Idea: conditonal parser, which takes a parser and a condition about chumsky HOT 4 CLOSED

Ah yes! Thank you those are exactly what I need

from chumsky.

Perhaps .filter(...) is what you're looking for? You can do a.filter(|a| a.is_foo()). I think you might be a bit confused about how parser outputs work though: Fallibility is built in to a parser, if the filter closure returns false, then the parser just won't parse anything.

from chumsky.

No, that's not really what i'm looking for. I'm not looking for a fallible parser, but for one that will parse something or not parse something based on a condition independent of what is being parsed. So condition(true, just('a')) is a parser with output Option<char>, specifically Some(a).

from chumsky.

Ah, are you trying to dynamically change the behaviour of your parser at runtime based on some configuration for the parser? I assumed that the condition was dependent on the output of the parser. In that case, there are a few approaches you can take:

1. Parsers can be dynamically dispatched, so you can do if condition { a.map(Some).boxed() } else { empty().to(None).boxed() } and this should type-check just fine (the parser in the else branch always succeeds while parsing no tokens)

2. Use custom like so:

custom(|inp| if condition { inp.parse(a).map(Some) } else { Ok(None) })

this approach avoids the dynamic dispatch of the previous approach.

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