Nom currently supports custom error types, but that requires a lot of boilerplate on user side that I believe could simply be part of nom itself:

nom::error::Error and nom::error::VerboseError only embeds an ErrorKind enum. They could instead allow a user-defined type to be embedded instead of having to create a new type and implement all the necessary nom traits on it:
```
pub struct Error<I, E=()> {
pub input: I,
pub code: ErrorKind,
pub inner: E
}
```
ParseError::from_error_kind() could use Default::default() to create a value for E, so that the user can simply use e.g. E=Option<TheUserType>.
Provide combinator that can create such an error from the user-defined type, in the style of nom::combinator::fail(). Otherwise returning a semantic error becomes really unwieldy and loses a lot of readability. Some of that can be improved by using map_res(), but it does not always have the right semantic (https://github.com/rust-bakery/nom/issues/1624) and this forces to split out the parsing and validation code, often leading to more dynamic allocation than necessary. I ended up with the following:
```
#[inline]
pub fn error<I, O, E, E2>(input: I, err: E) -> nom::IResult<I, O, E2>
where
E2: FromExternalError<I, E>,
{
Err(nom::Err::Error(E2::from_external_error(
    input,
    ErrorKind::Fail,
    err,
)))
}
```

[inline]

pub fn failure<I, O, E, E2>(input: I, err: E) -> nom::IResult<I, O, E2> where E2: FromExternalError<I, E>, { Err(nom::Err::Failure(E2::from_external_error( input, ErrorKind::Fail, err, ))) }



nom version : 7.1.3

rust-bakery / nom

Ease integration of custom error #1661

[inline]