The big thorn is closures, which show up all over the place in FP. You either need to limit closures vs ordinary functions (as e.g. Rust does), have manual memo

The big thorn is closures, which show up all over the place in FP. You either need to limit closures vs ordinary functions (as e.g. Rust does), have manual memory annotations (such as e.g. what Swift does) or you basically need a GC. The former two choices are annoying if you really take advantage of functions as first-class citizens.