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Source for wiki ModuleSystems version 8
Added link to ModuleSystemSurvey.
== Introduction ==
As the charter states, "the purpose of this work is to facilitate
sharing of Scheme code." To this effect, a module system is
explicitly given as a requirement in the charter. We need to consider
all the different types of module systems supported by R5RS and R6RS
implementations and come up with a proposal that can gain widespread
Module systems serve the purpose of encapsulating code and managing
namespaces. Although `lambda` lets us manage local identifiers,
it doesn't allow us to encapsulate macros, and it wants for a
friendlier interface for module-like uses.
There are a number of fundamentally different ways in which modules
can be provided. The major types of module systems include:
First-class environments that can be passed as a second argument
to `eval` (or the equivalent) can be used as a form of
encapsulation, where a module is the set of bindings defined in
the environment. If macros are not first-class, the environments
would need some interface for passing back and forth macro
bindings between environments. The disadvantages of this approach
are that many implementations do not support such first-class
environments, separate compilation and phasing becomes more
complicated, and static analysis is almost impossible.
Another natural approach to creating modules is to build them on
top of, and allow them to be composed with, macros, and allowing
import forms to be expanded from macros. It is relatively easy to
implement this for non-syntactic bindings only, and one sample
portable implementation can be found in
Allowing importing and exporting syntax requires non-portable
extensions. Chez Scheme uses this approach.
An alternative approach to extending a module system is to provide
one or more extensible languages in which to compose the modules.
Generally, these languages are described within the module itself.
PLT takes this approach with multiple languages, which can also
modify the read syntax and other features such as the semantics of
function application. Scheme48 takes this approach with a single
meta module called the `config` module, which is globally extended
when you want to add new module syntax. In either of these cases,
you obtain the flexibility to extend the module syntax with new
features, but at the same time provide a separation between the
module meta-info and the actual body of the module itself,
allowing a certain amount of static analysis without actually
expanding the module.
R6RS provides a simple static module syntax which provides just
the basics needed for importing and exporting (optionally renamed)
identifiers from other modules. Neither the module itself nor the
import or export forms may be expanded from macros, so the only
way to extend the syntax is with future standardization efforts.
The primary reason a static module system is desirable is because
by specifying only the syntax it can be implemented by any of the
above module systems. Implementations can then extend the syntax
as needed when not concerned with portability, but a common ground
exists when no advanced features are needed.
== Additional Issues ==
== Existing Proposals ==
* [http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-10.html#node_chap_7 R6RS] (the default choice)