So the Supreme Court said in Google v Oracle that the use of APIs is fair use. Does that mean HelloSystem could re-implement all the Cocoa APIs and have Mac apps compile & run seamlessly?

Theoretically, sure! But that's an enormous effort. This is actually what the goal of GNUstep is and has been from the start, and I kind of doubt there's even a single codebase (apart from, maybe, example code) that will compile on both Xcode and GNUstep without major changes.

Edit: To be clear, GNUstep is something like 6 years _older_ than Mac OS X, so it's had plenty of time to catch up. The problem is really that these APIs are constantly changing and being extended. The goalpost is moving far too fast to ever meet it.

Microsoft tried porting GNUStep to Windows as part of the Windows 10 "iOS Bridge". They gave up, which is actually kind of sad.

Wow, do you have any source on that? Would love to read more about it!

Ok, I'm very much paraphrasing from "shit I read on Wikipedia", but WinObjC does exist and it did use parts of GNUStep: https://github.com/Microsoft/WinObjC/issues/116

On the heyday of trying to cater devs into WinRT, Microsoft had a couple of projects:

- Project Astoria[0], for porting Android apps

- Project Centennial[1], for porting Win32 apps

- Windows Bridge for iOS[2], for porting iOS apps

Project Astoria was later rebooted into WSL, and now with Windows Subsystem for Android we are back at Astoria's original goal.

[0] - https://en.wikipedia.org/wiki/Windows_10_Mobile#Project_Asto...

[1] - https://blogs.windows.com/windowsdeveloper/tag/project-cente...

[2] - https://blogs.windows.com/windowsdeveloper/2015/08/06/open-s...

There was actually an official implementation of OPENSTEP for Windows NT. And later on, you could still get the runtime (called Yellow Box) via WebObjects for example. Not sure if it still exists anymore nowadays.

As far as I know they largely based this on Cocotron[1], without giving credit.

And of course it wasn't so much "giving up" (which sorta implies it was the difficulty of the task), but more "losing interest", because they discontinued Windows Phone, where they were behind Apple's iOS in terms of market share. In that context, getting iOS apps on their system is economically interesting.

On desktop, Windows is way ahead of macOS in terms of market share, so there isn't much of a point.

[1] https://cocotron.org

Considering Wine and ReactOS exist, I assume that'd be doable.

Well, there is the Darling project. https://github.com/darlinghq/darling

You would have achieved the same via Delphi, C++ Builder, VB6, .NET.

I do Windows since version 3.0, and very seldom have touched raw Win32 other than for some university projects where I had no choice to do otherwise.

Having said that, NeXT and Apple frameworks are great.

.NET wasn't even close to ready when I tried it back then. The very first thing I tried (a customized table/listview) was directly enormously inferior to the corresponding class in Cocoa.

I have a rule that if a platform fails the very first thing I try, I'm gonna assume it can't really be awesome for all other things, statistically :P