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rust-bakery / nom

Rust parser combinator framework
MIT License
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Change fail output type to unit #1647

Closed MariaSolOs closed 1 year ago

MariaSolOs commented 1 year ago

Hello! First of all, thank you for the amazing project. I've been using nom for writing a new parser and it's been a joy <3

While working in my project, I noticed that every time I use the fail combinator I have to specify the output type despite the parser always failing, hence such type feels irrelevant (?). In this PR I change the output type to () and hence simplify the use of the combinator.

epage commented 1 year ago

While working in my project, I noticed that every time I use the fail combinator I have to specify the output type despite the parser always failing, hence such type feels irrelevant (?). In this PR I change the output type to () and hence simplify the use of the combinator.

I'm curious where you are using fail that the output type isn't relevant. For me, it almost always is. For example, in the last case of an alt, I might use a fail to specialize the error message. The output type needs to be the same type as all of the other cases in my alt.

MariaSolOs commented 1 year ago

@epage oh you're right, this change would break the usage of fail in alt. You can dismiss this PR then.

To answer your question, my use cases for fail involve parsing something, performing some verification which may involve modifying the input span (I'm using nom_locate with a LocatedSpan that has some state inside its extra), and if such verification fails, I directly use the fail parser with let (span, _) = fail::<_, (), _>(span)?. I know that this looks odd, but since my verifications might require access to the span, I cannot use something like verify (since there I only have access to the output type).