Architecture that extends types using extension methods (Microsoft patent)

The expression problem has been known for quite some time [0].

Final tagless[1] is an fp approach and Object algebras[2] is an oop approach to solving this problem.

Setting aside the timelime, reading the various claims it seems like Eric Meijer, Anders Hejlsberg et al still had a hard time obfuscating the math to make patentable claims. A naive read (on my part) of the claims looks like they wished to patent Object Algebras.

[0] https://homepages.inf.ed.ac.uk/wadler/papers/expression/expr...

[1] https://okmij.org/ftp/tagless-final/

[2] https://blog.acolyer.org/2015/11/13/scrap-your-boilerplate-w...

Welcome to the bizarro world of software patents where prior art no longer matters.

So much for C# being open-source, I guess? Is it even safe to fork this repo? https://github.com/dotnet/csharpstandard