Statically-typed, purely functional programming language with a sophisticated type system cited by Schirp as the canonical example of a very high ecosystem threshold — code literally won't compile unless many invariants hold. Schirp would always pick Haskell if he had no economic constraints; in practice he can't sell it to VCs or recruiters (would spend 80% of his time arguing for it, 20% working), so he reaches for Rust as an 'easy sell 80% Haskell'. Property-based testing tools originated in Haskell, and he found them a god-send when working on Haskell in finance.