WG1 Module Proposal
Aaron W. Hsu [arcfide] <email@example.com>
A simple two-level module system is proposed that permits fine grained binding control at any level of Scheme code as well as providing for the creation of complete packaging units for distribution and code management. The system extends the Scheme language without introducing any specialized contexts. Rather the system promotes the free use and rebinding of these forms anywhere in Scheme code, permitting very expressive extensions to the Core Scheme language.
A simple but expressive module system is needed that enables the extension of Scheme without undue complexity. The charter indicates that a module system for WG1 should be defined to "promote extensibility." Extensibility ranges in more than one direction, and a module system ought to address both the issues of packaging as well as the issues of binding visibility and scope at all levels of Scheme code.
This document proposes the following forms:
The following syntax describes their general use.<library> := (library <library name> <export> <import> <expression> ...) <library name> := (<library name element> <library name element> ...) <library name element> := <symbol> | <exact integer> <module> := (module [<identifier>] <expression> <expression> ...) <export> := (export <export-spec> ...) <export-spec> := <identifier> | (rename (<idientifier_1> <identifier_2>) ...) <co-export> := (co-export <identifier> <identifier> ...) <implicit-export> := (implicit-export <identifier> <identifier> ...) <import> := (import <import-spec> ...) <import-spec> := <identifier> | <library name> | (library <library name>) | (only <import-spec> <identifier> ...) | (except <import-spec> <identifier> ...) | (prefix <import-spec> <identifier>) | (rename <import-spec> (<identifier_1> <identifier_2>) ...) <include> := (include <string>)
A library form may only occur at the top-level of a program. The library name is the unique identifier of the library defined by the library form. This proposal extends the R6RS naming convention to allow for exact integers as well as symbols.
A module form may occur at any definition context. That is, it may appear anywhere that a definition may appear. The module form evaluates to an unspecified value. When a module form with the optional name element is encountered, the given identifier is visible as a valid import for all import forms that occur within the scope where the module form occurs. That is, the identifier is bound to a module entity that may be imported using an import form. If no optional identifier is given, then the exports associated with the module form are exported and made visible in the scope where the module form occurs.
Macros may expand into both module and library forms. In the case of libraries, the libraries are only legal at the top-level.
Both modules and libraries may have exports associated with them using the export form. Each export spec indicates one or more identifiers that are exports from the library or module. When the export spec is simply an identifier, then that identifier is exported. When the export spec is a rename form, the latter identifier of each pair is exported.
Exports apply to the nearest module or library form where they are encountered. That is, if a library form encapsulates a module form wherein occurs an export form, that export form applies to the module form, rather than the library form which wraps or contains the module form.
Identifiers may be associated with a set of co-exports or implicit exports. In the case of co-exports, when an identifier with co-exports is exported by an export form, then all of the co-exports associated with that identifier are also exported. In the case of implicit-exports, if a macro is bound to an identifier with implicit-exports, if that identifier is exported, then the macro may expand into identifiers that are implicitly exported without it being an unbound identifier error. This allows bindings to be implicitly visible to the outside world without requiring the module or library form to explicitly export them. Rather, a macro may expand into them, instead. By default, if no implicit-export form indicates the set of implicit identifiers associated with a given syntax, and that syntax is exported, then all of the definitions and bindings visible to the macro are implicitly exported (this corresponds to the R6RS behavior). However, if an implicit-export form does exist, the set of identifiers implicitly exported for that syntax is completely and fully specified by the implicit-export form. It is an error to have more than one implicit-export form attaching names to a single syntax export.
The import form makes bindings exported by libraries or named modules visible at some scope. An import form may occur anywhere a definition may occur. The exports associated with the libraries and modules referenced by the import form are made visible in the scope where the import form occurs. By default, all of the exports are made visible. However, the exports may be limited or renamed using only, prefix, except, and rename, according to the R6RS rules. The library form allows for the import of libraries whose names class with those of the operations just listed.
It is sometimes useful to separate the source code of a library or module from the declaration of that module or library. This can be achieved by the use of the 'include' form. The include form expects a single string as its only argument. The include form reads in the forms that it finds by resolving the string to some location, possibly using the source-directories parameter to do so. It expands into the forms that it reads in, with each form wrapped with as if they had been entered where the include occured. That is, the include form wraps the incoming forms with its own wraps before expanding into those forms.
The source-directories function behaves as if it had parameter semantics, whether or not actual parameters are used to implement this procedure. While other values may be stored, the source-directories parameter is expected to at least contain a list of strings. Implementations must at least support some interpretation of this source-directories parameter value, and it is permissible for them to ignore it.
Multiple bindings of the same name may not be imported into the same scope. Moreover, the bindings exported by a library form are considered to be immuatable, whereas the bindings exported by a module form are immutable or mutable only based on their original definition. That is, by default, if a binding is mutable inside the module form, then it is also mutable outside of the module form when exported and subsequently imported.
All module forms are expanded and evaluated once. In this respect they are similar to LET forms that export definitions from inside their form, and return (void) as their value. Bindings are visible in the same contexts that they were defined in.
On the other hand, library forms may be evaluated many times, possibly zero times, if an implementation may determine that the library is not needed. Additionally, for forwards compatibility, it is expected that exports from a library may be made visible in arbitrary contexts, though this has no effect on WG1.
This system is explicitly designed to take advantage of and encourage the full range of expressive power from syntactic abstractions. To this end, the imports and exports are separate forms, rather than being tied directly to the library and module forms. Moreover, it is recognized that different applications require different semantics for their modules, and thus the library and modules forms are defined to have clearly distinct semantics that serve two different purposes in the language.
A number of decisions were taken to allow for the efficient implementation and compilation of libraries and modules. Firstly, the use of the implicit-export form allows for the optimization of modules without requiring whole program analysis, which is not always possible. This encourage more dynamic programming without the associated cost of inefficient code. Additionally, the ability for a library to never be evaluated is a feature. This allows a more sophisticated implementation to determine that a given library will never be used, and to elide it from the final program, potentially eliminating many costly evaluations and saving on space. Even on implicit phasing systems, this does not cause a problem because it is still possible to force the visitation or invokation of a library.
In the end, this system was designed to be both expressive and efficient, while remaining as compatible as possible with existing systems, and not straying from already well documented and implemented semantics. The differences are largely syntactic, and the author is not aware of any new semantic work in these forms.
R5RS does not specify a module system, but some Scheme implementations provided their own. The module form is similar in construction to Chez Scheme's module form.
A subset of R6RS libraries will run without modification in this system. In particular, those libraries which do not take advantage of explicit phasing and/or versioning are expected to run without incident.
This proposal allows for the handling of phasing issues, which is not currently being discussed in WG1. Additionally, it permits any sorts of extensions to the forms by the creation of new forms in the same manner that export and import are defined. This system is compatible with both explicit and implicit phasing provided that the system is properly specified and clarified for those semantics.