tap 0.3.1

Generic extensions for tapping values in Rust


Documentation crates.io version Language (Rust)

A simple crate exposing tapping functionality for all types, and extended functionality for Option, Result & Future. Often useful for logging.

The tap operation takes, and then returns, full ownership of the variable being tapped. This means that the closure may have mutable access to the variable, even if the variable is otherwise immutable. Mutating closures require the |mut name| pattern, and so can be easily distinguished from non-mutating taps.


  • future - Exposes the TapFutureOps trait, providing tap_ready, tap_not_ready & tap_err (requires the futures crate).

    Futures do not provide mutable access for tap closures.

  • nom3 - Exposes the TapNomOps trait, which provides tap_done, tap_error, and tap_incomplete on their respective variants of nom::IResult.


extern crate tap;

use tap::{TapOps, TapResultOps, TapOptionOps};

fn filter_map() {
    let values: &[Result<i32, &str>] = &[Ok(3), Err("foo"), Err("bar"), Ok(8)];

    let _ = values.iter().filter_map(|result| {
        // It is especially useful in filter maps, allowing error information to
        // be logged/printed before the information is discarded.
        result.tap_err(|error| println!("Invalid entry: {}", error)).ok()

fn basic() {
    let mut foo = 5;

    // The `tap` extension can be used on all types
    if 10.tap(|v| foo += *v) > 0 {
        assert_eq!(foo, 15);

    // Results have `tap_err` & `tap_ok` available.
    let _: Result<i32, i32> = Err(5).tap_err(|e| foo = *e);
    assert_eq!(foo, 5);

    // Options have `tap_some` & `tap_none` available.
    let _: Option<i32> = None.tap_none(|| foo = 10);
    assert_eq!(foo, 10);

fn mutable() {
    let base = [1, 2, 3];
    let mutated = base.tap(|mut arr| for elt in arr.iter_mut() {
        *elt *= 2;
    assert_eq!(mutated, [2, 4, 6]);
    //  base was consumed, and then returned into mutated, and is out of scope.