Trait Drain

pub trait Drain {
    type Ok;
    type Err;

    // Required method
    fn log(
        &self,
        record: &Record<'_>,
        values: &OwnedKVList,
    ) -> Result<Self::Ok, Self::Err>;

    // Provided methods
    fn is_enabled(&self, level: Level) -> bool { ... }
    fn is_critical_enabled(&self) -> bool { ... }
    fn is_error_enabled(&self) -> bool { ... }
    fn is_warning_enabled(&self) -> bool { ... }
    fn is_info_enabled(&self) -> bool { ... }
    fn is_debug_enabled(&self) -> bool { ... }
    fn is_trace_enabled(&self) -> bool { ... }
    fn map<F, R>(self, f: F) -> R
       where Self: Sized,
             F: FnOnce(Self) -> R { ... }
    fn filter<F>(self, f: F) -> Filter<Self, F>
       where Self: Sized,
             F: FilterFn { ... }
    fn filter_level(self, level: Level) -> LevelFilter<Self>
       where Self: Sized { ... }
    fn map_err<F, E>(self, f: F) -> MapError<Self, E>
       where Self: Sized,
             F: MapErrFn<Self::Err, E> { ... }
    fn ignore_res(self) -> IgnoreResult<Self>
       where Self: Sized { ... }
    fn fuse(self) -> Fuse<Self>
       where Self::Err: Debug,
             Self: Sized { ... }
}

Logging drain

Drains typically mean destination for logs, but slog generalizes the term.

Drains are responsible for handling logging statements (Records) from Loggers associated with them: filtering, modifying, formatting and writing the log records into given destination(s).

It’s a typical pattern to parametrize Drains over Drain traits to allow composing Drains.

Implementing this trait allows writing custom Drains. Slog users should not be afraid of implementing their own Drains. Any custom log handling logic should be implemented as a Drain.

Required Associated Types

type Ok

Type returned by this drain

It can be useful in some circumstances, but rarely. It will probably default to () once https://github.com/rust-lang/rust/issues/29661 is stable.

type Err

Type of potential errors that can be returned by this Drain

Required Methods

fn log( &self, record: &Record<'_>, values: &OwnedKVList, ) -> Result<Self::Ok, Self::Err>

Handle one logging statement (Record)

Every logging Record built from a logging statement (eg. info!(...)), and key-value lists of a Logger it was executed on will be passed to the root drain registered during Logger::root.

Typically Drains:

• pass this information (or not) to the sub-logger(s) (filters)
• format and write the information to a destination (writers)
• deal with the errors returned from the sub-logger(s)

Provided Methods

fn is_enabled(&self, level: Level) -> bool

Avoid: Check if messages at the specified log level are maybe enabled for this logger.

The purpose of it so to allow imprecise detection if a given logging level has any chance of actually being logged. This might be used to explicitly skip needless computation.

It is best effort, can return false positives, but not false negatives.

The logger is still free to ignore records even if the level is enabled, so an enabled level doesn’t necessarily guarantee that the record will actually be logged.

This function is somewhat needless, and is better expressed by using lazy values (see FnValue). A FnValue is more precise and does not require additional (potentially recursive) calls to do something that log will already do anyways (making decision if something should be logged or not).

let logger = Logger::root(Discard, o!());
if logger.is_enabled(Level::Debug) {
    let num = 5.0f64;
    let sqrt = num.sqrt();
    debug!(logger, "Sqrt"; "num" => num, "sqrt" => sqrt);
}

fn is_critical_enabled(&self) -> bool

Avoid: See is_enabled

fn is_error_enabled(&self) -> bool

Avoid: See is_enabled

fn is_warning_enabled(&self) -> bool

Avoid: See is_enabled

fn is_info_enabled(&self) -> bool

Avoid: See is_enabled

fn is_debug_enabled(&self) -> bool

Avoid: See is_enabled

fn is_trace_enabled(&self) -> bool

Avoid: See is_enabled

fn map<F, R>(self, f: F) -> R

where Self: Sized, F: FnOnce(Self) -> R,

Pass Drain through a closure, eg. to wrap into another Drain.

#[macro_use]
extern crate slog;
use slog::*;

fn main() {
    let _drain = Discard.map(Fuse);
}

fn filter<F>(self, f: F) -> Filter<Self, F>

where Self: Sized, F: FilterFn,

Filter logging records passed to Drain

Wrap Self in Filter

This will convert self to a Drain that ignores Records for which f returns false.

fn filter_level(self, level: Level) -> LevelFilter<Self>

where Self: Sized,

Filter logging records passed to Drain (by level)

Wrap Self in LevelFilter

This will convert self to a Drain that ignores Records of logging lever smaller than level.

fn map_err<F, E>(self, f: F) -> MapError<Self, E>

where Self: Sized, F: MapErrFn<Self::Err, E>,

Map logging errors returned by this drain

f is a closure that takes Drain::Err returned by a given drain, and returns new error of potentially different type

fn ignore_res(self) -> IgnoreResult<Self>

where Self: Sized,

Ignore results returned by this drain

Wrap Self in IgnoreResult

fn fuse(self) -> Fuse<Self>

where Self::Err: Debug, Self: Sized,

Make Self panic when returning any errors

Wrap Self in Map

Implementations on Foreign Types

impl<'a, D: Drain + 'a> Drain for &'a D

type Ok = <D as Drain>::Ok

type Err = <D as Drain>::Err

fn log( &self, record: &Record<'_>, values: &OwnedKVList, ) -> Result<Self::Ok, Self::Err>

fn is_enabled(&self, level: Level) -> bool

impl<'a, D: Drain + 'a> Drain for &'a mut D

type Ok = <D as Drain>::Ok

type Err = <D as Drain>::Err

fn log( &self, record: &Record<'_>, values: &OwnedKVList, ) -> Result<Self::Ok, Self::Err>

fn is_enabled(&self, level: Level) -> bool

impl<D: Drain + ?Sized> Drain for Box<D>

type Ok = <D as Drain>::Ok

type Err = <D as Drain>::Err

fn log(&self, record: &Record<'_>, o: &OwnedKVList) -> Result<Self::Ok, D::Err>

fn is_enabled(&self, level: Level) -> bool

impl<D: Drain + ?Sized> Drain for Arc<D>

type Ok = <D as Drain>::Ok

type Err = <D as Drain>::Err

fn log(&self, record: &Record<'_>, o: &OwnedKVList) -> Result<Self::Ok, D::Err>

fn is_enabled(&self, level: Level) -> bool

impl<D: Drain> Drain for Mutex<D>

type Ok = <D as Drain>::Ok

type Err = MutexDrainError<D>

fn log( &self, record: &Record<'_>, logger_values: &OwnedKVList, ) -> Result<Self::Ok, Self::Err>

fn is_enabled(&self, level: Level) -> bool

Implementors

impl Drain for Discard

type Ok = ()

type Err = NeverStruct

impl<D1: Drain, D2: Drain> Drain for Duplicate<D1, D2>

type Ok = (<D1 as Drain>::Ok, <D2 as Drain>::Ok)

type Err = (Result<<D1 as Drain>::Ok, <D1 as Drain>::Err>, Result<<D2 as Drain>::Ok, <D2 as Drain>::Err>)

impl<D> Drain for Logger<D>

where D: SendSyncUnwindSafeDrain<Ok = (), Err = Never>,

type Ok = ()

type Err = NeverStruct

impl<D: Drain> Drain for Fuse<D>

where D::Err: Debug,

type Ok = ()

type Err = NeverStruct

impl<D: Drain> Drain for IgnoreResult<D>

type Ok = ()

type Err = NeverStruct

impl<D: Drain> Drain for LevelFilter<D>

type Ok = Option<<D as Drain>::Ok>

type Err = <D as Drain>::Err

impl<D: Drain, E> Drain for MapError<D, E>

type Ok = <D as Drain>::Ok

type Err = E

impl<D: Drain, F> Drain for Filter<D, F>

where F: FilterFn,

type Ok = Option<<D as Drain>::Ok>

type Err = <D as Drain>::Err