### Modules

To allow some control over visible bindings and to organize code at the global level, a simple module system is available. A module defines a set of toplevel expressions that are initially evaluated in an empty syntactical environment. By importing other modules, exported value- and macro-bindings are made visible inside the environment of the module that imports them.

Note that modules are purely syntactical - they do not change the control flow or delay the execution of the contained toplevel forms. The body of a module is executed at load-time, when code is loaded or accessed via the `uses` declaration, just like normal toplevel expressions. Exported macro-definitions are compiled as well, and can be accessed in interpreted or compiled code by loading and importing the compiled file that contains the module.

Imported toplevel bindings can be assigned (with `set!`), any modifications to these will change the global value and will be visible to other modules that export or import the same toplevel binding.

A module is initially empty (has no visible bindings). You must at least import the `scheme` module to do anything useful. To access any of the non-standard macros and procedures, import the `chicken` module.

CHICKEN's module system has the following features:

- Separation of compile/expansion-time and run-time code is provided, which allows cross compilation
- Module-generating code is only created, when needed
- Supports batch-compilation of separate compilation units
- No separate "identifier" type is used, all identifiers appearing in code and processed in expansions are symbols
- The module system is fully optional
- Parameterized modules are supported

#### module

`(module NAME (EXPORT ...) BODY ...)` syntax
`(module NAME (EXPORT ...) FILENAME)` syntax
`(module NAME * BODY ...)` syntax
`(module NAME1 = NAME2 [BODY ...])` syntax
`(module NAME = (FUNCTORNAME MODULENAME1 ...))` syntax

  Defines a module with the name `NAME`, a set of exported bindings and a contained sequence of toplevel expressions that are evaluated in an empty syntactical environment.

  `(EXPORT ...)` should be an export-specification which holds a list of identifiers to be exported from the module and which should be visible when imported into another module or the toplevel environment. `EXPORT` may have any of the following forms:

  `IDENTIFIER` names a value- or syntax binding to be exported.

  `(IDENTIFIER1 ...)` or `(syntax: IDENTIFIER1 ...)` exports `IDENTIFIER1` (which should name a macro) and also arranges for the remaining identifiers in the list to be visible in the expansion of the macro (this is a hint to the module expander to export bindings referenced by syntax-definitions which make use of them, but which would normally be internal to the module - which gives more opportunities for optimization).

  `(interface: INTERFACENAME)` adds all exports defined for the given interface to be added to the list of exported identifiers of this module.

  As a special case, specifying `*` instead of an export-list will export all definitions. As another special case, the export-list may be a symbol naming an interface.

  When the `BODY` consists of a single string, it is treated like `(include FILENAME)`.

  `(module NAME = (FUNCTORNAME MODULENAME1 ...))` instantiates a functor (see below for information about functors).

  The syntax `(module NAME1 = NAME2)` defines an alias `NAME1` for the module `NAME2`, so `NAME1` can be used in place of `NAME2` in all forms that accept module names. Module aliases defined inside a module are local to that module. If followed by a module body, then this is a special form of functor instantiation.

  Nested modules, modules not at toplevel (i.e. local modules) or mutually recursive modules are not supported. As an exception module alias definitions are allowed inside a module definition.

  When compiled, the module information, including exported macros is stored in the generated binary and available when loading it into interpreted or compiled code. Note that this is different to normal macros (outside of module declarations), which are normally not exported from compiled code.

  Note that the module system is only a device for controlling the mapping of identifiers to value or syntax bindings. Modules do not instantiate separate environments that contain their own bindings, as do many other module systems. Redefinition or assignment of value or syntax bindings will modify the original, imported definition.

  Syntax expansions may result in module-definitions, but must be at toplevel.

#### export

`(export EXPORT ...)` syntax

  Allows augmenting module-exports from inside the module-body. `EXPORT` is if the same form as an export-specifier in a `module` export list. An export must precede its first occurrence (either use or definition).

  If used outside of a module, then this form does nothing.

#### import

`(import IMPORT ...)` syntax

  Imports module bindings into the current syntactical environment. The visibility of any imported bindings is limited to the current module, if used inside a module-definition, or to the current compilation unit, if compiled and used outside of a module.

  Importing a module does not load or link it - this is a separate operation from importing its bindings.

  `IMPORT` may be a module name or an import specifier, where a module name is either a symbol or a list of the form `(srfi N)`. An `IMPORT` defines a set of bindings that are to be made visible in the current scope.

  Note that the imported bindings are only visible in the next toplevel expression (regardless of whether the import appears inside or outside a module):

       (begin
         (import m1)
         ...)              ; imports not visible here

       ...                ; imports visible here

##### only

    [import specifier] (only IMPORT IDENTIFIER ...)

Only import the listed value- or syntax bindings from the set given by `IMPORT`.

##### except

    [import specifier] (except IMPORT IDENTIFIER ...)

Remove the listed identifiers from the import-set defined by `IMPORT`.

##### rename

    [import specifier] (rename IMPORT (OLD1 NEW1) ...)

Renames identifiers imported from `IMPORT`.

##### prefix

    [import specifier] (prefix IMPORT SYMBOL)

Prefixes all imported identifiers with `SYMBOL`.

#### import-for-syntax

`(import-for-syntax IMPORT ...)` syntax

  Similar to `import`, but imports exported bindings of a module into the environment in which macro transformers are evaluated.

  Note: currently this isn't fully correct - value bindings are still imported into the normal environment because a separate import environment for syntax has not been implemented (syntactic bindings are kept separate correctly).

#### reexport

`(reexport IMPORT ...)` syntax

  Imports `IMPORT ...` and automatically exports all imported identifiers. This can be used to build compound modules: modules that just extend other modules:

      (module r4rs ()
        (import scheme chicken)
        (reexport
          (except scheme
            dynamic-wind values call-with-values eval scheme-report-environment
            null-environment interaction-environment)))

### define-interface

`(define-interface INTERFACENAME (EXPORT ...))` syntax

  Defines an interface, a group of exports that can be used in module-definitions using the `(interface: INTERFACE)` syntax. See the definition of `module` above for an explanation of `EXPORT` specifications.

  Interface names use a distinct global namespace. Interfaces defined inside modules are not visible outside of the module body.

### import libraries

import libraries allow the syntactical (compile-time) and run-time parts of a compiled module to be separated into a normal compiled file and a shared library that only contains macro definitions and module information. This reduces the size of executables and simplifies compiling code that uses modules for a different architecture than the machine the compiler is executing on (i.e. "cross" compilation).

By using the `emit-import-library` compiler-option or declaration, a separate file is generated that only contains syntactical information (including macros) for a module. `import` will automatically find and load an import library for a currently unknown module, if the import- library is either in the extension repository or the current include path. Import libraries may also be explicitly loaded into the compiler by using the `-extend` compiler option. Interpreted code can simply load the import library to make the module-definition available. Macro-support definitions defined with `define-for-syntax` and expansion-time expressions of the form `(begin-for-syntax ...)` will be added to import libraries to make them available for exported macros. Note that these definitions will ruthlessly pollute the toplevel namespace and so they should be used sparingly.

### Using modules as evaluation environments

#### module-environment

`(module-environment MODULENAME)` procedure

  Locates the module with the name `MODULENAME` and returns an environment that can be passed as the second argument to `eval`. The evaluated expressions have only access to the bindings that are visible inside the module. Note that the environment is not mutable.

  If the module is not registered in the current process, `module-environment` will try to locate meta-information about the module by loading any existing import library with the name `MODULENAME.import.[scm|so]`, if possible.

  In compiled modules, only exported bindings will be visible to interactively entered code. In interpreted modules all bindings are visible.

### Predefined modules

Import libraries for the following modules are initially available:

    [module] scheme
    [module] r4rs
    [module] r5rs

Exports the definitions given in R4RS or R5RS. `r5rs` is a module alias for `scheme`.

    [module] chicken

Everything from the `library`, `eval` and `expand` library units.

    [module] extras
    [module] data-structures
    [module] ports
    [module] lolevel
    [module] posix
    [module] regex
    [module] srfi-1
    [module] srfi-4
    [module] srfi-13
    [module] srfi-14
    [module] srfi-18
    [module] srfi-69
    [module] tcp
    [module] utils

Modules exporting the bindings from the respective library units.

    [module] foreign

Exports all macros and procedures that are used to access foreign C/C++ code.

### Examples of using modules

Here is a silly little test module to demonstrate how modules are defined and used:

    ;; hello.scm

    (module test (hello greet)
      (import scheme)

      (define-syntax greet
        (syntax-rules ()
          ((_ whom)
           (begin
             (display "Hello, ")
             (display whom)
             (display " !\n") ) ) ) )

      (define (hello)
        (greet "world") )  )

The module `test` exports one value (`hello`) and one syntax binding (`greet`). To use it in `csi`, the interpreter, simply load and import it:

     #;1> ,l hello.scm
     ; loading hello.scm ...
     ; loading /usr/local/lib/chicken/4/scheme.import.so ...
     #;1> (import test)
     #;2> (hello)
     Hello, world !
     #;3> (greet "you")
     Hello, you !

The module can easily be compiled

     % csc -s hello.scm

and used in an identical manner:

     #;1> ,l hello.so
     ; loading hello.so ...
     #;1> (import test)
     #;2> (hello)
     Hello, world !
     #;3> (greet "you")
     Hello, you !

If you are depending on external libraries inside your module, follow the general rule of `(import chicken scheme) (use anything-else)` like so:

    (module test (hello greet)
      (import chicken scheme)
      (use posix)
      (use srfi-4)
      ...)

If you want to keep macro-definitions in a separate file, use import libraries:

     % csc -s hello.scm -j test
     % csc -s test.import.scm

     #;1> ,l hello.so
     ; loading hello.so ...
     #;1> (import test)
     ; loading ./test.import.so ...
     #;2> (hello)
     Hello, world !
     #;3> (greet "you")
     Hello, you !

If an import library (compiled or in source-form) is located somewhere in the extensions-repository or include path, it is automatically loaded on import. Otherwise you have to load it manually:

     #;1> ,l hello.so
     ; loading hello.so ...
     #;1> ,l test.import.so
     ; loading test.import.so ...
     #;1> (import test)
     #;2>

Note that you must use import libraries if you compile code that depends on other modules. The compiler will not execute the modules that are referred to by compiled code, and thus the binding information and exported syntax of the former must be available separately.

### Functors

A functor is a higher-order module that can be parameterized with other modules. A functor defines the body of a module for a set of argument modules and can be instantiated with concrete module names specializing the code contained in the functor. This is best explained with a silly and pointless example:

    (functor (squaring-functor (M (multiply))) (square)
      (import scheme M)
      (define (square x) (multiply x x)))

This defines a generic "squaring" operation that uses `multiply`, a procedure (or macro!) exported by the as-yet-unknown module `M`. Now let's instantiate the functor for a specific input module:

    (module nums (multiply)
      (import scheme)
      (define (multiply x y) (* x y)))

    (module number-squarer = (squaring-functor nums))

    (import number-squarer)
    (square 3)                ===>  9

We can easily instantiate the functor for other inputs:

    (module stars (multiply)
      (import scheme)
      (use srfi-1)
      (define (multiply x y)
        (list-tabulate x (lambda _ (list-tabulate y (lambda _ '*))))))

    (module star-squarer = (squaring-functor stars))

    (import star-squarer)
    (square 3)                ===>  ((* * *)
                                     (* * *)
                                     (* * *))

So whenever you have a generic algorithm it can be packaged into a functor and specialized for specific input modules. The instantiation will check that the argument modules match the required signature, `(multiply)` in the case above. The argument module must export at least the signature given in the functor definition. You can use `define-interface` to reduce typing and give a more meaningful name to a set of exports.

The general syntax of a functor definition looks like this:

`(functor (FUNCTORNAME (ARGUMENTMODULE1 EXPORTS1) ...) FUNCTOREXPORTS BODY)` syntax

  Defines a "functor", a parameterized module.

  This functor definition does not generate any code. This is done by instantiating the functor for specific input modules:

      (module MODULENAME = (FUNCTORNAME MODULENAME1 ...))

  Inside `BODY`, references to `ARGUMENTMODULE` will be replaced by the corresponding `MODULENAME` argument. The instantiation expands into the complete functor-code `BODY` and as such can be considered a particular sort of macro-expansion. Note that there is no requirement that a specific export of an argument-module must be syntax or non-syntax - it can be syntax in one instantiation and a procedure definition in another.

  `ARGUMENTMODULE` may also be a list of the form `(ALIAS DEFAULT)` to allow specifying a default- or optional functor argument in case the instanation doesn't provide one. Optional functor arguments may only be followed by non-optional functor arguments.

  The common case of using a functor with a single argument module that is not used elsewhere can be expressed in the following way:

      (module NAME = FUNCTORNAME BODY ...)

  which is the same as

      (begin
        (module _NAME * BODY ...)
        (module NAME = (FUNCTORNAME _NAME)))

  Since functors exist at compile time, they can be stored in import-libraries via `-emit-import-library FUNCTORNAME` or `-emit-all-import-libraries` (see Using the compiler for more information about this). That allows you to import functors for later instantiation. Internally, a functor-definition also defines a module with the same name, but importing this module has no effect. It also has no runtime code, so it is sufficient to merely `import` it (as opposed to using `require-extension` or one of its variants, which also loads the run-time part of a module).

  Note that functor-instantiation creates a complete copy of the functor body.

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