Be OS API
The BeOS API, often called the Be API after Be Inc., is the application programming interface required to write graphical native applications on BeOS, and hence its derivatives such as ZETA and Haiku.
The API is divided into a number of "kits", which collect related classes together and bear some relation to the library which contains the supporting code.
It is almost entirely C++, with third-party bindings for most classes in Python and a tiny subset of classes, mostly Storage kit related, in Perl.
Read more about Be OS API: Kits, Non-BeOS Implementations, Documentation