cmp/options.go
func Reporter(r interface {
	// PushStep is called when a tree-traversal operation is performed.
	// The PathStep itself is only valid until the step is popped.
	// The PathStep.Values are valid for the duration of the entire traversal
	// and must not be mutated.
	//
	// Equal always calls PushStep at the start to provide an operation-less
	// PathStep used to report the root values.
	//
	// Within a slice, the exact set of inserted, removed, or modified elements
	// is unspecified and may change in future implementations.
	// The entries of a map are iterated through in an unspecified order.
	PushStep(PathStep)

	// Report is called exactly once on leaf nodes to report whether the
	// comparison identified the node as equal, unequal, or ignored.
	// A leaf node is one that is immediately preceded by and followed by
	// a pair of PushStep and PopStep calls.
	Report(Result)

	// PopStep ascends back up the value tree.
	// There is always a matching pop call for every push call.
	PopStep()
}) Option {
	return reporter{r}
}
func Reporter(r interface {
	// PushStep is called when a tree-traversal operation is performed.
	// The PathStep itself is only valid until the step is popped.
	// The PathStep.Values are valid for the duration of the entire traversal
	// and must not be mutated.
	//
	// Equal always calls PushStep at the start to provide an operation-less
	// PathStep used to report the root values.
	//
	// Within a slice, the exact set of inserted, removed, or modified elements
	// is unspecified and may change in future implementations.
	// The entries of a map are iterated through in an unspecified order.
	PushStep(PathStep)

	// Report is called exactly once on leaf nodes to report whether the
	// comparison identified the node as equal, unequal, or ignored.
	// A leaf node is one that is immediately preceded by and followed by
	// a pair of PushStep and PopStep calls.
	Report(Result)

	// PopStep ascends back up the value tree.
	// There is always a matching pop call for every push call.
	PopStep()
}) Option {
	return reporter{r}
}
// Reporter is an Option that can be passed to Equal. When Equal traverses
// the value trees, it calls PushStep as it descends into each node in the
// tree and PopStep as it ascend out of the node. The leaves of the tree are
// either compared (determined to be equal or not equal) or ignored and reported
// as such by calling the Report method.