You can only code with Haxe in OpenFL.
Read in Developer section of OpenFL website
At the heart of OpenFL is the Haxe programming language. Haxe uniquely targets other programming languages as the output. As a result, a Haxe application has the power to target C++ (for native platforms), JavaScript (for HTML5) and Flash from the source. This is mature, well-supported, and is what enables OpenFL to take a different approach -- instead of compromising, OpenFL creates a middle-ground between scripting languages and native languages. You can get the performance and strict typing you need for excellent results on mobile or desktop, while leveraging your investment for the web. OpenFL provides the tools and libraries necessary to make it simple to wrap this cross-language power and make it easy to deliver great results, everywhere.