"Marzipan" is the code name for UIKit apps on the Mac. Traditional Mac apps are built in AppKit, something that traces back to Steve Jobs' NeXT, but with Marzipan, the goal is to open Mac development up to the much bigger, broader, iOS community. So far, only Apple has deployed only a few Marzipan apps, including News, Voice Recorder, Stocks, and Home for macOS Mojave. This year, Apple has said they'll be opening up the frameworks to developers, likely at WWDC in June. But that's just the beginning.

Mark Gurman, writing for Bloomberg:

Later this year, Apple plans to let developers port their iPad apps to Mac computers via a new software development kit that the company will release as early as June at its annual developer conference. Developers will still need to submit separate versions of the app to Apple's iOS and Mac App Stores, but the new kit will mean they don't have to write the underlying software code twice, said the people familiar with the plan.

In 2020, Apple plans to expand the kit so iPhone applications can be converted into Mac apps in the same way. Apple engineers have found this challenging because iPhone screens are so much smaller than Mac computer displays.

By 2021, developers will be able to merge iPhone, iPad, and Mac applications into one app or what is known as a "single binary." This means developers won't have to submit their work to different Apple App Stores, allowing iOS apps to be downloaded directly from Mac computers -- effectively combining the stores.

I've always wanted iPhone app windows on iPad and, yeah, Mac, so sign me up.