Parsec中的用户状态 [英] User state in Parsec

问题描述

我使用Parsec解析一个表达式，并且我想使用Parsec中的用户状态跟踪这些表达式中的变量。不幸的是，我真的不知道该怎么做。

I'm parsing an expression using Parsec and I want to keep track of variables in these expressions using the user state in Parsec. Unfortunately I don't really get how to do it.

给出以下代码：

Given the following code:

import Data.Set as Set
inp = "$x = $y + $z"

data Var = V String

var = do char '$'
      n <- many1 letter
      let v = Var n
       -- I want to modify the set of variables here
      return v

parseAssignment = ... -- parses the above assignment

run = case runIdentity $ runParserT parseAssignment Set.empty "" inp of
                   Left err -> ...
                   Right -> ...

因此， u in ParsecT suma 应该是 Set.Set 。但是，如何将状态更新集成到 var ？

So, the u in ParsecT s u m a would be Set.Set. But how would I integrate the state update into var?

我尝试了类似于修改$ Set.insert v ，但这不起作用，因为 Set.Set 不是状态monad。

I tried something like modify $ Set.insert v, but this doesn't work, since Set.Set is not a state monad.

推荐答案

不幸的是，Yuras建议 updateParserState 不是最优的如果你想修改Parsec的内部状态 ）;相反，你应该传递一个可以在你的自定义用户状态（即 u - > u 类型）上运行的函数，以 modifyState ，例如在这个例子中：

Unfortunately, Yuras' suggestion of updateParserState is not optimal (you'd use that function if you're looking to modify Parsec's internal state as well); instead you should pass a function that works over your custom user state (i.e. of type u -> u) to modifyState, such as in this example:

expr  = do
  x <- identifier
  modifyState (+1)
  -- ^ in this example, our type u is Int
  return (Id x)

或使用 getState 和 putState 函数的任意组合。对于你的情况，你可以这样做：

or use any combination of the getState and putState functions. For your case, you'd do something like:

modifyState (Set.insert v)

请参阅此链接以获取更多信息。

See this link for more info.

有关更多教程在Parsec中介绍如何使用用户状态，此文档虽然陈旧，应该是相关的。

For a more tutorial-like introduction to working with user state in Parsec, this document, though old, should be relevant.

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