According to this [issue comment](https://github.com/elixir-lang/elixir/issues/4103#issuecomment-166843877), Windows users can get tab completion by starting the special `werl` console. To start the REPL with werl, run `iex --werl` at the command prompt. I don't know if this console is significantly different than the standard `iex` console, but dang tab completion is handy.