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,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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