Fail parser immediately (skip other choices)

[zaynetro]

Let's say that I have this parser: it either parses a function definition or an expression.

let fn_ = keyword("fn").padded()
    .ignore_then(ident_)
    .then(args)
    .then(fn_body);

choice((
    fn_,
    expr,
))

How do I prevent my parser from falling back to expression parsing after it has encountered a fn keyword and then failed?

Nom has a cut combinator that fails the parser completely. Is there a similar thing in chumsky?

Why do I need this? If there was a problem with function arguments I want to see the error there instead of an error due it my parser failing to parse an expression.

[zesterer]

Hello,

If there was a problem with function arguments I want to see the error there instead of an error due it my parser failing to parse an expression.

Chumsky will automatically prioritise error branches that make the most progress into the input. This means that if the parser sees fn <malformed>, the error you will see will be that generated by the fn_ parser (unless, of course, the expr parser contains some syntax rule that can parse the fn too).

The exact mechanism for this is kept as an implementation detail (there are some future changes we might be able to do to improve error prioritisation even further), but the parser will already exhibit the behaviour you're looking for without any extra combinators :)

[zaynetro]

oh wow. That's pretty cool! I think you can put this answer into error_and_recovery guide.

I guess I need to play around more with error cases to see how it works in practice.