They all have distiguishing features that can be emulated in other languages (to a degree) but will throw off even senior developers. Lisp has its macros. El

They all have distiguishing features that can be emulated in other languages (to a degree) but will throw off even senior developers. Lisp has its macros. Elm has its type system. Go, Elixir, and JS have rather idiosyncratic concurrency models. Rust has its borrow checker. Perl seems to be SO special that even proficient Perl devs aren't able to write performant code in it. /s Sure, Prolog and SQL are much more different than the rest. But I didn't want to strawman here.