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Source for wiki ModuleSystems version 4











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.  Some of the existing
general strategies for implementations include:

  * StaticModules

    Many implementations provide a static module syntax that cannot
    be extended.  They just provide the basics needed for importing and
    exporting (optionally renamed) identifiers from other modules.
    The module form itself, and any auxiliary forms such as `import`
    and `export`, may or may not be expandable as syntax.

  * SyntacticModules

    A natural approach to creating modules is to build them on top of,
    and allow them to be composed with, macros.  It is relatively easy
    to implement this for variables only (not importing or exporting
    syntax), and one sample implementation can be found in
    [ lexmod].
    Allowing importing and exporting syntax requires non-portable
    extensions.  Chez Scheme uses this approach.

  * ConfigModules

    An alternative approach to extending a module system is to use a
    DSL for the module syntax that can extend itself.  Scheme48 and
    Chibi take this approach, supplying a DSL that is essentially
    Scheme itself.  This allows equivalent extensibility to
    SyntacticModules while keeping the module extensions clearly
    separate from the language they describe, and making it easier to
    statically analyze modules.

  * FirstClassEnvironments

    First-class environments that can be passed as a second argument
    to `eval` (or the equivalent) can be used to implement modules
    directly.  If macros are not first-class, the environments would
    need some interface for passing back and forth macro bindings.
    The disadvantage of this approach is that many implementations do
    not support first-class environments.

Whatever implementation strategy is used, a syntax for the standard
module form must be chosen.  Modulo keyword names, options include:

  * Unwrapped:

    (module <declarations ...)
    <body> ...

    Seen in most other programming languages, the `module` form is
    just a single declaration, and the rest of the file contains the
    actual code.  This has the distinct disadvantage that it can't be
    implemented easily in many strategies, or on top of existing
    module systems.

  * Simple wrapper:

    (module <declarations> ... <body> ...)

    Used in the R6RS proposal and many Scheme implementations, this is
    simple but opens questions of whether declarations may be expanded
    from macros, and if so makes any static analysis of the module
    impossible without expanding the body.  Sometimes disliked because
    it requires indenting the body of the module.

  * Delimited wrapper:

    (module (<declarations> ...) <body> ...)

    This avoids the issues above, simply by delimiting the
    declarations with a pair of parentheses so they are all known in
    advance.  If they are static (can't be expanded from macros), then
    a simple rule of allowing and ignoring any unknown declaration
    keyword allows for easy forward-compatibility and
    implementation-specific extensions.

  * Declaration-only:

    (module <declarations> ...)

    The module form only allows declarations - any code needs to be
    specified with declarations such as (include <file>) or (body
    <code> ...).  This is the syntax used in Scheme48 and
    Chibi-Scheme.  It is equivalent in expressiveness to the delimited
    wrapper approach, trading an additional level of indentation for
    no extra parens around the declarations.

  * Declaration-only with embedded body shortcut:

    (module <declarations> ... ---- <body> ...)

    where `----` is some arbitrary symbol chosen to act as a delimiter
    between the declarations and the body of the library.  Otherwise
    the same as above, the shortcut syntax allows the body to have
    only one level of indentation instead of two.

In addition, with all of these syntaxes, some declarations
that are frequently used may get fixed positions.  The name
of the module is almost universally the first argument after
the `module` keyword.  In some systems, such as Chez and
Chicken, the exports list is given as the second positional

One frequent debate with respect to syntax is whether to
keep the module declaration and source in one file or to
split them across separate files.  However, whichever
module system is used this largely boils down to user
preference.  The syntaxes which include an implicit body
suggest a single file, but so long as an (include <file>)
or similar form is provided, any such system can move the
body to a separate file.  From the other side, syntaxes
with no <body> suggest an `include` is required, but all
such systems provide a way to inline the body in the
module declaration (`begin` in Scheme48 and `body` in Chibi).
So this preference shouldn't affect your choice of syntax.

Existing Proposals:

  * [ R6RS] (the default choice)

  * ModulesAndPackagesArcfide


2010-03-05 12:39:46