This module system was not designed as the be-all and end-all of Scheme module systems; it was only intended to help Richard Kelsey and me to organize the Scheme 48 system. Not only does the module system help avoid name clashes by keeping different subsystems in different namespaces, it has also helped us to tighten up and generalize Scheme 48 's internal interfaces. Scheme 48 is unusual among Lisp implementations in admitting many different possible modes of operation. Examples of such multiple modes include the following:
The need to make structures and interfaces second-class instead of first-class results from the requirements of static program analysis: it must be possible for the compiler and linker to expand macros and resolve variable bindings before the program is executed. Structures could be made first-class (as in FX) if a type system were added to Scheme and the definitions of exported macros were defined in interfaces instead of in module bodies, but even in that case types and interfaces would remain second-class.
The prohibition on assignment to imported bindings makes substitution a valid optimization when a module is compiled as a block. The block compiler first scans the entire module body, noting which variables are assigned. Those that aren't assigned (only defined) may be assumed never assigned, even if they are exported. The optimizer can then perform a very simple-minded analysis to determine automatically that some procedures can and should have their calls compiled in line.
The programming style encouraged by the module system is consistent with the unextended Scheme language. Because module system features do not generally show up within module bodies, an individual module may be understood by someone who is not familiar with the module system. This is a great aid to code presentation and portability. If a few simple conditions are met (no name conflicts between packages, no use of structure-ref, and use of files in preference to begin), then a multi-module program can be loaded into a Scheme implementation that does not support the module system. The Scheme 48 static linker satisfies these conditions, and can therefore run in other Scheme implementations. Scheme 48 's bootstrap process, which is based on the static linker, is therefore nonincestuous. This contrasts with most other integrated programming environments, such as Smalltalk-80, where the system can only be built using an existing version of the system itself.
Like ML modules, but unlike Scheme Xerox modules, this module system is compositional. That is, structures are constructed by single syntactic units that compose existing structures with a body of code. In Scheme Xerox, the set of modules that can contribute to an interface is open-ended --- any module can contribute bindings to any interface whose name is in scope. The module system implementation is a cross-bar that channels definitions from modules to interfaces. The module system described here has simpler semantics and makes dependencies easier to trace. It also allows for higher-order modules, which Scheme Xerox considers unimportant.