Even in Haskell, writing state is as easy as += or <- and this is used in nearly every actual program. A cryptocurrency implementation like this doesn't need a

Even in Haskell, writing state is as easy as += or <- and this is used in nearly every actual program. A cryptocurrency implementation like this doesn't need a language that makes state impossible. It needs static analysis, model checking, and proofs. This would be true even if it were written in your favorite functional language.