I've been using Revo Dev to implement a typechecker paper using #Haskell. It's pretty impressive! Runs tests, finds mistakes, iterates. Code is not that good (e.g. reimplements existing functions, uses partial pattern matches) but not awful. Luckily it's Haskell so refactoring is easy.

GHC's codegen+runtime is one of the seven wonders of the programming world