Have yet to find a better choice than OCaml:

- Strongly typed, including GADTs and various flavors of polymorphism, but not as inscrutable as Haskell

- (Mostly) pure functions, but multiple imperative/OO escape hatches

- The base language is surprisingly simple

- Very fast to build/test (the bytecode target, at least)

- Can target WASM/JS

- All code in a file is always evaluated in order, which means it has to be defined in order. Circular dependencies between functions or types have to be explicitly called out, or build fails.

I should add, it's also very fun to work with as a human! Finding refactors with pure code that's this readable is a real joy.