Module tap::tap

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Point-Free Inspection

The standard library does not provide a way to view or modify an expression without binding it to a name. This module provides extension methods that take and return a value, allowing it to be temporarily bound without creating a new let-statement in the enclosing scope.

The two main uses of these methods are to temporarily attach debugging tracepoints to an expression without modifying its surrounding code, or to temporarily mutate an otherwise-immutable object.

For convenience, methods are available that will modify the view of the tapped object that is passed to the effect function, by using the value’s Borrow/BorrowMut, AsRef/AsMut, or Index/IndexMut trait implementations. For example, the Vec collection has no fn sort method: this is actually implemented on slices, to which Vec dereferences.

use tap::tap::*;

// taps take ordinary closures, which can use deref coercion
make_vec().tap_mut(|v| v.sort());
// `Vec<T>` implements `BorrowMut<[T]>`,
// and `AsMut<[T]>`,
// and `DerefMut<Target = [T]>,
// but has no inherent method `sort`.
// make_vec().tap_mut(Vec::sort);



Point-free value inspection and modification.
Fallible tapping, conditional on the optional success of an expression.
Optional tapping, conditional on the optional presence of a value.