Many Lisps (like ANSI Common Lisps, and many of its predecessors) have optional chaining in their list processing; it has existed before Apple Inc.

https://news.ycombinator.com/item?id=19002470

no.

Patents are written to be as broad as possible. The most careful interpretation is that if anyone does option chaining anywhere, it is covered by the patent, and actionable by the patent holder.

Companies that are mere consumers of IT have been sued by patent trolls, and paid up.

Take a look at the claims. The patent does not claim option chaining. It claims a few ways to implement a system that may support option chaining.

If whatever language implementation you are using implements that, and infringes, then you infringe that by running programs in that language. All unlicensed users of anything emanating from the patent infringe on it.

Could you please elaborate on this? Is there any direct confirmation of this? There wasn't any single answer from anyone affiliated with the Swift core team on this specific point. Or from any lawyer for that matter.

Patent claims define the legal scope: if "using Swift" isn't in the claims then using another language, provided it fits the language of the claims, is also covered by the patent.

ok, does it mean that if you implement only parts of the patent (say only a single feature) there's no infringement because then it doesn't "fit the language of the claims"?

Also, "using Swift" isn't in the claims ¯\_(ツ)_/¯

If you look at the main claims (any that don't add to a previous claim, ie don't include "the invention of claim X wherein ...") then you need to be doing something that matches all clauses in the claim to infringe.

When drafting the claims writer (patent attorney or agent usually) will add clauses to work around prior art. You can't just add unrelated features though, the features need to have synergy to represent a single invention.

[I've read a lot on USA patents but only worked directly with UK and EU patents.

This is not legal advice.]