用户状态（秒差距）
我正在使用秒差距解析一个表达式，并且我想使用秒差距中的用户状态来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。
给出以下代码：
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，ParsecT sum a 中的 u 将是 Set.Set。但是我如何将状态更新集成到 var 中？
我尝试了类似 modify $ Set.insert v 的方法，但这不起作用，因为 Set.Set 不是状态单子。
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.
Given the following code:
import Data.Set as Set
inp = "$x = $y + $z"
data Var = V String
var = do char '
So, the u in ParsecT s u m a would be Set.Set. But how would I integrate the state update into var?
I tried something like modify $ Set.insert v, but this doesn't work, since Set.Set is not a state monad.
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 -> ...
So, the u in ParsecT s u m a would be Set.Set. But how would I integrate the state update into var?
I tried something like modify $ Set.insert v, but this doesn't work, since Set.Set is not a state monad.
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评论（2）
不幸的是，Yuras 对 updateParserState 的建议并不是最佳选择（如果您想修改 Parsec 的内部状态，您也可以使用该函数）；相反，您应该将一个可处理自定义用户状态（即
u -> u类型）的函数传递给modifyState，如本例所示：或使用任意组合
getState和putState函数。对于您的情况，您需要执行以下操作：请参阅 此链接了解更多信息。
有关以秒差距处理用户状态的更多类似教程的介绍，此文档虽然旧，但应该是相关的。
Unfortunately, Yuras' suggestion of
updateParserStateis 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 typeu -> u) tomodifyState, such as in this example:or use any combination of the
getStateandputStatefunctions. For your case, you'd do something like:See this link for more info.
For a more tutorial-like introduction to working with user state in Parsec, this document, though old, should be relevant.
您可以使用 更新解析器状态
You can use updateParserState