解析器示例不会与我点击(Parser example isn't clicking with me)

解析器示例不会与我点击(Parser example isn't clicking with me)

我正在通过Real World Haskell进行阅读，并且作为函子/ monads的介绍，它给出了以下示例：

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

（其余部分可以在页面的四分之一处阅读： http ： //book.realworldhaskell.org/read/code-case-study-parsing-a-binary-data-format.html ）

对我来说没有任何意义的是，为什么这个函数不带任何参数？ 我希望它接受包含要解析的文本的Parse对象，然后返回解析的文本和新的Parse对象。 相反，（我认为）它“神奇地”访问Parser，弹出一个字节，然后返回一个“修改过的”Parser。 物体来自哪里？ 我一直在盯着现在一天，仍然不知道这个功能如何工作。

任何指导在这里将不胜感激。

I'm reading through Real World Haskell, and as an intro into functors/monads, it gives the following example:

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

(The rest of it can be read about a quarter of the way down the page at: http://book.realworldhaskell.org/read/code-case-study-parsing-a-binary-data-format.html)

The thing that doesn't make any sense to me is, why does this function not take any parameters? I'd expect it to accept a Parse object containing the text to be parsed, then return the parsed text, and a new Parse object. Instead, (as I see it) it's "magically" accessing a Parser, popping a byte, then returning a "modified" Parser. Where does the object come from? I've been staring at at for a day now and still have no clue how this function works.

Any guidance here would be appreciated.

最满意答案

Parse类型定义为

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

因此，如果您想知道输入来自哪里，那就是类型的定义！ 每个Parse值包含一个函数，然后我们在这个例子中使用==>通过合成将两个Parse到一个新的Parse 。 然后最终使用runParse运行。 此函数需要ParseState ，定义为

data ParseState = ParseState { string :: L.ByteString , offset :: Int64 } deriving (Show)

这是正在解析的字符串。

您可以将Parse类型视为该类型的别名

ParseState -> Either String (a, ParseState)

这是一个你期望的功能。 随着==>函数的类型（删除了newtype包装器）

(==>) :: (ParseState -> Either String (a, ParseState)) -> (a -> (ParseState -> Either String (b, ParseState))) -> (ParseState -> Either String (b, ParseState))

我们可以接受一个Parse并将它提供给另一个Parse来创建一个新的Parse 。 所有这些仅仅是围绕常规功能组合的一个奇特封装。 它从初始状态调用runParse地方获取它的输入。

The Parse type is defined as

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

So if you're wondering where the input comes from, it's in the definition of the type! Each Parse value wraps up a function, and then we use ==> in this example to chain two Parses together through composition into a new Parse. This is then finally run using runParse. This function requires a ParseState, being defined as

data ParseState = ParseState { string :: L.ByteString , offset :: Int64 } deriving (Show)

this is what carries the string being parsed.

You can think of the Parse type as an alias for the type

ParseState -> Either String (a, ParseState)

which is a function like you'd expect. With the ==> function having the type (with the newtype wrapper removed)

(==>) :: (ParseState -> Either String (a, ParseState)) -> (a -> (ParseState -> Either String (b, ParseState))) -> (ParseState -> Either String (b, ParseState))

we can then take one Parse and feed it into another Parse to make a new Parse. All of this is no more than a fancy wrapper around regular function composition. It gets it input from where-ever runParse is called from the initial state.