I recently learned about that too, how Lean provides ways to create new language syntax and DSLs, which is (on the surface) similar to Lisps like Racket. It's like operator overloading but way more flexible and general purpose. I'm wary of such language features, convenient for specific purposes (like working with vectors or matrices) but I'm afraid it's too powerful for normal usage, if everyone starts building their own DSLs and syntaxes, it would likely make the typical codebase difficult to read and understand. For example, the C++ template metaprogramming madness, it can be used responsibly but in my opinion it was detrimental to the language ecosystem.

And then there are sealed DSLs...

Byte encoder/decoder in Elm are opaque, so their implementation is hidden. A smart compiler, such as the one I am writing, can take advantage of that with compiler intrinsics that replace the implementation with something that the compiler itself optimizes.