Usually, you just declare the module interface separately from the implementation. I don't recall there being any to "work around" e.g. declaring mutually recur

Usually, you just declare the module interface separately from the implementation. I don't recall there being any to "work around" e.g. declaring mutually recursive types in two different modules[1]. (And I don't think that's in any way sane, FWIW.) [1] Of course you can parametrize one of the modules with a signature declared elsewhere and the other module implement that signature, but that's really just regular old dependency-breaking, so I'm not sure that counts.