For convenience, I'd like to define some variables. But I worry that I may forget their names. How can I get a list of them?
You can use command
:let on its own to list the contents of all defined variables.
:let: List the values of all variables.
If you have an idea of what the beginning of the name of the variable might be, you can also use tab completion. For example, for a variable starting with "open", you can use
:let open<Tab> to start cycling over all of those, or
:let open<C-A><CR> to list the contents of all of them.