState { string = remainder,
                                     offset = newOffset }
              newOffset = offset initState + 1

在哪里

newtype Parse a = Parse {
      runParse :: ParseState -> Either String (a, ParseState)
    }

identity :: a -> Parse a
identity a = Parse (\s -> Right (a, s))
 
(==>) :: Parse a -> (a -> Parse b) -> Parse b
firstParser ==> secondParser  =  Parse chainedParser
  where chainedParser initState   =
          case runParse firstParser initState of
            Left errMessage ->
                Left errMessage
            Right (firstResult, newState) ->
                runParse (secondParser firstResult) newState

在parseByte，我不明白如何initState不取代newState。

haskell
Martin Hope
planarian
Asked: 2024-05-19 21:14:21 +0800 CST
  • 7

在用 Haskell 进行函数式思考（第 45 页）中，Richard Bird 说有 6 个类型为 的函数Maybe a -> Maybe a，尽管他只提到了 5 个。

应用于Nothing函数只能返回Nothing或undefined。应用于Just x函数只能返回NothingorJust x或 undefined。关键是我们对底层类型一无所知，因此无法发明新值。这总共有六种可能的功能。

我数了七：

first, second, third, fourth, fifth, sixth, seventh :: Maybe a -> Maybe a

first (Just x) = Just x
second (Just x) = Nothing
third (Just x) = undefined
fourth Nothing = Nothing
fifth Nothing = undefined
sixth undefined = Nothing
seventh undefined = undefined

一个相关的难题是，当我尝试将其中三个定义放在一个函数下时，我收到编译错误“模式匹配是多余的”。删除三行中的任何一行都会使其消失

foo :: Maybe a -> Maybe a
foo (Just x) = Nothing
foo Nothing = undefined
foo undefined = Nothing
haskell