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 separately compilable,
mutually recursive components. Mixin modules facilitate
recursive linking of separately compiled components, but they are
not hierarchically composable and typically do not support type
abstraction. 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 MLstyle
data 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) can be understood
simply as stylized uses of a small set of orthogonal underlying constructs,
with mixin composition playing a central role.