Alternatively remove the include statement from functions.jl
and only include structs.jl from main.
That depends a little bit on your desired program structure.
Do you want doubly nested modules by making structs a submodule of functions which itself lives in main?
Alternatively both functions and structs could live in main directly.
Note however that all these modules are not necessary at all in your case here. In particular your main module does not seem to serve an purpose.