> - You can't use a function before it's defined.

I'm not sure what you mean by this. You obviously can't call a function before it is defined, but you can use a function in another function without any problems:

    julia> foo(x) = bar(x)
    foo (generic function with 1 method)
    
    julia> bar(x) = x+2
    bar (generic function with 1 method)
    
    julia> foo(3)
    5