hh_client can be used directly on the command line, having the immediate feedback directly in an editor can be an amazing productivity boost. If your preferred editor or IDE is unsupported, contributions of additional plugins are very welcome; the existing plugins are decent examples of how to script the
The Nuclide editor has first-class support for Hack. It not only provides inline typechecking, but it also supports autocomplete, jump-to-definition and project search.
Emacs users will find a plugin inside
/usr/share/hhvm/hack/emacs, when installing Hack from a supported HHVM package. And this is the source code