The secret of Lean4 is that its actually a great programming language, sort like a super Haskell. In fact I like it more as a programming language then as a proof checker!

Dependent types are awesome for regular programming in addition to formal verification. For proof checking I actually prefer Agda.

I wanted to know if Lean 4 is usable for an everyday web app yet, not just for proofs, so I built RealWorld (the Medium clone) in it, frontend included, using Datastar. Looks like it's feasible now with the new libraries. This post describes the experience