What (typed as :what in the HACTRN prompt) was a small information utility available in the Incompatible Timesharing System.
It could provide information about incoming email, bus schedule on the MIT campus, executable source files or answer the user in a humorous manner.
:what was written in the MIDAS assembly language.
The last traceable edit of the source code was by Ken L. Harrenstien on 16 May 1988.
Without arguments :what would print information about inbox status: With the argument bus it would print out information about the next few buses leaving from the MIT campus: Asked about source for NAME, :what responded with paths to source files corresponding to NAME: Not knowing the answer, it would often resort to humor: Finally, :what displayed some amount of introspection: