Single-pass compile was a basic requirement back when your compiler only had access to a 64KB address space for its code and data. But it forces program organization into a clunky bottom-up sequential shape that just doesn't jive with the most basic concerns for larger-scale, professional software development. There's a reason why most of us don't code in FORTH, and why more recently Rust even chose the 'crate' rather than the single program file as its compilation unit.