Open mkrasnitski opened 3 weeks ago
When you pass clap::value_parser!(u8).map(CleanLevel::from_val
to clap, you are passing in an adapter from an OsStr
to a CleanLevel
. The intermediate u8
is transient and never shared with clap and clap doesn't know how to increment CleanLevel
.
For this to work, we'd need to do one of the following
dyn
trait design to allow all of the interactions to play out.As for workarounds...
We take the "intermediate" struct approach in https://docs.rs/clap-verbosity-flag/latest/clap_verbosity_flag/
The conversion can also be a helper method on Config
which I do for other types of data in my CLIs.
The intermediate
u8
is transient and never shared with clap and clap doesn't know how to incrementCleanLevel
.
This seems solvable by introducing a trait called something like Counter
, with an increment
method, like you mentioned. In the case of CleanLevel
, the state machine implements a kind of saturating addition, and auto-implementing this functionality would be hard via #[derive(Counter)]
, but a manual implementation would work fine. Default implementations for integers and potentially one for Option<T>
(0 => None
) would work well IMO.
However, this is a fairly specialized operation and doesn't scale to other needs
Could you elaborate on this point? Yes, likely this new trait would be closely coupled to ArgAction::Count
, but that could be made clear with a good name. Arguably this is a relatively simple generalization to make to allow using arbitrary types as counters, enabling better state machines while staying pretty self-contained.
This seems solvable by introducing a trait called something like Counter, with an increment method, like you mentioned. In the case of CleanLevel, the state machine implements a kind of saturating addition, and auto-implementing this functionality would be hard via #[derive(Counter)], but a manual implementation would work fine. Default implementations for integers and potentially one for Option
(0 => None) would work well IMO.
We are effectively operating on a Box<Any>
for the intermediate state due to the dynamic nature of clap. We can't easily access a trait like Counter
.
Could you elaborate on this point? Yes, likely this new trait would be closely coupled to ArgAction::Count, but that could be made clear with a good name. Arguably this is a relatively simple generalization to make to allow using arbitrary types as counters, enabling better state machines while staying pretty self-contained.
I was specifically referring to baking in the information for this behavior in ValueParser
which is what has knowledge of the concrete type. This is unrelated to a "new trait" solution.
Please complete the following tasks
Rust Version
rustc 1.82.0 (f6e511eec 2024-10-15)
Clap Version
4.5.19
Minimal reproducible code
playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ba6ae5eae9e9514d276dc40d5479cf5e
Steps to reproduce the bug with the above code
cargo run
Actual Behaviour
At runtime, clap panics with the following message:
Expected Behaviour
I would expect that the value parser is called to produce a
u8
and thenCleanLevel::from_val
is called on the parsed value to map to anOption<CleanLevel>
. It seems the sequence of events in parsing is somehow wrong, or maybe I'm misunderstanding howMapValueParser
works.Additional Context
The alternative solution is to make use of an intermediate struct and a method on that struct, like so:
However, this introduces extra clutter and indirection. It would be great to avoid using an intermediate representation.
Debug Output
No response