`ErrorKind::Input`

[dhardy]

ErrorKind was designed with RNG errors in mind, but has also found some use in other code (#518). I suggest we add a new enum variant: Input (or InvalidInput or UserInput or something), meaning that a function was called with invalid parameters.

[dhardy]

Regarding weighted sampling:

@sicking's #518 introduced fn choose_weighted<R, F, B, X>(&self, rng: &mut R, weight: F) -> Option<&Self::Item>, but does not address error handling of these convenience methods

https://github.com/rust-lang-nursery/rand/blob/0faff206d21515e3fd5233cfbd6899efd673a46b/src/seq.rs#L112

I think the best option would be to change the return type to Result<&Self::Item, WeightedError> where:

enum WeightedError {
    NoItem,
    InvalidWeights,
}

impl From<WeightedError> for Error { .. }

or similar (possibly separate variants for negative weights, zero weights and no weights?).

WeightedIndex::new should probably return the same error type.

[dhardy]

@sicking you offered to look into this I think?

[sicking]

I'll definitely get to this this week.