The hardest thing is the load() function, particularly in the REPL. It looks dynamic, but is actually static. Pulling off this slight-of-hand requires both type providers and automatic compile-time function evaluation on arbitrary expressions.

F# is the only other language I know of that has type providers. They invented it.

As for CTFE, languages like Zig and D require the user to indicate when to evaluate something ahead of time. I wanted this to happen automatically and still be available for compound expressions, user-defined functions, user-defined types, etc. Doing that requires tracking purity (no state or IO) in an expression, plus a mechanism to actually do the evaluation. I've never seen a language take it to the extreme that Empirical does.

So an existing statically typed language would need (1) a REPL interface, (2) purity tracking, (3) compile-time function evaluation, (4) some kind of types-as-parameters setup, and (5) array notation. Most existing statically typed languages don't have a REPL; the ones that do generally lack array notation. I couldn't find a language that did all of that plus type providers and automated CTFE on arbitrary expressions.

Hence, I had to create my own language.