thanks. by full io system, does this mean no opening/closing files/ports, etc?

No (with-output-to-file "foo" (lambda () (write "heyya"))) works, so we have “some” of the IO, (implemented in RPython), but we don’t have everything. We will have the full IO when we start using the IO linklet that Racket provides, which has the “full io system” (in Racket)