guile/doc/ref/api-modules.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000-2004, 2007-2014, 2019, 2020
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Modules
@section Modules
@cindex modules

When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file.  Even just a @emph{similarity}
between function names can cause hard-to-find bugs, since a programmer
might type the wrong function name.

The approach used to tackle this problem is called @emph{information
encapsulation}, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.
@cindex encapsulation
@cindex information encapsulation
@cindex name space

The language features that allow this are usually called @emph{the
module system} because programs are broken up into modules that are
compiled separately (or loaded separately in an interpreter).

Older languages, like C, have limited support for name space
manipulation and protection.  In C a variable or function is public by
default, and can be made local to a module with the @code{static}
keyword.  But you cannot reference public variables and functions from
another module with different names.

More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
@emph{renaming} of objects from a foreign module, so they will not
clutter the global name space.
@cindex name space - private

In addition, Guile offers variables as first-class objects.  They can
be used for interacting with the module system.

@menu
* General Information about Modules::  Guile module basics.
* Using Guile Modules::         How to use existing modules.
* Creating Guile Modules::      How to package your code into modules.
* Modules and the File System:: Installing modules in the file system.
* R6RS Version References::     Using version numbers with modules.
* R6RS Libraries::              The library and import forms.
* Variables::                   First-class variables.
* Module System Reflection::    First-class modules.
* Declarative Modules::         Allowing Guile to reason about modules.
* Accessing Modules from C::    How to work with modules with C code.
* provide and require::         The SLIB feature mechanism.
* Environments::                R5RS top-level environments.
@end menu

@node General Information about Modules
@subsection General Information about Modules

A Guile module can be thought of as a collection of named procedures,
variables and macros.  More precisely, it is a set of @dfn{bindings}
of symbols (names) to Scheme objects.

Within a module, all bindings are visible.  Certain bindings
can be declared @dfn{public}, in which case they are added to the
module's so-called @dfn{export list}; this set of public bindings is
called the module's @dfn{public interface} (@pxref{Creating Guile
Modules}).

A client module @dfn{uses} a providing module's bindings by either
accessing the providing module's public interface, or by building a
custom interface (and then accessing that).  In a custom interface, the
client module can @dfn{select} which bindings to access and can also
algorithmically @dfn{rename} bindings.  In contrast, when using the
providing module's public interface, the entire export list is available
without renaming (@pxref{Using Guile Modules}).

All Guile modules have a unique @dfn{module name}, for example
@code{(ice-9 popen)} or @code{(srfi srfi-11)}.  Module names are lists
of one or more symbols.

When Guile goes to use an interface from a module, for example
@code{(ice-9 popen)}, Guile first looks to see if it has loaded
@code{(ice-9 popen)} for any reason.  If the module has not been loaded
yet, Guile searches a @dfn{load path} for a file that might define it,
and loads that file.

The following subsections go into more detail on using, creating,
installing, and otherwise manipulating modules and the module system.

@node Using Guile Modules
@subsection Using Guile Modules

To use a Guile module is to access either its public interface or a
custom interface (@pxref{General Information about Modules}).  Both
types of access are handled by the syntactic form @code{use-modules},
which accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded, following @code{%load-path}
(@pxref{Modules and the File System}).

An @dfn{interface specification} has one of two forms.  The first
variation is simply to name the module, in which case its public
interface is the one accessed.  For example:

@lisp
(use-modules (ice-9 popen))
@end lisp

Here, the interface specification is @code{(ice-9 popen)}, and the
result is that the current module now has access to @code{open-pipe},
@code{close-pipe}, @code{open-input-pipe}, and so on (@pxref{Pipes}).

Note in the previous example that if the current module had already
defined @code{open-pipe}, that definition would be overwritten by the
definition in @code{(ice-9 popen)}.  For this reason (and others), there
is a second variation of interface specification that not only names a
module to be accessed, but also selects bindings from it and renames
them to suit the current module's needs.  For example:

@cindex binding renamer
@lisp
(use-modules ((ice-9 popen)
              #:select ((open-pipe . pipe-open) close-pipe)
              #:renamer (symbol-prefix-proc 'unixy:)))
@end lisp

@noindent
or more simply:

@cindex prefix
@lisp
(use-modules ((ice-9 popen)
              #:select ((open-pipe . pipe-open) close-pipe)
              #:prefix unixy:))
@end lisp

Here, the interface specification is more complex than before, and the
result is that a custom interface with only two bindings is created and
subsequently accessed by the current module.  The mapping of old to new
names is as follows:

@c Use `smallexample' since `table' is ugly.  --ttn
@smallexample
(ice-9 popen) sees:             current module sees:
open-pipe                       unixy:pipe-open
close-pipe                      unixy:close-pipe
@end smallexample

This example also shows how to use the convenience procedure
@code{symbol-prefix-proc}.

You can also directly refer to bindings in a module by using the
@code{@@} syntax.  For example, instead of using the
@code{use-modules} statement from above and writing
@code{unixy:pipe-open} to refer to the @code{pipe-open} from the
@code{(ice-9 popen)}, you could also write @code{(@@ (ice-9 popen)
open-pipe)}.  Thus an alternative to the complete @code{use-modules}
statement would be

@lisp
(define unixy:pipe-open (@@ (ice-9 popen) open-pipe))
(define unixy:close-pipe (@@ (ice-9 popen) close-pipe))
@end lisp

There is also @code{@@@@}, which can be used like @code{@@}, but does
not check whether the variable that is being accessed is actually
exported.  Thus, @code{@@@@} can be thought of as the impolite version
of @code{@@} and should only be used as a last resort or for
debugging, for example.

Note that just as with a @code{use-modules} statement, any module that
has not yet been loaded will be loaded when referenced by a @code{@@} or
@code{@@@@} form.

You can also use the @code{@@} and @code{@@@@} syntaxes as the target
of a @code{set!} when the binding refers to a variable.

@deffn {Scheme Procedure} symbol-prefix-proc prefix-sym
Return a procedure that prefixes its arg (a symbol) with
@var{prefix-sym}.
@end deffn

@deffn syntax use-modules spec @dots{}
Resolve each interface specification @var{spec} into an interface and
arrange for these to be accessible by the current module.  The return
value is unspecified.

@var{spec} can be a list of symbols, in which case it names a module
whose public interface is found and used.

@var{spec} can also be of the form:

@cindex binding renamer
@lisp
 (MODULE-NAME [#:select SELECTION]
              [#:prefix PREFIX]
              [#:renamer RENAMER])
@end lisp

in which case a custom interface is newly created and used.
@var{module-name} is a list of symbols, as above; @var{selection} is a
list of selection-specs; @var{prefix} is a symbol that is prepended to
imported names; and @var{renamer} is a procedure that takes a symbol and
returns its new name.  A selection-spec is either a symbol or a pair of
symbols @code{(ORIG . SEEN)}, where @var{orig} is the name in the used
module and @var{seen} is the name in the using module.  Note that
@var{seen} is also modified by @var{prefix} and @var{renamer}.

The @code{#:select}, @code{#:prefix}, and @code{#:renamer} clauses are
optional.  If all are omitted, the returned interface has no bindings.
If the @code{#:select} clause is omitted, @var{prefix} and @var{renamer}
operate on the used module's public interface.

In addition to the above, @var{spec} can also include a @code{#:version} 
clause, of the form:

@lisp
 #:version VERSION-SPEC
@end lisp

where @var{version-spec} is an R6RS-compatible version reference.  An
error will be signaled in the case in which a module with the same name
has already been loaded, if that module specifies a version and that
version is not compatible with @var{version-spec}.  @xref{R6RS Version
References}, for more on version references.

If the module name is not resolvable, @code{use-modules} will signal an
error.
@end deffn

@deffn syntax @@ module-name binding-name
Refer to the binding named @var{binding-name} in module
@var{module-name}.  The binding must have been exported by the module.
@end deffn

@deffn syntax @@@@ module-name binding-name
Refer to the binding named @var{binding-name} in module
@var{module-name}.  The binding must not have been exported by the
module.  This syntax is only intended for debugging purposes or as a
last resort.  @xref{Declarative Modules}, for some limitations on the
use of @code{@@@@}.
@end deffn

@node Creating Guile Modules
@subsection Creating Guile Modules

When you want to create your own modules, you have to take the following
steps:

@itemize @bullet
@item
Create a Scheme source file and add all variables and procedures you wish
to export, or which are required by the exported procedures.

@item
Add a @code{define-module} form at the beginning.

@item
Export all bindings which should be in the public interface, either
by using @code{define-public} or @code{export} (both documented below).
@end itemize

@deffn syntax define-module module-name option @dots{}
@var{module-name} is a list of one or more symbols.

@lisp
(define-module (ice-9 popen))
@end lisp

@code{define-module} makes this module available to Guile programs under
the given @var{module-name}.

@var{option} @dots{} are keyword/value pairs which specify more about the
defined module.  The recognized options and their meaning are shown in
the following table.

@table @code
@item #:use-module @var{interface-specification}
Equivalent to a @code{(use-modules @var{interface-specification})}
(@pxref{Using Guile Modules}).

@item #:autoload @var{module} @var{symbol-list}
@cindex autoload
Load @var{module} when any of @var{symbol-list} are accessed.  For
example,

@example
(define-module (my mod)
  #:autoload (srfi srfi-1) (partition delete-duplicates))
...
(when something
  (set! foo (delete-duplicates ...)))
@end example

When a module is autoloaded, only the bindings in @var{symbol-list}
become available@footnote{In Guile 2.2 and earlier, @emph{all} the
module bindings would become available; @var{symbol-list} was just the
list of bindings that will first trigger the load.}.

An autoload is a good way to put off loading a big module until it's
really needed, for instance for faster startup or if it will only be
needed in certain circumstances.

@item #:export @var{list}
@cindex export
Export all identifiers in @var{list} which must be a list of symbols
or pairs of symbols.  This is equivalent to @code{(export @var{list})} 
in the module body.

@item #:re-export @var{list}
@cindex re-export
Re-export all identifiers in @var{list} which must be a list of
symbols or pairs of symbols.  The symbols in @var{list} must be 
imported by the current module from other modules.  This is equivalent
to @code{re-export} below.

@item #:replace @var{list}
@cindex replace
@cindex replacing binding
@cindex overriding binding
@cindex duplicate binding
Export all identifiers in @var{list} (a list of symbols or pairs of
symbols) and mark them as @dfn{replacing bindings}.  In the module 
user's name space, this will have the effect of replacing any binding 
with the same name that is not also ``replacing''.  Normally a 
replacement results in an ``override'' warning message, 
@code{#:replace} avoids that.

In general, a module that exports a binding for which the @code{(guile)}
module already has a definition should use @code{#:replace} instead of
@code{#:export}.  @code{#:replace}, in a sense, lets Guile know that the
module @emph{purposefully} replaces a core binding.  It is important to
note, however, that this binding replacement is confined to the name
space of the module user.  In other words, the value of the core binding
in question remains unchanged for other modules.

Note that although it is often a good idea for the replaced binding to
remain compatible with a binding in @code{(guile)}, to avoid surprising
the user, sometimes the bindings will be incompatible.  For example,
SRFI-19 exports its own version of @code{current-time} (@pxref{SRFI-19
Time}) which is not compatible with the core @code{current-time}
function (@pxref{Time}).  Guile assumes that a user importing a module
knows what she is doing, and uses @code{#:replace} for this binding
rather than @code{#:export}.

A @code{#:replace} clause is equivalent to @code{(export! @var{list})}
in the module body.

The @code{#:duplicates} (see below) provides fine-grain control about
duplicate binding handling on the module-user side.

@item #:re-export-and-replace @var{list}
@cindex re-export-and-replace
Like @code{#:re-export}, but also marking the bindings as replacements
in the sense of @code{#:replace}.

@item #:version @var{list}
@cindex module version
Specify a version for the module in the form of @var{list}, a list of
zero or more exact, non-negative integers.  The corresponding
@code{#:version} option in the @code{use-modules} form allows callers
to restrict the value of this option in various ways.

@item #:duplicates @var{list}
@cindex duplicate binding handlers
@cindex duplicate binding
@cindex overriding binding
Tell Guile to handle duplicate bindings for the bindings imported by
the current module according to the policy defined by @var{list}, a
list of symbols.  @var{list} must contain symbols representing a
duplicate binding handling policy chosen among the following:

@table @code
@item check
Raises an error when a binding is imported from more than one place.
@item warn
Issue a warning when a binding is imported from more than one place
and leave the responsibility of actually handling the duplication to
the next duplicate binding handler.
@item replace
When a new binding is imported that has the same name as a previously
imported binding, then do the following:

@enumerate
@item
@cindex replacing binding
If the old binding was said to be @dfn{replacing} (via the
@code{#:replace} option above) and the new binding is not replacing,
the keep the old binding.
@item
If the old binding was not said to be replacing and the new binding is
replacing, then replace the old binding with the new one.
@item
If neither the old nor the new binding is replacing, then keep the old
one.
@end enumerate

@item warn-override-core
Issue a warning when a core binding is being overwritten and actually
override the core binding with the new one.
@item first
In case of duplicate bindings, the firstly imported binding is always
the one which is kept.
@item last
In case of duplicate bindings, the lastly imported binding is always
the one which is kept.
@item noop
In case of duplicate bindings, leave the responsibility to the next
duplicate handler.
@end table

If @var{list} contains more than one symbol, then the duplicate
binding handlers which appear first will be used first when resolving
a duplicate binding situation.  As mentioned above, some resolution
policies may explicitly leave the responsibility of handling the
duplication to the next handler in @var{list}.

If GOOPS has been loaded before the @code{#:duplicates} clause is
processed, there are additional strategies available for dealing with
generic functions.  @xref{Merging Generics}, for more information.

@findex default-duplicate-binding-handler
The default duplicate binding resolution policy is given by the
@code{default-duplicate-binding-handler} procedure, and is

@lisp
(replace warn-override-core warn last)
@end lisp

@item #:pure
@cindex pure module
Create a @dfn{pure} module, that is a module which does not contain any
of the standard procedure bindings except for the syntax forms.  This is
useful if you want to create @dfn{safe} modules, that is modules which
do not know anything about dangerous procedures.
@end table

@end deffn

@deffn syntax export variable @dots{}
Add all @var{variable}s (which must be symbols or pairs of symbols) to 
the list of exported bindings of the current module.  If @var{variable}
is a pair, its @code{car} gives the name of the variable as seen by the
current module and its @code{cdr} specifies a name for the binding in
the current module's public interface.
@end deffn

@deffn syntax define-public @dots{}
Equivalent to @code{(begin (define foo ...) (export foo))}.
@end deffn

@deffn syntax re-export variable @dots{}
Add all @var{variable}s (which must be symbols or pairs of symbols) to 
the list of re-exported bindings of the current module.  Pairs of 
symbols are handled as in @code{export}.  Re-exported bindings must be
imported by the current module from some other module.
@end deffn

@deffn syntax export! variable @dots{}
Like @code{export}, but marking the exported variables as replacing.
Using a module with replacing bindings will cause any existing bindings
to be replaced without issuing any warnings.  See the discussion of
@code{#:replace} above.
@end deffn

@node Modules and the File System
@subsection Modules and the File System

Typical programs only use a small subset of modules installed on a Guile
system.  In order to keep startup time down, Guile only loads modules
when a program uses them, on demand.

When a program evaluates @code{(use-modules (ice-9 popen))}, and the
module is not loaded, Guile searches for a conventionally-named file
in the @dfn{load path}.

In this case, loading @code{(ice-9 popen)} will eventually cause Guile
to run @code{(primitive-load-path "ice-9/popen")}.
@code{primitive-load-path} will search for a file @file{ice-9/popen} in
the @code{%load-path} (@pxref{Load Paths}).  For each directory in
@code{%load-path}, Guile will try to find the file name, concatenated
with the extensions from @code{%load-extensions}.  By default, this will
cause Guile to @code{stat} @file{ice-9/popen.scm}, and then
@file{ice-9/popen}.  @xref{Load Paths}, for more on
@code{primitive-load-path}.

If a corresponding compiled @file{.go} file is found in the
@code{%load-compiled-path} or in the fallback path, and is as fresh as
the source file, it will be loaded instead of the source file.  If no
compiled file is found, Guile may try to compile the source file and
cache away the resulting @file{.go} file.  @xref{Compilation}, for more
on compilation.

Once Guile finds a suitable source or compiled file is found, the file
will be loaded.  If, after loading the file, the module under
consideration is still not defined, Guile will signal an error.

For more information on where and how to install Scheme modules,
@xref{Installing Site Packages}.


@node R6RS Version References
@subsection R6RS Version References

Guile's module system includes support for locating modules based on
a declared version specifier of the same form as the one described in
R6RS (@pxref{Library form, R6RS Library Form,, r6rs, The Revised^6 
Report on the Algorithmic Language Scheme}).  By using the 
@code{#:version} keyword in a @code{define-module} form, a module may
specify a version as a list of zero or more exact, non-negative integers.

This version can then be used to locate the module during the module
search process.  Client modules and callers of the @code{use-modules} 
function may specify constraints on the versions of target modules by
providing a @dfn{version reference}, which has one of the following
forms:

@lisp
 (@var{sub-version-reference} ...)
 (and @var{version-reference} ...)
 (or @var{version-reference} ...)
 (not @var{version-reference})
@end lisp

in which @var{sub-version-reference} is in turn one of:

@lisp
 (@var{sub-version})
 (>= @var{sub-version})
 (<= @var{sub-version})
 (and @var{sub-version-reference} ...)
 (or @var{sub-version-reference} ...)
 (not @var{sub-version-reference})
@end lisp

in which @var{sub-version} is an exact, non-negative integer as above. A
version reference matches a declared module version if each element of
the version reference matches a corresponding element of the module 
version, according to the following rules:

@itemize @bullet
@item
The @code{and} sub-form matches a version or version element if every 
element in the tail of the sub-form matches the specified version or 
version element.

@item
The @code{or} sub-form matches a version or version element if any 
element in the tail of the sub-form matches the specified version or
version element.

@item
The @code{not} sub-form matches a version or version element if the tail
of the sub-form does not match the version or version element.  

@item
The @code{>=} sub-form matches a version element if the element is 
greater than or equal to the @var{sub-version} in the tail of the 
sub-form.

@item
The @code{<=} sub-form matches a version element if the version is less
than or equal to the @var{sub-version} in the tail of the sub-form.

@item
A @var{sub-version} matches a version element if one is @var{eqv?} to
the other.
@end itemize

For example, a module declared as:

@lisp
 (define-module (mylib mymodule) #:version (1 2 0))
@end lisp

would be successfully loaded by any of the following @code{use-modules}
expressions:

@lisp
 (use-modules ((mylib mymodule) #:version (1 2 (>= 0))))
 (use-modules ((mylib mymodule) #:version (or (1 2 0) (1 2 1))))
 (use-modules ((mylib mymodule) #:version ((and (>= 1) (not 2)) 2 0)))
@end lisp


@node R6RS Libraries
@subsection R6RS Libraries

In addition to the API described in the previous sections, you also
have the option to create modules using the portable @code{library} form
described in R6RS (@pxref{Library form, R6RS Library Form,, r6rs, The
Revised^6 Report on the Algorithmic Language Scheme}), and to import 
libraries created in this format by other programmers.  Guile's R6RS 
library implementation takes advantage of the flexibility built into the
module system by expanding the R6RS library form into a corresponding 
Guile @code{define-module} form that specifies equivalent import and 
export requirements and includes the same body expressions.  The library
expression:

@lisp
  (library (mylib (1 2))
    (export mybinding)
    (import (otherlib (3))))
@end lisp

is equivalent to the module definition:

@lisp
  (define-module (mylib)
    #:version (1 2)
    #:use-module ((otherlib) #:version (3))
    #:export (mybinding))
@end lisp

Central to the mechanics of R6RS libraries is the concept of import
and export @dfn{levels}, which control the visibility of bindings at
various phases of a library's lifecycle --- macros necessary to 
expand forms in the library's body need to be available at expand 
time; variables used in the body of a procedure exported by the
library must be available at runtime.  R6RS specifies the optional
@code{for} sub-form of an @emph{import set} specification (see below)
as a mechanism by which a library author can indicate that a
particular library import should take place at a particular phase 
with respect to the lifecycle of the importing library.  

Guile's library implementation uses a technique called 
@dfn{implicit phasing} (first described by Abdulaziz Ghuloum and R. 
Kent Dybvig), which allows the expander and compiler to automatically 
determine the necessary visibility of a binding imported from another 
library.  As such, the @code{for} sub-form described below is ignored by
Guile (but may be required by Schemes in which phasing is explicit).

@deffn {Scheme Syntax} library name (export export-spec ...) (import import-spec ...) body ...
Defines a new library with the specified name, exports, and imports,
and evaluates the specified body expressions in this library's 
environment.

The library @var{name} is a non-empty list of identifiers, optionally
ending with a version specification of the form described above
(@pxref{Creating Guile Modules}).

Each @var{export-spec} is the name of a variable defined or imported
by the library, or must take the form 
@code{(rename (internal-name external-name) ...)}, where the 
identifier @var{internal-name} names a variable defined or imported 
by the library and @var{external-name} is the name by which the
variable is seen by importing libraries.

Each @var{import-spec} must be either an @dfn{import set} (see below)
or must be of the form @code{(for import-set import-level ...)}, 
where each @var{import-level} is one of:

@lisp
  run
  expand
  (meta @var{level})
@end lisp

where @var{level} is an integer.  Note that since Guile does not
require explicit phase specification, any @var{import-set}s found 
inside of @code{for} sub-forms will be ``unwrapped'' during 
expansion and processed as if they had been specified directly.

Import sets in turn take one of the following forms:

@lisp
  @var{library-reference}
  (library @var{library-reference})
  (only @var{import-set} @var{identifier} ...)
  (except @var{import-set} @var{identifier} ...)
  (prefix @var{import-set} @var{identifier})
  (rename @var{import-set} (@var{internal-identifier} @var{external-identifier}) ...)
@end lisp

where @var{library-reference} is a non-empty list of identifiers
ending with an optional version reference (@pxref{R6RS Version 
References}), and the other sub-forms have the following semantics,
defined recursively on nested @var{import-set}s:

@itemize @bullet

@item
The @code{library} sub-form is used to specify libraries for import
whose names begin with the identifier ``library.''

@item
The @code{only} sub-form imports only the specified @var{identifier}s
from the given @var{import-set}.

@item
The @code{except} sub-form imports all of the bindings exported by 
@var{import-set} except for those that appear in the specified list
of @var{identifier}s.

@item
The @code{prefix} sub-form imports all of the bindings exported
by @var{import-set}, first prefixing them with the specified
@var{identifier}.

@item
The @code{rename} sub-form imports all of the identifiers exported
by @var{import-set}.  The binding for each @var{internal-identifier}
among these identifiers is made visible to the importing library as
the corresponding @var{external-identifier}; all other bindings are
imported using the names provided by @var{import-set}.

@end itemize

Note that because Guile translates R6RS libraries into module 
definitions, an import specification may be used to declare a
dependency on a native Guile module --- although doing so may make 
your libraries less portable to other Schemes.

@end deffn

@deffn {Scheme Syntax} import import-spec ...
Import into the current environment the libraries specified by the
given import specifications, where each @var{import-spec} takes the
same form as in the @code{library} form described above.
@end deffn


@node Variables
@subsection Variables
@tpindex Variables

Each module has its own hash table, sometimes known as an @dfn{obarray},
that maps the names defined in that module to their corresponding
variable objects.

A variable is a box-like object that can hold any Scheme value.  It is
said to be @dfn{undefined} if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example @code{#f}); otherwise the variable is
@dfn{defined}.

On its own, a variable object is anonymous.  A variable is said to be
@dfn{bound} when it is associated with a name in some way, usually a
symbol in a module obarray.  When this happens, the name is said to be
bound to the variable, in that module.

(That's the theory, anyway.  In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated --- or deliberately drawn no distinction between --- a
name that is unbound and a name that is bound to a variable whose value
is undefined.  We will try to be clear about the difference and explain
any confusion where it is unavoidable.)

Variables do not have a read syntax.  Most commonly they are created and
bound implicitly by @code{define} expressions: a top-level @code{define}
expression of the form

@lisp
(define @var{name} @var{value})
@end lisp

@noindent
creates a variable with initial value @var{value} and binds it to the
name @var{name} in the current module.  But they can also be created
dynamically by calling one of the constructor procedures
@code{make-variable} and @code{make-undefined-variable}.

@deffn {Scheme Procedure} make-undefined-variable
@deffnx {C Function} scm_make_undefined_variable ()
Return a variable that is initially unbound.
@end deffn

@deffn {Scheme Procedure} make-variable init
@deffnx {C Function} scm_make_variable (init)
Return a variable initialized to value @var{init}.
@end deffn

@deffn {Scheme Procedure} variable-bound? var
@deffnx {C Function} scm_variable_bound_p (var)
Return @code{#t} if @var{var} is bound to a value, or @code{#f}
otherwise.  Throws an error if @var{var} is not a variable object.
@end deffn

@deffn {Scheme Procedure} variable-ref var
@deffnx {C Function} scm_variable_ref (var)
Dereference @var{var} and return its value.
@var{var} must be a variable object; see @code{make-variable}
and @code{make-undefined-variable}.
@end deffn

@deffn {Scheme Procedure} variable-set! var val
@deffnx {C Function} scm_variable_set_x (var, val)
Set the value of the variable @var{var} to @var{val}.
@var{var} must be a variable object, @var{val} can be any
value. Return an unspecified value.
@end deffn

@deffn {Scheme Procedure} variable-unset! var
@deffnx {C Function} scm_variable_unset_x (var)
Unset the value of the variable @var{var}, leaving @var{var} unbound.
@end deffn

@deffn {Scheme Procedure} variable? obj
@deffnx {C Function} scm_variable_p (obj)
Return @code{#t} if @var{obj} is a variable object, else return
@code{#f}.
@end deffn


@node Module System Reflection
@subsection Module System Reflection

The previous sections have described a declarative view of the module
system.  You can also work with it programmatically by accessing and
modifying various parts of the Scheme objects that Guile uses to
implement the module system.

At any time, there is a @dfn{current module}.  This module is the one
where a top-level @code{define} and similar syntax will add new
bindings.  You can find other module objects with @code{resolve-module},
for example.

These module objects can be used as the second argument to @code{eval}.

@deffn {Scheme Procedure} current-module
@deffnx {C Function} scm_current_module ()
Return the current module object.
@end deffn

@deffn {Scheme Procedure} set-current-module module
@deffnx {C Function} scm_set_current_module (module)
Set the current module to @var{module} and return
the previous current module.
@end deffn

@deffn {Scheme Procedure} save-module-excursion thunk
Call @var{thunk} within a @code{dynamic-wind} such that the module that
is current at invocation time is restored when @var{thunk}'s dynamic
extent is left (@pxref{Dynamic Wind}).

More precisely, if @var{thunk} escapes non-locally, the current module
(at the time of escape) is saved, and the original current module (at
the time @var{thunk}'s dynamic extent was last entered) is restored.  If
@var{thunk}'s dynamic extent is re-entered, then the current module is
saved, and the previously saved inner module is set current again.
@end deffn

@deffn {Scheme Procedure} resolve-module name [autoload=#t] [version=#f] @
                          [#:ensure=#t]
@deffnx {C Function} scm_resolve_module (name)
Find the module named @var{name} and return it.  When it has not already
been defined and @var{autoload} is true, try to auto-load it.  When it
can't be found that way either, create an empty module if @var{ensure}
is true, otherwise return @code{#f}.  If @var{version} is true, ensure
that the resulting module is compatible with the given version reference
(@pxref{R6RS Version References}).  The name is a list of symbols.
@end deffn

@deffn {Scheme Procedure} resolve-interface name [#:select=#f] @
                          [#:hide='()] [#:prefix=#f] @
                          [#:renamer=#f] [#:version=#f]
Find the module named @var{name} as with @code{resolve-module} and
return its interface.  The interface of a module is also a module
object, but it contains only the exported bindings.
@end deffn

@deffn {Scheme Procedure} module-uses module
Return a list of the interfaces used by @var{module}.
@end deffn

@deffn {Scheme Procedure} module-use! module interface
Add @var{interface} to the front of the use-list of @var{module}.  Both
arguments should be module objects, and @var{interface} should very
likely be a module returned by @code{resolve-interface}.
@end deffn

@deffn {Scheme Procedure} reload-module module
Revisit the source file that corresponds to @var{module}.  Raises an
error if no source file is associated with the given module.
@end deffn

As mentioned in the previous section, modules contain a mapping between
identifiers (as symbols) and storage locations (as variables).  Guile
defines a number of procedures to allow access to this mapping.  If you
are programming in C, @ref{Accessing Modules from C}.

@deffn {Scheme Procedure} module-variable module name
Return the variable bound to @var{name} (a symbol) in @var{module}, or
@code{#f} if @var{name} is unbound.
@end deffn

@deffn {Scheme Procedure} module-add! module name var
Define a new binding between @var{name} (a symbol) and @var{var} (a
variable) in @var{module}.
@end deffn

@deffn {Scheme Procedure} module-ref module name
Look up the value bound to @var{name} in @var{module}.  Like
@code{module-variable}, but also does a @code{variable-ref} on the
resulting variable, raising an error if @var{name} is unbound.
@end deffn

@deffn {Scheme Procedure} module-define! module name value
Locally bind @var{name} to @var{value} in @var{module}.  If @var{name}
was already locally bound in @var{module}, i.e., defined locally and not
by an imported module, the value stored in the existing variable will be
updated.  Otherwise, a new variable will be added to the module, via
@code{module-add!}.
@end deffn

@deffn {Scheme Procedure} module-set! module name value
Update the binding of @var{name} in @var{module} to @var{value}, raising
an error if @var{name} is not already bound in @var{module}.
@end deffn

There are many other reflective procedures available in the default
environment.  If you find yourself using one of them, please contact the
Guile developers so that we can commit to stability for that interface.


@node Declarative Modules
@subsection Declarative Modules

The first-class access to modules and module variables described in the
previous subsection is very powerful and allows Guile users to build
many tools to dynamically learn things about their Guile systems.
However, as Scheme godparent Mathias Felleisen wrote in ``On the
Expressive Power of Programming Languages'', a more expressive language
is necessarily harder to reason about.  There are transformations that
Guile's compiler would like to make which can't be done if every
top-level definition is subject to mutation at any time.

Consider this module:

@example
(define-module (boxes)
  #:export (make-box box-ref box-set! box-swap!))

(define (make-box x) (list x))
(define (box-ref box) (car box))
(define (box-set! box x) (set-car! box x))
(define (box-swap! box x)
  (let ((y (box-ref box)))
    (box-set! box x)
    y))
@end example

Ideally you'd like for the @code{box-ref} in @code{box-swap!} to be
inlined to @code{car}.  Guile's compiler can do this, but only if it
knows that @code{box-ref}'s definition is what it appears to be in the
text.  However, in the general case it could be that a programmer could
reach into the @code{(boxes)} module at any time and change the value of
@code{box-ref}.

@cindex declarative
@cindex modules, declarative
@cindex definitions, declarative
To allow Guile to reason about the values of top-levels from a module, a
module can be marked as @dfn{declarative}.  This flag applies only to
the subset of top-level definitions that are themselves declarative:
those that are defined within the compilation unit, and not assigned
(@code{set!}) or redefined within the compilation unit.

To explicitly mark a module as being declarative, pass the
@code{#:declarative?} keyword argument when declaring a module:

@example
(define-module (boxes)
  #:export (make-box box-ref box-set! box-swap!)
  #:declarative? #t)
@end example

By default, modules are compiled declaratively if the
@code{user-modules-declarative?} parameter is true when the
module is compiled.

@deffn {Scheme Parameter} user-modules-declarative?
A boolean indicating whether definitions in modules created by
@code{define-module} or implicitly as part of a compilation unit without
an explicit module can be treated as declarative.
@end deffn

Because it's usually what you want, the default value of
@code{user-modules-declarative?} is @code{#t}.

@subsubheading Should I Mark My Module As Declarative?

In the vast majority of use cases, declarative modules are what you
want.  However, there are exceptions.

Consider the @code{(boxes)} module above.  Let's say you want to be able
to go in and change the definition of @code{box-set!} at run-time:

@example
scheme@@(guile-user)> (use-modules (boxes))
scheme@@(guile-user)> ,module boxes
scheme@@(boxes)> (define (box-set! x y) (set-car! x (pk y)))
@end example

However, considering that @code{(boxes)} is a declarative module, it
could be that @code{box-swap!} inlined the call to @code{box-set!} -- so
it may be that you are surprised if you call @code{(box-swap! x y)} and
you don't see the new definition being used.  (Note, however, that Guile
has no guarantees about what definitions its compiler will or will not
inline.)

If you want to allow the definition of @code{box-set!} to be changed and
to have all of its uses updated, then probably the best option is to
edit the module and reload the whole thing:

@example
scheme@@(guile-user)> ,reload (boxes)
@end example

The advantage of the reloading approach is that you maintain the
optimizations that declarative modules enable, while also being able to
live-update the code.  If the module keeps precious program state, those
definitions can be marked as @code{define-once} to prevent reloads from
overwriting them.  @xref{Top Level}, for more on @code{define-once}.
Incidentally, @code{define-once} also prevents declarative-definition
optimizations, so if there's a limited subset of redefinable bindings,
@code{define-once} could be an interesting tool to mark those
definitions as works-in-progress for interactive program development.

To users, whether a module is declarative or not is mostly immaterial:
besides normal use via @code{use-modules}, users can reference and
redefine public or private bindings programmatically or interactively.
The only difference is that changing a declarative definition may not
change all of its uses.  If this use-case is important to you, and if
reloading whole modules is insufficient, then you can mark all
definitions in a module as non-declarative by adding
@code{#:declarative? #f} to the module definition.

The default of whether modules are declarative or not can be controlled
via the @code{(user-modules-declarative?)} parameter mentioned above,
but care should be taken to set this parameter when the modules are
compiled, e.g. via @code{(eval-when (expand) (user-modules-declarative?
#f))}.  @xref{Eval When}.

Alternately you can prevent declarative-definition optimizations by
compiling at the @code{-O1} optimization level instead of the default
@code{-O2}, or via explicitly passing @code{-Ono-letrectify} to the
@code{guild compile} invocation.  @xref{Compilation}, for more on
compiler options.

@cindex inlining
One final note.  Currently, definitions from declarative modules can
only be inlined within the module they are defined in, and within a
compilation unit.  This may change in the future to allow Guile to
inline imported declarative definitions as well (cross-module inlining).
To Guile, whether a definition is inlinable or not is a property of the
definition, not its use.  We hope to improve compiler tooling in the
future to allow the user to identify definitions that are out of date
when a declarative binding is redefined.


@node Accessing Modules from C
@subsection Accessing Modules from C

The last sections have described how modules are used in Scheme code,
which is the recommended way of creating and accessing modules.  You
can also work with modules from C, but it is more cumbersome.

The following procedures are available.

@deftypefn {C Function} SCM scm_c_call_with_current_module (SCM @var{module}, SCM (*@var{func})(void *), void *@var{data})
Call @var{func} and make @var{module} the current module during the
call.  The argument @var{data} is passed to @var{func}.  The return
value of @code{scm_c_call_with_current_module} is the return value of
@var{func}.
@end deftypefn

@deftypefn {C Function} SCM scm_public_variable (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_public_variable ({const char *}@var{module_name}, {const char *}@var{name})
Find a the variable bound to the symbol @var{name} in the public
interface of the module named @var{module_name}.

@var{module_name} should be a list of symbols, when represented as a
Scheme object, or a space-separated string, in the @code{const char *}
case.  See @code{scm_c_define_module} below, for more examples.

Signals an error if no module was found with the given name.  If
@var{name} is not bound in the module, just returns @code{#f}.
@end deftypefn

@deftypefn {C Function} SCM scm_private_variable (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_private_variable ({const char *}@var{module_name}, {const char *}@var{name})
Like @code{scm_public_variable}, but looks in the internals of the
module named @var{module_name} instead of the public interface.
Logically, these procedures should only be called on modules you write.
@end deftypefn

@deftypefn {C Function} SCM scm_public_lookup (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_public_lookup ({const char *}@var{module_name}, {const char *}@var{name})
@deftypefnx {C Function} SCM scm_private_lookup (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_private_lookup ({const char *}@var{module_name}, {const char *}@var{name})
Like @code{scm_public_variable} or @code{scm_private_variable}, but if
the @var{name} is not bound in the module, signals an error.  Returns a
variable, always.

@example
static SCM eval_string_var;

/* NOTE: It is important that the call to 'my_init'
   happens-before all calls to 'my_eval_string'. */
void my_init (void)
@{
  eval_string_var = scm_c_public_lookup ("ice-9 eval-string",
                                         "eval-string");
@}

SCM my_eval_string (SCM str)
@{
  return scm_call_1 (scm_variable_ref (eval_string_var), str);
@}
@end example
@end deftypefn

@deftypefn {C Function} SCM scm_public_ref (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_public_ref ({const char *}@var{module_name}, {const char *}@var{name})
@deftypefnx {C Function} SCM scm_private_ref (SCM @var{module_name}, SCM @var{name})
@deftypefnx {C Function} SCM scm_c_private_ref ({const char *}@var{module_name}, {const char *}@var{name})
Like @code{scm_public_lookup} or @code{scm_private_lookup}, but
additionally dereferences the variable.  If the variable object is
unbound, signals an error.  Returns the value bound to @var{name} in
@var{module_name}.
@end deftypefn

In addition, there are a number of other lookup-related procedures.  We
suggest that you use the @code{scm_public_} and @code{scm_private_}
family of procedures instead, if possible.

@deftypefn {C Function} SCM scm_c_lookup ({const char *}@var{name})
Return the variable bound to the symbol indicated by @var{name} in the
current module.  If there is no such binding or the symbol is not
bound to a variable, signal an error.
@end deftypefn

@deftypefn {C Function} SCM scm_lookup (SCM @var{name})
Like @code{scm_c_lookup}, but the symbol is specified directly.
@end deftypefn

@deftypefn {C Function} SCM scm_c_module_lookup (SCM @var{module}, {const char *}@var{name})
@deftypefnx {C Function} SCM scm_module_lookup (SCM @var{module}, SCM @var{name})
Like @code{scm_c_lookup} and @code{scm_lookup}, but the specified
module is used instead of the current one.
@end deftypefn

@deftypefn {C Function} SCM scm_module_variable (SCM @var{module}, SCM @var{name})
Like @code{scm_module_lookup}, but if the binding does not exist, just
returns @code{#f} instead of raising an error.
@end deftypefn

To define a value, use @code{scm_define}:

@deftypefn {C Function} SCM scm_c_define ({const char *}@var{name}, SCM @var{val})
Bind the symbol indicated by @var{name} to a variable in the current
module and set that variable to @var{val}.  When @var{name} is already
bound to a variable, use that.  Else create a new variable.
@end deftypefn

@deftypefn {C Function} SCM scm_define (SCM @var{name}, SCM @var{val})
Like @code{scm_c_define}, but the symbol is specified directly.
@end deftypefn

@deftypefn {C Function} SCM scm_c_module_define (SCM @var{module}, {const char *}@var{name}, SCM @var{val})
@deftypefnx {C Function} SCM scm_module_define (SCM @var{module}, SCM @var{name}, SCM @var{val})
Like @code{scm_c_define} and @code{scm_define}, but the specified
module is used instead of the current one.
@end deftypefn

In some rare cases, you may need to access the variable that
@code{scm_module_define} would have accessed, without changing the
binding of the existing variable, if one is present.  In that case, use
@code{scm_module_ensure_local_variable}:

@deftypefn {C Function} SCM scm_module_ensure_local_variable (SCM @var{module}, SCM @var{sym})
Like @code{scm_module_define}, but if the @var{sym} is already locally
bound in that module, the variable's existing binding is not reset.
Returns a variable.
@end deftypefn

@deftypefn {C Function} SCM scm_module_reverse_lookup (SCM @var{module}, SCM @var{variable})
Find the symbol that is bound to @var{variable} in @var{module}.  When no such binding is found, return @code{#f}.
@end deftypefn

@deftypefn {C Function} SCM scm_c_define_module ({const char *}@var{name}, void (*@var{init})(void *), void *@var{data})
Define a new module named @var{name} and make it current while
@var{init} is called, passing it @var{data}.  Return the module.

The parameter @var{name} is a string with the symbols that make up
the module name, separated by spaces.  For example, @samp{"foo bar"} names
the module @samp{(foo bar)}.

When there already exists a module named @var{name}, it is used
unchanged, otherwise, an empty module is created.
@end deftypefn

@deftypefn {C Function} SCM scm_c_resolve_module ({const char *}@var{name})
Find the module name @var{name} and return it.  When it has not
already been defined, try to auto-load it.  When it can't be found
that way either, create an empty module.  The name is interpreted as
for @code{scm_c_define_module}.
@end deftypefn

@deftypefn {C Function} SCM scm_c_use_module ({const char *}@var{name})
Add the module named @var{name} to the uses list of the current
module, as with @code{(use-modules @var{name})}.  The name is
interpreted as for @code{scm_c_define_module}.
@end deftypefn

@deftypefn {C Function} void scm_c_export ({const char *}@var{name}, ...)
Add the bindings designated by @var{name}, ... to the public interface
of the current module.  The list of names is terminated by
@code{NULL}.
@end deftypefn


@node provide and require
@subsection provide and require

Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB @emph{provide} a feature, and when user programs
@emph{require} that feature, the library file is loaded in.

For example, the file @file{random.scm} in the SLIB package contains the
line

@lisp
(provide 'random)
@end lisp

so to use its procedures, a user would type

@lisp
(require 'random)
@end lisp

and they would magically become available, @emph{but still have the same
names!}  So this method is nice, but not as good as a full-featured
module system.

When SLIB is used with Guile, provide and require can be used to access
its facilities.

@node Environments
@subsection Environments
@cindex environment

Scheme, as defined in R5RS, does @emph{not} have a full module system.
However it does define the concept of a top-level @dfn{environment}.
Such an environment maps identifiers (symbols) to Scheme objects such
as procedures and lists: @ref{About Closure}.  In other words, it
implements a set of @dfn{bindings}.

Environments in R5RS can be passed as the second argument to
@code{eval} (@pxref{Fly Evaluation}).  Three procedures are defined to
return environments: @code{scheme-report-environment},
@code{null-environment} and @code{interaction-environment} (@pxref{Fly
Evaluation}).

In addition, in Guile any module can be used as an R5RS environment,
i.e., passed as the second argument to @code{eval}.

Note: the following two procedures are available only when the 
@code{(ice-9 r5rs)} module is loaded:

@lisp
(use-modules (ice-9 r5rs))
@end lisp

@deffn {Scheme Procedure} scheme-report-environment version
@deffnx {Scheme Procedure} null-environment version
@var{version} must be the exact integer `5', corresponding to revision
5 of the Scheme report (the Revised^5 Report on Scheme).
@code{scheme-report-environment} returns a specifier for an
environment that is empty except for all bindings defined in the
report that are either required or both optional and supported by the
implementation. @code{null-environment} returns a specifier for an
environment that is empty except for the (syntactic) bindings for all
syntactic keywords defined in the report that are either required or
both optional and supported by the implementation.

Currently Guile does not support values of @var{version} for other
revisions of the report.

The effect of assigning (through the use of @code{eval}) a variable
bound in a @code{scheme-report-environment} (for example @code{car})
is unspecified.  Currently the environments specified by
@code{scheme-report-environment} are not immutable in Guile.
@end deffn



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