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Source for wiki ModuleSystems version 4

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alexshinn

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118.21.204.67

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ModuleSystems

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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
use.

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
[http://mumble.net/~campbell/scheme/lexmod.tar.gz 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
argument.

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:

* [http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-10.html#node_chap_7 R6RS] (the default choice)

* ModulesAndPackagesArcfide

time

2010-03-05 12:39:46