I haven't done anything ≥10k lines yet, but I do have a few projects between 1k and 5k lines that I've been working on and I've easily written 50k+ lines of OCaml in the past year. The development experience I've had is probably the best I've dealt with so far. There isn't a ton of infrastructure around the language, but everything that exists feels very high quality.There's no standard IDE really (although I did recently find OCamlEditor, which seems very nice and even runs on Windows!), but Merlin basically turns any compatible text editor into a full-fledged OCaml IDE (with features like quick-checking code, auto completion, linting, and automatic indentation/formatting). Honestly, debugging isn't something I've had to think of very often. The type system does a really good job of rejecting buggy programs, so whenever I write code it usually works as intended immediately (or I've made a small mistake that I'll notice pretty quickly).