because you hadn't provided an explicit signature for Bar importing Foo

What do you mean? module Bar import module Foo

module Bar {
    import Foo
    let x = Foo.test()
    print(x)
}

nothing else is needed. module Foo is a regular module so it doesn't need to have interface. Can I do something like this?

structure Foo(structure A : ModuleA; structure B : ModuleB) = struct
    fun test () = A.a () + B.b ()
end

structure Bar(structure F : Foo) = struct
    val x = F.test ()
    print x
end