What does "(#o4, #x4, ?\C-d)" mean here?

Question

I use m-x eval-expression to evaluate a very simple lisp code:

 ( + 1 3)

And I got my result like this

What does the symbols (#o4, #x4, ?\C-d) mean in this context?

Answer 1

These are alternative representations of an integer with their reader literals for octal, hexadecimal and char notation. In other words, evaluating the octal number #o4, hexadecimal number #x4 or char ?\C-d yields 4.

edit: FWIW, this behaviour seems to be a bug and should be fixed on master (and hopefully become part of the upcoming 25.1 release).