Lazy context

[9999years]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

winnow version

master

Describe your use case

I'd like to add context to parsers without immediately allocating a Vec for them

Describe the solution you'd like

Would it be possible to have a .context_from(|| vec!["context1", "context2"]) function which only computes the context on error branches? Ideally this would be a middle-ground for performance between allocating the contexts up-front and omitting them entirely.

Alternatives, if applicable

Not sure! I'm interested to hear discussion.

Additional Context

See #421.

[epage]

Could you share your error type and context type?

[9999years]

I'm using PResult<ContextError<StrContext>>, but I'll probably create my own context type like this so I can dynamically construct informative parse context messages:

#[derive(Debug, Clone)]
pub enum ParseContext {
    Description(Cow<'static, str>),
    Expected(Cow<'static, str>),
}

[epage]

ContextError doesn't immediately allocate but does only pushes on error.

I'm confused by the example because you don't pass a Vec to context but a single context.

[9999years]

I'm confused by the example because you don't pass a Vec to context but a single context.

Yeah, sorry, to be clear I mean a method which computes multiple contexts up-front.

ContextError doesn't immediately allocate but does only pushes on error.

You're right, thank you. I've looked at the code a bit more. But the Parser::context method does take the context up-front, which means that the cost of computing the context itself is always paid. Not much of an issue for &'static str contexts, but for anything else it could be expensive:

winnow/src/parser.rs

Lines 646 to 660 in 8674ed2

	/// If parsing fails, add context to the error
	///
	/// This is used mainly to add user friendly information
	/// to errors when backtracking through a parse tree.
	#[doc(alias = "labelled")]
	#[inline(always)]
	fn context<C>(self, context: C) -> Context<Self, I, O, E, C>
	where
	Self: core::marker::Sized,
	I: Stream,
	E: AddContext<I, C>,
	C: Clone + crate::lib::std::fmt::Debug,
	{
	Context::new(self, context)
	}

Maybe I'm just holding it wrong? Not sure.

[epage]

It would help if you gave a more complete example of what you are trying to accomplish.