DataFixerUpper VS qsharp-compiler

You don't need to load a world in every version and in fact this is not recommended because later versions fix various bugs with converting older worlds and with the exception of Beta 1.3 (Alpha to Region*) and release 1.2 (Region to Anvil*) only chunks you've loaded will be converted (since 1.13 there is an "optimize world" feature which can force a full conversion and is recommended mainly to avoid lag due to the game converting chunks on the fly; modern versions change data formats so often Mojang created an entire "data fixer" library and data versioning so it can tell exactly what version a file was last saved in and update it properly, anything without a version tag is assumed to be pre-1.9, which barely changed aside from Alpha-Region-Anvil).

No need to guess, we know pretty much exactly what this is referring to, because it was performance changes in one of the open-source components, DataFixerUpper. The code is open to the public, and you can look at the changes yourself if you want to. It's somewhat technical but I'll try to explain.

Read it and weep! https://github.com/Mojang/DataFixerUpper

java and generics mentioned in the same headline and noone's linked the funny library yet, smh

And I work on Minecraft.

DataFixerUpper, particularly codecs

You can write a library for profunctor optics, something that is usually used in functional languages like Haskell, in Java, but that doesn't mean it's convenient or even practical to use.

It was previously written in C# and F#: https://github.com/microsoft/qsharp-compiler

These are starting to get developed, for example Microsoft's Q#