@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2011, 2012, 2013, 2014, 2020
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Utility Functions
@section General Utility Functions

@c FIXME::martin: Review me!

This chapter contains information about procedures which are not cleanly
tied to a specific data type.  Because of their wide range of
applications, they are collected in a @dfn{utility} chapter.

@menu
* Equality::                    When are two values `the same'?
* Object Properties::           A modern interface to object properties.
* Sorting::                     Sort utility procedures.
* Copying::                     Copying deep structures.
* General Conversion::          Converting objects to strings.
* Hooks::                       User-customizable event lists.
@end menu


@node Equality
@subsection Equality
@cindex sameness
@cindex equality

There are three kinds of core equality predicates in Scheme, described
below.  The same kinds of comparisons arise in other functions, like
@code{memq} and friends (@pxref{List Searching}).

For all three tests, objects of different types are never equal.  So
for instance a list and a vector are not @code{equal?}, even if their
contents are the same.  Exact and inexact numbers are considered
different types too, and are hence not equal even if their values are
the same.

@code{eq?} tests just for the same object (essentially a pointer
comparison).  This is fast, and can be used when searching for a
particular object, or when working with symbols or keywords (which are
always unique objects).

@code{eqv?} extends @code{eq?} to look at the value of numbers and
characters.  It can for instance be used somewhat like @code{=}
(@pxref{Comparison}) but without an error if one operand isn't a
number.

@code{equal?} goes further, it looks (recursively) into the contents
of lists, vectors, etc.  This is good for instance on lists that have
been read or calculated in various places and are the same, just not
made up of the same pairs.  Such lists look the same (when printed),
and @code{equal?} will consider them the same.

@sp 1
@deffn {Scheme Procedure} eq? x y
@deffnx {C Function} scm_eq_p (x, y)
@rnindex eq?
Return @code{#t} if @var{x} and @var{y} are the same object, except
for numbers and characters.  For example,

@example
(define x (vector 1 2 3))
(define y (vector 1 2 3))

(eq? x x)  @result{} #t
(eq? x y)  @result{} #f
@end example

Numbers and characters are not equal to any other object, but the
problem is they're not necessarily @code{eq?} to themselves either.
This is even so when the number comes directly from a variable,

@example
(let ((n (+ 2 3)))
  (eq? n n))       @result{} *unspecified*
@end example

Generally @code{eqv?} below should be used when comparing numbers or
characters.  @code{=} (@pxref{Comparison}) or @code{char=?}
(@pxref{Characters}) can be used too.

It's worth noting that end-of-list @code{()}, @code{#t}, @code{#f}, a
symbol of a given name, and a keyword of a given name, are unique
objects.  There's just one of each, so for instance no matter how
@code{()} arises in a program, it's the same object and can be
compared with @code{eq?},

@example
(define x (cdr '(123)))
(define y (cdr '(456)))
(eq? x y) @result{} #t

(define x (string->symbol "foo"))
(eq? x 'foo) @result{} #t
@end example
@end deffn

@deftypefn {C Function} int scm_is_eq (SCM x, SCM y)
Return @code{1} when @var{x} and @var{y} are equal in the sense of
@code{eq?}, otherwise return @code{0}.

@findex ==
The @code{==} operator should not be used on @code{SCM} values, an
@code{SCM} is a C type which cannot necessarily be compared using
@code{==} (@pxref{The SCM Type}).
@end deftypefn

@sp 1
@deffn {Scheme Procedure} eqv? x y
@deffnx {C Function} scm_eqv_p (x, y)
@rnindex eqv?
Return @code{#t} if @var{x} and @var{y} are the same object, or for
characters and numbers the same value.

On objects except characters and numbers, @code{eqv?} is the same as
@code{eq?} above, it's true if @var{x} and @var{y} are the same
object.

If @var{x} and @var{y} are numbers or characters, @code{eqv?} compares
their type and value.  An exact number is not @code{eqv?} to an
inexact number (even if their value is the same).

@example
(eqv? 3 (+ 1 2)) @result{} #t
(eqv? 1 1.0)     @result{} #f
@end example
@end deffn

@sp 1
@deffn {Scheme Procedure} equal? x y
@deffnx {C Function} scm_equal_p (x, y)
@rnindex equal?
Return @code{#t} if @var{x} and @var{y} are the same type, and their
contents or value are equal.

For a pair, string, vector, array or structure, @code{equal?} compares the
contents, and does so using the same @code{equal?} recursively,
so a deep structure can be traversed.

@example
(equal? (list 1 2 3) (list 1 2 3))   @result{} #t
(equal? (list 1 2 3) (vector 1 2 3)) @result{} #f
@end example

For other objects, @code{equal?} compares as per @code{eqv?} above,
which means characters and numbers are compared by type and value (and
like @code{eqv?}, exact and inexact numbers are not @code{equal?},
even if their value is the same).

@example
(equal? 3 (+ 1 2)) @result{} #t
(equal? 1 1.0)     @result{} #f
@end example

Hash tables are currently only compared as per @code{eq?}, so two
different tables are not @code{equal?}, even if their contents are the
same.

@code{equal?} does not support circular data structures, it may go
into an infinite loop if asked to compare two circular lists or
similar.

GOOPS object types (@pxref{GOOPS}), including foreign object types
(@pxref{Defining New Foreign Object Types}), can have an @code{equal?}
implementation specialized on two values of the same type.  If
@code{equal?} is called on two GOOPS objects of the same type,
@code{equal?} will dispatch out to a generic function.  This lets an
application traverse the contents or control what is considered
@code{equal?} for two objects of such a type.  If there's no such
handler, the default is to just compare as per @code{eq?}.
@end deffn


@node Object Properties
@subsection Object Properties

It's often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored.  Object
properties allow you to do just that.

Guile's representation of an object property is a procedure-with-setter
(@pxref{Procedures with Setters}) that can be used with the generalized
form of @code{set!} (REFFIXME) to set and retrieve that property for any
Scheme object.  So, setting a property looks like this:

@lisp
(set! (my-property obj1) value-for-obj1)
(set! (my-property obj2) value-for-obj2)
@end lisp

@noindent
And retrieving values of the same property looks like this:

@lisp
(my-property obj1)
@result{}
value-for-obj1

(my-property obj2)
@result{}
value-for-obj2
@end lisp

To create an object property in the first place, use the
@code{make-object-property} procedure:

@lisp
(define my-property (make-object-property))
@end lisp

@deffn {Scheme Procedure} make-object-property
Create and return an object property.  An object property is a
procedure-with-setter that can be called in two ways.  @code{(set!
(@var{property} @var{obj}) @var{val})} sets @var{obj}'s @var{property}
to @var{val}.  @code{(@var{property} @var{obj})} returns the current
setting of @var{obj}'s @var{property}.
@end deffn

A single object property created by @code{make-object-property} can
associate distinct property values with all Scheme values that are
distinguishable by @code{eq?} (ruling out numeric values).

Internally, object properties are implemented using a weak key hash
table.  This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected.  When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.

Guile also implements a more traditional Lispy interface to properties,
in which each object has an list of key-value pairs associated with it.
Properties in that list are keyed by symbols.  This is a legacy
interface; you should use weak hash tables or object properties instead.

@deffn {Scheme Procedure} object-properties obj
@deffnx {C Function} scm_object_properties (obj)
Return @var{obj}'s property list.
@end deffn

@deffn {Scheme Procedure} set-object-properties! obj alist
@deffnx {C Function} scm_set_object_properties_x (obj, alist)
Set @var{obj}'s property list to @var{alist}.
@end deffn

@deffn {Scheme Procedure} object-property obj key
@deffnx {C Function} scm_object_property (obj, key)
Return the property of @var{obj} with name @var{key}.
@end deffn

@deffn {Scheme Procedure} set-object-property! obj key value
@deffnx {C Function} scm_set_object_property_x (obj, key, value)
In @var{obj}'s property list, set the property named @var{key}
to @var{value}.
@end deffn


@node Sorting
@subsection Sorting

@c FIXME::martin: Review me!

@cindex sorting
@cindex sorting lists
@cindex sorting vectors

Sorting is very important in computer programs.  Therefore, Guile comes
with several sorting procedures built-in.  As always, procedures with
names ending in @code{!} are side-effecting, that means that they may
modify their parameters in order to produce their results.

The first group of procedures can be used to merge two lists (which must
be already sorted on their own) and produce sorted lists containing
all elements of the input lists.

@deffn {Scheme Procedure} merge alist blist less
@deffnx {C Function} scm_merge (alist, blist, less)
Merge two already sorted lists into one.
Given two lists @var{alist} and @var{blist}, such that
@code{(sorted? alist less?)} and @code{(sorted? blist less?)},
return a new list in which the elements of @var{alist} and
@var{blist} have been stably interleaved so that
@code{(sorted? (merge alist blist less?) less?)}.
Note:  this does _not_ accept vectors.
@end deffn

@deffn {Scheme Procedure} merge! alist blist less
@deffnx {C Function} scm_merge_x (alist, blist, less)
Takes two lists @var{alist} and @var{blist} such that
@code{(sorted? alist less?)} and @code{(sorted? blist less?)} and
returns a new list in which the elements of @var{alist} and
@var{blist} have been stably interleaved so that
 @code{(sorted? (merge alist blist less?) less?)}.
This is the destructive variant of @code{merge}
Note:  this does _not_ accept vectors.
@end deffn

The following procedures can operate on sequences which are either
vectors or list.  According to the given arguments, they return sorted
vectors or lists, respectively.  The first of the following procedures
determines whether a sequence is already sorted, the other sort a given
sequence.  The variants with names starting with @code{stable-} are
special in that they maintain a special property of the input sequences:
If two or more elements are the same according to the comparison
predicate, they are left in the same order as they appeared in the
input.

@deffn {Scheme Procedure} sorted? items less
@deffnx {C Function} scm_sorted_p (items, less)
Return @code{#t} if @var{items} is a list or vector such that,
for each element @var{x} and the next element @var{y} of
@var{items}, @code{(@var{less} @var{y} @var{x})} returns
@code{#f}.  Otherwise return @code{#f}.
@end deffn

@deffn {Scheme Procedure} sort items less
@deffnx {C Function} scm_sort (items, less)
Sort the sequence @var{items}, which may be a list or a
vector.  @var{less} is used for comparing the sequence
elements.  This is not a stable sort.
@end deffn

@deffn {Scheme Procedure} sort! items less
@deffnx {C Function} scm_sort_x (items, less)
Sort the sequence @var{items}, which may be a list or a
vector.  @var{less} is used for comparing the sequence
elements.  The sorting is destructive, that means that the
input sequence is modified to produce the sorted result.
This is not a stable sort.
@end deffn

@deffn {Scheme Procedure} stable-sort items less
@deffnx {C Function} scm_stable_sort (items, less)
Sort the sequence @var{items}, which may be a list or a
vector. @var{less} is used for comparing the sequence elements.
This is a stable sort.
@end deffn

@deffn {Scheme Procedure} stable-sort! items less
@deffnx {C Function} scm_stable_sort_x (items, less)
Sort the sequence @var{items}, which may be a list or a
vector. @var{less} is used for comparing the sequence elements.
The sorting is destructive, that means that the input sequence
is modified to produce the sorted result.
This is a stable sort.
@end deffn

The procedures in the last group only accept lists or vectors as input,
as their names indicate.

@deffn {Scheme Procedure} sort-list items less
@deffnx {C Function} scm_sort_list (items, less)
Sort the list @var{items}, using @var{less} for comparing the
list elements. This is a stable sort.
@end deffn

@deffn {Scheme Procedure} sort-list! items less
@deffnx {C Function} scm_sort_list_x (items, less)
Sort the list @var{items}, using @var{less} for comparing the
list elements. The sorting is destructive, that means that the
input list is modified to produce the sorted result.
This is a stable sort.
@end deffn

@deffn {Scheme Procedure} restricted-vector-sort! vec less startpos endpos
@deffnx {C Function} scm_restricted_vector_sort_x (vec, less, startpos, endpos)
Sort the vector @var{vec}, using @var{less} for comparing
the vector elements.  @var{startpos} (inclusively) and
@var{endpos} (exclusively) delimit
the range of the vector which gets sorted.  The return value
is not specified.
@end deffn


@node Copying
@subsection Copying Deep Structures

The procedures for copying lists (@pxref{Lists}) only produce a flat
copy of the input list, and currently Guile does not even contain
procedures for copying vectors.  The @code{(ice-9 copy-tree)} module
contains a @code{copy-tree} function that can be used for this purpose,
as it does not only copy the spine of a list, but also copies any pairs
in the cars of the input lists.

@example
(use-modules (ice-9 copy-tree))
@end example

@deffn {Scheme Procedure} copy-tree obj
@deffnx {C Function} scm_copy_tree (obj)
Recursively copy the data tree that is bound to @var{obj}, and return
the new data structure.  @code{copy-tree} recurses down the
contents of both pairs and vectors (since both cons cells and vector
cells may point to arbitrary objects), and stops recursing when it hits
any other object.
@end deffn


@node General Conversion
@subsection General String Conversion

@c FIXME::martin: Review me!

When debugging Scheme programs, but also for providing a human-friendly
interface, a procedure for converting any Scheme object into string
format is very useful.  Conversion from/to strings can of course be done
with specialized procedures when the data type of the object to convert
is known, but with this procedure, it is often more comfortable.

@code{object->string} converts an object by using a print procedure for
writing to a string port, and then returning the resulting string.
Converting an object back from the string is only possible if the object
type has a read syntax and the read syntax is preserved by the printing
procedure.

@deffn {Scheme Procedure} object->string obj [printer]
@deffnx {C Function} scm_object_to_string (obj, printer)
Return a Scheme string obtained by printing @var{obj}.
Printing function can be specified by the optional second
argument @var{printer} (default: @code{write}).
@end deffn


@node Hooks
@subsection Hooks
@tpindex Hooks

A hook is a list of procedures to be called at well defined points in
time.  Typically, an application provides a hook @var{h} and promises
its users that it will call all of the procedures in @var{h} at a
defined point in the application's processing.  By adding its own
procedure to @var{h}, an application user can tap into or even influence
the progress of the application.

Guile itself provides several such hooks for debugging and customization
purposes: these are listed in a subsection below.

When an application first creates a hook, it needs to know how many
arguments will be passed to the hook's procedures when the hook is run.
The chosen number of arguments (which may be none) is declared when the
hook is created, and all the procedures that are added to that hook must
be capable of accepting that number of arguments.

A hook is created using @code{make-hook}.  A procedure can be added to
or removed from a hook using @code{add-hook!} or @code{remove-hook!},
and all of a hook's procedures can be removed together using
@code{reset-hook!}.  When an application wants to run a hook, it does so
using @code{run-hook}.

@menu
* Hook Example::                Hook usage by example.
* Hook Reference::              Reference of all hook procedures.
* C Hooks::                     Hooks for use from C code.
* GC Hooks::                    Garbage collection hooks.
* REPL Hooks::                  Hooks into the Guile REPL.
@end menu


@node Hook Example
@subsubsection Hook Usage by Example

Hook usage is shown by some examples in this section.  First, we will
define a hook of arity 2 --- that is, the procedures stored in the hook
will have to accept two arguments.

@lisp
(define hook (make-hook 2))
hook
@result{} #<hook 2 40286c90>
@end lisp

Now we are ready to add some procedures to the newly created hook with
@code{add-hook!}.  In the following example, two procedures are added,
which print different messages and do different things with their
arguments.

@lisp
(add-hook! hook (lambda (x y)
                    (display "Foo: ")
                    (display (+ x y))
                    (newline)))
(add-hook! hook (lambda (x y)
                    (display "Bar: ")
                    (display (* x y))
                    (newline)))
@end lisp

Once the procedures have been added, we can invoke the hook using
@code{run-hook}.

@lisp
(run-hook hook 3 4)
@print{} Bar: 12
@print{} Foo: 7
@end lisp

Note that the procedures are called in the reverse of the order with
which they were added.  This is because the default behaviour of
@code{add-hook!} is to add its procedure to the @emph{front} of the
hook's procedure list.  You can force @code{add-hook!} to add its
procedure to the @emph{end} of the list instead by providing a third
@code{#t} argument on the second call to @code{add-hook!}.

@lisp
(add-hook! hook (lambda (x y)
                    (display "Foo: ")
                    (display (+ x y))
                    (newline)))
(add-hook! hook (lambda (x y)
                    (display "Bar: ")
                    (display (* x y))
                    (newline))
                    #t)             ; @r{<- Change here!}

(run-hook hook 3 4)
@print{} Foo: 7
@print{} Bar: 12
@end lisp


@node Hook Reference
@subsubsection Hook Reference

When you create a hook with @code{make-hook}, you must specify the arity
of the procedures which can be added to the hook.  If the arity is not
given explicitly as an argument to @code{make-hook}, it defaults to
zero.  All procedures of a given hook must have the same arity, and when
the procedures are invoked using @code{run-hook}, the number of
arguments passed must match the arity specified at hook creation time.

The order in which procedures are added to a hook matters.  If the third
parameter to @code{add-hook!} is omitted or is equal to @code{#f}, the
procedure is added in front of the procedures which might already be on
that hook, otherwise the procedure is added at the end.  The procedures
are always called from the front to the end of the list when they are
invoked via @code{run-hook}.

The ordering of the list of procedures returned by @code{hook->list}
matches the order in which those procedures would be called if the hook
was run using @code{run-hook}.

Note that the C functions in the following entries are for handling
@dfn{Scheme-level} hooks in C.  There are also @dfn{C-level} hooks which
have their own interface (@pxref{C Hooks}).

@deffn {Scheme Procedure} make-hook [n_args]
@deffnx {C Function} scm_make_hook (n_args)
Create a hook for storing procedure of arity @var{n_args}.
@var{n_args} defaults to zero.  The returned value is a hook
object to be used with the other hook procedures.
@end deffn

@deffn {Scheme Procedure} hook? x
@deffnx {C Function} scm_hook_p (x)
Return @code{#t} if @var{x} is a hook, @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} hook-empty? hook
@deffnx {C Function} scm_hook_empty_p (hook)
Return @code{#t} if @var{hook} is an empty hook, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} add-hook! hook proc [append_p]
@deffnx {C Function} scm_add_hook_x (hook, proc, append_p)
Add the procedure @var{proc} to the hook @var{hook}. The
procedure is added to the end if @var{append_p} is true,
otherwise it is added to the front.  The return value of this
procedure is not specified.
@end deffn

@deffn {Scheme Procedure} remove-hook! hook proc
@deffnx {C Function} scm_remove_hook_x (hook, proc)
Remove the procedure @var{proc} from the hook @var{hook}.  The
return value of this procedure is not specified.
@end deffn

@deffn {Scheme Procedure} reset-hook! hook
@deffnx {C Function} scm_reset_hook_x (hook)
Remove all procedures from the hook @var{hook}.  The return
value of this procedure is not specified.
@end deffn

@deffn {Scheme Procedure} hook->list hook
@deffnx {C Function} scm_hook_to_list (hook)
Convert the procedure list of @var{hook} to a list.
@end deffn

@deffn {Scheme Procedure} run-hook hook arg @dots{}
@deffnx {C Function} scm_run_hook (hook, args)
Apply all procedures from the hook @var{hook} to the arguments @var{arg}
@enddots{}.  The order of the procedure application is first to last.
The return value of this procedure is not specified.
@end deffn

If, in C code, you are certain that you have a hook object and well
formed argument list for that hook, you can also use
@code{scm_c_run_hook}, which is identical to @code{scm_run_hook} but
does no type checking.

@deftypefn {C Function} void scm_c_run_hook (SCM hook, SCM args)
The same as @code{scm_run_hook} but without any type checking to confirm
that @var{hook} is actually a hook object and that @var{args} is a
well-formed list matching the arity of the hook.
@end deftypefn

For C code, @code{SCM_HOOKP} is a faster alternative to
@code{scm_hook_p}:

@deftypefn {C Macro} int SCM_HOOKP (x)
Return 1 if @var{x} is a Scheme-level hook, 0 otherwise.
@end deftypefn


@node C Hooks
@subsubsection Hooks For C Code.

The hooks already described are intended to be populated by Scheme-level
procedures.  In addition to this, the Guile library provides an
independent set of interfaces for the creation and manipulation of hooks
that are designed to be populated by functions implemented in C.

The original motivation here was to provide a kind of hook that could
safely be invoked at various points during garbage collection.
Scheme-level hooks are unsuitable for this purpose as running them could
itself require memory allocation, which would then invoke garbage
collection recursively @dots{}  However, it is also the case that these
hooks are easier to work with than the Scheme-level ones if you only
want to register C functions with them.  So if that is mainly what your
code needs to do, you may prefer to use this interface.

To create a C hook, you should allocate storage for a structure of type
@code{scm_t_c_hook} and then initialize it using @code{scm_c_hook_init}.

@deftp {C Type} scm_t_c_hook
Data type for a C hook.  The internals of this type should be treated as
opaque.
@end deftp

@deftp {C Enum} scm_t_c_hook_type
Enumeration of possible hook types, which are:

@table @code
@item SCM_C_HOOK_NORMAL
@vindex SCM_C_HOOK_NORMAL
Type of hook for which all the registered functions will always be called.
@item SCM_C_HOOK_OR
@vindex SCM_C_HOOK_OR
Type of hook for which the sequence of registered functions will be
called only until one of them returns C true (a non-NULL pointer).
@item SCM_C_HOOK_AND
@vindex SCM_C_HOOK_AND
Type of hook for which the sequence of registered functions will be
called only until one of them returns C false (a NULL pointer).
@end table
@end deftp

@deftypefn {C Function} void scm_c_hook_init (scm_t_c_hook *hook, void *hook_data, scm_t_c_hook_type type)
Initialize the C hook at memory pointed to by @var{hook}.  @var{type}
should be one of the values of the @code{scm_t_c_hook_type} enumeration,
and controls how the hook functions will be called.  @var{hook_data} is
a closure parameter that will be passed to all registered hook functions
when they are called.
@end deftypefn

To add or remove a C function from a C hook, use @code{scm_c_hook_add}
or @code{scm_c_hook_remove}.  A hook function must expect three
@code{void *} parameters which are, respectively:

@table @var
@item hook_data
The hook closure data that was specified at the time the hook was
initialized by @code{scm_c_hook_init}.

@item func_data
The function closure data that was specified at the time that that
function was registered with the hook by @code{scm_c_hook_add}.

@item data
The call closure data specified by the @code{scm_c_hook_run} call that
runs the hook.
@end table

@deftp {C Type} scm_t_c_hook_function
Function type for a C hook function: takes three @code{void *}
parameters and returns a @code{void *} result.
@end deftp

@deftypefn {C Function} void scm_c_hook_add (scm_t_c_hook *hook, scm_t_c_hook_function func, void *func_data, int appendp)
Add function @var{func}, with function closure data @var{func_data}, to
the C hook @var{hook}.  The new function is appended to the hook's list
of functions if @var{appendp} is non-zero, otherwise prepended.
@end deftypefn

@deftypefn {C Function} void scm_c_hook_remove (scm_t_c_hook *hook, scm_t_c_hook_function func, void *func_data)
Remove function @var{func}, with function closure data @var{func_data},
from the C hook @var{hook}.  @code{scm_c_hook_remove} checks both
@var{func} and @var{func_data} so as to allow for the same @var{func}
being registered multiple times with different closure data.
@end deftypefn

Finally, to invoke a C hook, call the @code{scm_c_hook_run} function
specifying the hook and the call closure data for this run:

@deftypefn {C Function} {void *} scm_c_hook_run (scm_t_c_hook *hook, void *data)
Run the C hook @var{hook} will call closure data @var{data}.  Subject to
the variations for hook types @code{SCM_C_HOOK_OR} and
@code{SCM_C_HOOK_AND}, @code{scm_c_hook_run} calls @var{hook}'s
registered functions in turn, passing them the hook's closure data, each
function's closure data, and the call closure data.

@code{scm_c_hook_run}'s return value is the return value of the last
function to be called.
@end deftypefn


@node GC Hooks
@subsubsection Hooks for Garbage Collection

Whenever Guile performs a garbage collection, it calls the following
hooks in the order shown.

@defvr {C Hook} scm_before_gc_c_hook
C hook called at the very start of a garbage collection, after setting
@code{scm_gc_running_p} to 1, but before entering the GC critical
section.

If garbage collection is blocked because @code{scm_block_gc} is
non-zero, GC exits early soon after calling this hook, and no further
hooks will be called.
@end defvr

@defvr {C Hook} scm_before_mark_c_hook
C hook called before beginning the mark phase of garbage collection,
after the GC thread has entered a critical section.
@end defvr

@defvr {C Hook} scm_before_sweep_c_hook
C hook called before beginning the sweep phase of garbage collection.
This is the same as at the end of the mark phase, since nothing else
happens between marking and sweeping.
@end defvr

@defvr {C Hook} scm_after_sweep_c_hook
C hook called after the end of the sweep phase of garbage collection,
but while the GC thread is still inside its critical section.
@end defvr

@defvr {C Hook} scm_after_gc_c_hook
C hook called at the very end of a garbage collection, after the GC
thread has left its critical section.
@end defvr

@defvr {Scheme Hook} after-gc-hook
@vindex scm_after_gc_hook
Scheme hook with arity 0.  This hook is run asynchronously
(@pxref{Asyncs}) soon after the GC has completed and any other events
that were deferred during garbage collection have been processed.  (Also
accessible from C with the name @code{scm_after_gc_hook}.)
@end defvr

All the C hooks listed here have type @code{SCM_C_HOOK_NORMAL}, are
initialized with hook closure data NULL, are invoked by
@code{scm_c_hook_run} with call closure data NULL.

@cindex guardians, testing for GC'd objects
The Scheme hook @code{after-gc-hook} is particularly useful in
conjunction with guardians (@pxref{Guardians}).  Typically, if you are
using a guardian, you want to call the guardian after garbage collection
to see if any of the objects added to the guardian have been collected.
By adding a thunk that performs this call to @code{after-gc-hook}, you
can ensure that your guardian is tested after every garbage collection
cycle.


@node REPL Hooks
@subsubsection Hooks into the Guile REPL


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