What does `abspath(PROGRAM_FILE) == @__FILE__` do?

Question

Given this code:

function main()
    println("hello")
end

if abspath(PROGRAM_FILE) == @__FILE__
    main()
end

What does the last part do, and how does it work? Where are those variables defined?

Answer 1

PROGRAM_FILE is a global constant defined as the string containing the script name passed to the executing Julia process from the command line (the first element of the argv in C terms, I think).

@__FILE__ is a macro expanding to the name of the file it is expanded in.

The combination shown is the Julia variant of Python's if __name__ == '__main__': when the file is run as a script, PROGRAM_FILE will contain the script name, equal to the current file, and main will be called. When the file is included or imported from somewhere else, PROGRAM_FILE will contain the name of whatever script used the current file, and differ from the current file name.