Late last year, a Bloomberg report suggested that an Apple project, codenamed Marzipan, was exploring the convergence of macOS and iOS apps. Seeing as a successful project with this goal would enable app developers to build an app once, and launch it cross-platform, the alleged Project Marzipan gained significant buzz in the Apple developer world. Recent updates suggest that the convergence of macOS and iOS might not be on an immediate roadmap; rather, Apple might be exploring declarative APIs for both macOS and iOS.
A declarative API allows a developer to write an app for multiple user interfaces at the same time. While a declarative API would assist developers needing to build apps for multiple platforms from an interface perspective, a declarative API would not allow a developer to port a macOS app to iOS, nor vice versa. A declarative API still requires coding for both macOS and iOS separately.
When the news of the MacOS/iOS convergence initially sprung, many anticipated the announcement would arrive at WWDC 2018, next month in San Jose. While there will be no shortage of big announcements at this year's event (perhaps mention of a declarative API), don't expect the convergence of macOS and iOS to be one of them. Just last month, Apple's CEO, Tim Cook, addressed the rumor with the Sydney Morning Herald:
"So this merger thing that some folks are fixated on, I don't think that's what users want....Wd don't believe in sort of watering down one for the other. Both [The Mac and iPad] are incredible. One of the reasons that both of them are incredible is because we pushed them to do what they do well. And if you being to merge the two...you being to make trade offs and compromises."