debug.getinfo ([thread,] function [, what])
Returns a table with information about a function. You can give the function directly, or you can give a number as the value of function, which means the function running at level function of the call stack of the given thread: level 0 is the current function (getinfo itself); level 1 is the function that called getinfo; and so on. If function is a number larger than the number of active functions, then getinfo returns nil.
The returned table may contain all the fields returned by lua_getinfo, with the string what describing which fields to fill in. The default for what is to get all information available, except the table of valid lines. If present, the option 'f' adds a field named func with the function itself. If present, the option 'L' adds a field named activelines with the table of valid lines.
For instance, the expression debug.getinfo(1,"n").name returns a name of the current function, if a reasonable name can be found, and debug.getinfo(print) returns a table with all available information about the print function.