CompactMap

.compactMap (Publishers.CompactMap) is closely related both to .map and to .filter. Like .map, it has the power to transform the value received from upstream into a different value. Like .filter, it has the power to prevent a value from proceeding further down the pipeline. Its function is both a map function and a filter function.

So this function has to say two things at once: what value should proceed down the pipeline, and should that value proceed down the pipeline. It does that by following a rule that the value it produces must be an Optional. If that Optional is not nil, it is unwrapped and the unwrapped value proceeds down the pipeline. But if it is nil, the value stops and doesn’t proceed further.

.compactMap is useful particularly when you need to cast down safely. A common use case is in connection with a Notification that is supposed to wrap a value in its userInfo dictionary. The userInfo dictionary itself is an Optional, and fetching a value by its key from a dictionary produces an Optional. Moreover, the userInfo value type is Any, so we need to cast it down safely to whatever it’s supposed to be. All of that can be elegantly expressed using .compactMap. In this example, my Notification’s userInfo is supposed to contain a "zip" key whose value is a String, which is what we’re really interested in:

NotificationCenter.default.publisher(for: .zipCodeDidChange)
    .compactMap { $0.userInfo?["zip"] as? String }

If the Notification has no userInfo, or if it doesn’t contain a "zip" key, or if the "zip" key’s value is not a String, we’ll get nil and the value just won’t proceed down the pipeline. Conversely, if everything happens the way we expect, what proceeds down the pipeline is that String.

There is also a .tryCompactMap (Publishers.TryCompactMap); it works similarly to .tryMap, so I won’t say more about it.