Why OCaml? It's a great language to write programs that works on complex data structures, e.g. ASTs. This choice was actually not very original: people in academia at stanford, berkeley, Microsoft research used OCaml for program analysis (CCured, Saturn, CIL, SLAM). And now and now the industry is also using it (Facebook Infer, Facebook Hack/Flow/Pyre, MS Static Device Verifier, etc.)