ML modules provide hierarchical namespace management, as well as fine-grained control over the propagation of type information, but they do not allow modules to be broken up into mutually recursive, separately compilable components. Mixin modules facilitate recursive linking of separately compiled components, but they are not hierarchically composable and typically do not support type ion. We synthesize the complementary advantages of these two mechanisms in a novel module system design we call MixML. A MixML module is like an ML structure in which some of the components are specified but not defined. In other words, it unifies the ML structure and signature languages into one. MixML seamlessly integrates hierarchical composition, translucent MLta abstraction, and mixin-style recursive linking. Moreover, the design of MixML is clean and minimalist; it emphasizes how all the salient, semantically interesting features of the ML module system (as well as several proposed extensions to it...