doc/: Miscellaneous clarifications and rewordings.

[sod] / doc / concepts.tex

%%% -*-latex-*-
%%%
%%% Conceptual background
%%%
%%% (c) 2015 Straylight/Edgeware
%%%

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\chapter{Concepts} \label{ch:concepts}

%%%--------------------------------------------------------------------------
\section{Operational model} \label{sec:concepts.model}

The Sod translator runs as a preprocessor, similar in nature to the
traditional Unix \man{lex}{1} and \man{yacc}{1} tools.  The translator reads
a \emph{module} file containing class definitions and other information, and
writes C~source and header files.  The source files contain function
definitions and static tables which are fed directly to a C~compiler; the
header files contain declarations for functions and data structures, and are
included by source files -- whether hand-written or generated by Sod -- which
makes use of the classes defined in the module.

Sod is not like \Cplusplus: it makes no attempt to `enhance' the C language
itself.  Sod module files describe classes, messages, methods, slots, and
other kinds of object-system things, and some of these descriptions need to
contain C code fragments, but this code is entirely uninterpreted by the Sod
translator.\footnote{%
  As long as a code fragment broadly follows C's lexical rules, and properly
  matches parentheses, brackets, and braces, the Sod translator will copy it
  into its output unchanged.  It might, in fact, be some other kind of C-like
  language, such as Objective~C or \Cplusplus.  Or maybe even
  Objective~\Cplusplus, because if having an object system is good, then
  having three must be really awesome.} %

The Sod translator is not a closed system.  It is written in Common Lisp, and
can load extension modules which add new input syntax, output formats, or
altered behaviour.  The interface for writing such extensions is described in
\xref{p:lisp}.  Extensions can change almost all details of the Sod object
system, so the material in this manual must be read with this in mind: this
manual describes the base system as provided in the distribution.

%%%--------------------------------------------------------------------------
\section{Modules} \label{sec:concepts.modules}

A \emph{module} is the top-level syntactic unit of input to the Sod
translator.  As described above, given an input module, the translator
generates C source and header files.

A module can \emph{import} other modules.  This makes the type names and
classes defined in those other modules available to class definitions in the
importing module.  Sod's module system is intentionally very simple.  There
are no private declarations or attempts to hide things.

As well as importing existing modules, a module can include a number of
different kinds of \emph{items}:
\begin{itemize}
\item \emph{class definitions} describe new classes, possibly in terms of
  existing classes;
\item \emph{type name declarations} introduce new type names to Sod's
  parser;\footnote{%
    This is unfortunately necessary because C syntax, upon which Sod's input
    language is based for obvious reasons, needs to treat type names
    differently from other kinds of identifiers.} %
  and
\item \emph{code fragments} contain literal C code to be dropped into an
  appropriate place in an output file.
\end{itemize}
Each kind of item, and, indeed, a module as a whole, can have a collection of
\emph{properties} associated with it.  A property has a \emph{name} and a
\emph{value}.  Properties are an open-ended way of attaching additional
information to module items, so extensions can make use of them without
having to implement additional syntax.

%%%--------------------------------------------------------------------------
\section{Classes, instances, and slots} \label{sec:concepts.classes}

For the most part, Sod takes a fairly traditional view of what it means to be
an object system.

An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.  An
object's state is maintained in named \emph{slots}, each of which can store a
C value of an appropriate (scalar or aggregate) type.  An object's behaviour
is stimulated by sending it \emph{messages}.  A message has a name, and may
carry a number of arguments, which are C values; sending a message may result
in the state of receiving object (or other objects) being changed, and a C
value being returned to the sender.

Every object is a (direct) instance of some \emph{class}.  The class
determines which slots its instances have, which messages its instances can
be sent, and which methods are invoked when those messages are received.  The
Sod translator's main job is to read class definitions and convert them into
appropriate C declarations, tables, and functions.  An object cannot
(usually) change its direct class, and the direct class of an object is not
affected by, for example, the static type of a pointer to it.


\subsection{Superclasses and inheritance}
\label{sec:concepts.classes.inherit}

\subsubsection{Class relationships}
Each class has zero or more \emph{direct superclasses}.

A class with no direct superclasses is called a \emph{root class}.  The Sod
runtime library includes a root class named @|SodObject|; making new root
classes is somewhat tricky, and won't be discussed further here.

Classes can have more than one direct superclass, i.e., Sod supports
\emph{multiple inheritance}.  A Sod class definition for a class~$C$ lists
the direct superclasses of $C$ in a particular order.  This order is called
the \emph{local precedence order} of $C$, and the list which consists of $C$
follows by $C$'s direct superclasses in local precedence order is called the
$C$'s \emph{local precedence list}.

The multiple inheritance in Sod works similarly to multiple inheritance in
Lisp-like languages, such as Common Lisp, EuLisp, Dylan, and Python, which is
very different from how multiple inheritance works in \Cplusplus.\footnote{%
  The latter can be summarized as `badly'.  By default in \Cplusplus, an
  instance receives an additional copy of superclass's state for each path
  through the class graph from the instance's direct class to that
  superclass, though this behaviour can be overridden by declaring
  superclasses to be @|virtual|.  Also, \Cplusplus\ offers only trivial
  method combination (\xref{sec:concepts.methods}), leaving programmers to
  deal with delegation manually and (usually) statically.} %

If $C$ is a class, then the \emph{superclasses} of $C$ are
\begin{itemize}
\item $C$ itself, and
\item the superclasses of each of $C$'s direct superclasses.
\end{itemize}
The \emph{proper superclasses} of a class $C$ are the superclasses of $C$
except for $C$ itself.  If a class $B$ is a (direct, proper) superclass of
$C$, then $C$ is a \emph{(direct, proper) subclass} of $B$.  If $C$ is a root
class then the only superclass of $C$ is $C$ itself, and $C$ has no proper
superclasses.

If an object is a direct instance of class~$C$ then the object is also an
(indirect) instance of every superclass of $C$.

If $C$ has a proper superclass $B$, then $B$ is not allowed to have $C$ has a
direct superclass.  In different terms, if we construct a graph, whose
vertices are classes, and draw an edge from each class to each of its direct
superclasses, then this graph must be acyclic.  In yet other terms, the `is a
superclass of' relation is a partial order on classes.

\subsubsection{The class precedence list}
This partial order is not quite sufficient for our purposes.  For each class
$C$, we shall need to extend it into a total order on $C$'s superclasses.
This calculation is called \emph{superclass linearization}, and the result is
a \emph{class precedence list}, which lists each of $C$'s superclasses
exactly once.  If a superclass $B$ precedes (resp.\ follows) some other
superclass $A$ in $C$'s class precedence list, then we say that $B$ is a more
(resp.\ less) \emph{specific} superclass of $C$ than $A$ is.

The superclass linearization algorithm isn't fixed, and extensions to the
translator can introduce new linearizations for special effects, but the
following properties are expected to hold.
\begin{itemize}
\item The first class in $C$'s class precedence list is $C$ itself; i.e.,
  $C$ is always its own most specific superclass.
\item If $A$ and $B$ are both superclasses of $C$, and $A$ is a proper
  superclass of $B$ then $A$ appears after $B$ in $C$'s class precedence
  list, i.e., $B$ is a more specific superclass of $C$ than $A$ is.
\end{itemize}
The default linearization algorithm used in Sod is the \emph{C3} algorithm,
which has a number of good properties described in~\cite{FIXME:C3}.
It works as follows.
\begin{itemize}
\item A \emph{merge} of some number of input lists is a single list
  containing each item that is in any of the input lists exactly once, and no
  other items; if an item $x$ appears before an item $y$ in any input list,
  then $x$ also appears before $y$ in the merge.  If a collection of lists
  have no merge then they are said to be \emph{inconsistent}.
\item The class precedence list of a class $C$ is a merge of the local
  precedence list of $C$ together with the class precedence lists of each of
  $C$'s direct superclasses.
\item If there are no such merges, then the definition of $C$ is invalid.
\item Suppose that there are multiple candidate merges.  Consider the
  earliest position in these candidate merges at which they disagree.  The
  \emph{candidate classes} at this position are the classes appearing at this
  position in the candidate merges.  Each candidate class must be a
  superclass of distinct direct superclasses of $C$, since otherwise the
  candidates would be ordered by their common subclass's class precedence
  list.  The class precedence list contains, at this position, that candidate
  class whose subclass appears earliest in $C$'s local precedence order.
\end{itemize}

\subsubsection{Class links and chains}
The definition for a class $C$ may distinguish one of its proper superclasses
as being the \emph{link superclass} for class $C$.  Not every class need have
a link superclass, and the link superclass of a class $C$, if it exists, need
not be a direct superclass of $C$.

Superclass links must obey the following rule: if $C$ is a class, then there
must be no three superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$ is the
link superclass of both $X$ and $Y$.  As a consequence of this rule, the
superclasses of $C$ can be partitioned into linear \emph{chains}, such that
superclasses $A$ and $B$ are in the same chain if and only if one can trace a
path from $A$ to $B$ by following superclass links, or \emph{vice versa}.

Since a class links only to one of its proper superclasses, the classes in a
chain are naturally ordered from most- to least-specific.  The least specific
class in a chain is called the \emph{chain head}; the most specific class is
the \emph{chain tail}.  Chains are often named after their chain head
classes.

\subsection{Names}
\label{sec:concepts.classes.names}

Classes have a number of other attributes:
\begin{itemize}
\item A \emph{name}, which is a C identifier.  Class names must be globally
  unique.  The class name is used in the names of a number of associated
  definitions, to be described later.
\item A \emph{nickname}, which is also a C identifier.  Unlike names,
  nicknames are not required to be globally unique.  If $C$ is any class,
  then all the superclasses of $C$ must have distinct nicknames.
\end{itemize}


\subsection{Slots} \label{sec:concepts.classes.slots}

Each class defines a number of \emph{slots}.  Much like a structure member, a
slot has a \emph{name}, which is a C identifier, and a \emph{type}.  Unlike
many other object systems, different superclasses of a class $C$ can define
slots with the same name without ambiguity, since slot references are always
qualified by the defining class's nickname.

\subsubsection{Slot initializers}
As well as defining slot names and types, a class can also associate an
\emph{initial value} with each slot defined by itself or one of its
subclasses.  A class $C$ provides an \emph{initialization function} (see
\xref{sec:concepts.classes.c}, and \xref{sec:structures.root.sodclass}) which
sets the slots of a \emph{direct} instance of the class to the correct
initial values.  If several of $C$'s superclasses define initializers for the
same slot then the initializer from the most specific such class is used.  If
none of $C$'s superclasses define an initializer for some slot then that slot
will be left uninitialized.

The initializer for a slot with scalar type may be any C expression.  The
initializer for a slot with aggregate type must contain only constant
expressions if the generated code is expected to be processed by a
implementation of C89.  Initializers will be evaluated once each time an
instance is initialized.


\subsection{C language integration} \label{sec:concepts.classes.c}

For each class~$C$, the Sod translator defines a C type, the \emph{class
type}, with the same name.  This is the usual type used when considering an
object as an instance of class~$C$.  No entire object will normally have a
class type,\footnote{%
  In general, a class type only captures the structure of one of the
  superclass chains of an instance.  A full instance layout contains multiple
  chains.  See \xref{sec:structures.layout} for the full details.} %
so access to instances is almost always via pointers.

\subsubsection{Access to slots}
The class type for a class~$C$ is actually a structure.  It contains one
member for each class in $C$'s superclass chain, named with that class's
nickname.  Each of these members is also a structure, containing the
corresponding class's slots, one member per slot.  There's nothing special
about these slot members: C code can access them in the usual way.

For example, if @|MyClass| has the nickname @|mine|, and defines a slot @|x|
of type @|int|, then the simple function
\begin{prog}
  int get_x(MyClass *m) \{ return (m@->mine.x); \}
\end{prog}
will extract the value of @|x| from an instance of @|MyClass|.

All of this means that there's no such thing as `private' or `protected'
slots.  If you want to hide implementation details, the best approach is to
stash them in a dynamically allocated private structure, and leave a pointer
to it in a slot.  (This will also help preserve binary compatibility, because
the private structure can grow more members as needed.  See
\xref{sec:fixme.compatibility} for more details.

\subsubsection{Class objects}
In Sod's object system, classes are objects too.  Therefore classes are
themselves instances; the class of a class is called a \emph{metaclass}.  The
consequences of this are explored in \xref{sec:concepts.metaclasses}.  The
\emph{class object} has the same name as the class, suffixed with
`@|__class|'\footnote{%
  This is not quite true.  @|$C$__class| is actually a macro.  See
  \xref{sec:structures.layout.additional} for the gory details.} %
and its type is usually @|SodClass|; @|SodClass|'s nickname is @|cls|.

A class object's slots contain or point to useful information, tables and
functions for working with that class's instances.  (The @|SodClass| class
doesn't define any messages, so it doesn't have any methods.  In Sod, a class
slot containing a function pointer is not at all the same thing as a method.)

\subsubsection{Instance allocation, imprinting, and initialization}
It is in general not sufficient to declare (or @|malloc|) an object of the
appropriate class type and fill it in, since the class type only describes an
instance's layout from the point of view of a single superclass chain.  The
correct type to allocate, to store a direct instance of some class is a
structure whose tag is the class name suffixed with `@|__ilayout|'; e.g., the
correct layout structure for a direct instance of @|MyClass| would be
@|struct MyClass__ilayout|.

Instance layouts may be declared as objects with automatic storage duration
(colloquially, `allocated on the stack') or allocated dynamically, e.g.,
using @|malloc|.  Sod's runtime system doesn't retain addresses of instances,
so, for example, Sod doesn't make using a fancy allocator which sometimes
moves objects around in memory any more difficult than it needs to be.

Once storage for an instance has been allocated, it must be \emph{imprinted}
before it can be used.  Imprinting an instance stores some metadata about its
direct class in the instance structure, so that the rest of the program (and
Sod's runtime library) can tell what sort of object it is, and how to use
it.\footnote{%
  Specifically, imprinting an instance's storage involves storing the
  appropriate vtable pointers in the right places in it.} %
A class object's @|imprint| slot points to a function which will correctly
imprint storage for one of that class's instances.

Once an instance's storage has been imprinted, it is possible to send the
instance messages; however, the instance's slots are uninitialized at this
point, so most methods are unlikely to do much of any use.  So, usually, you
don't just want to imprint instance storage, but to \emph{initialize} an
instance.  Initialization includes imprinting, but also sets the new
instance's slots to their initial values, as defined by the class.  If
neither the class nor any of its superclasses defines an initializer for a
slot then it will not be initialized.

There is currently no facility for providing parameters to the instance
initialization process (e.g., for use by slot initializer expressions).
Instance initialization is a complicated matter and for now I want to
experiment with various approaches before committing to one.  My current
interim approach is to specify slot initializers where appropriate and send
class-specific messages for more complicated parametrized initialization.

Automatic-duration instances can be conveniently constructed and initialized
using the \descref{SOD_DECL}[macro]{mac}.  No special support is currently
provided for dynamically allocated instances.  A simple function using
@|malloc| might work as follows.
\begin{prog}
  void *new_instance(const SodClass *c) \\
  \{ \\ \ind
    void *p = malloc(c@->cls.initsz); \\
    if (!p) return (0); \\
    c@->cls.init(p); \\
    return (p); \- \\
  \}
\end{prog}

\subsubsection{Instance finalization and deallocation}
There is currently no provided assistance for finalization or deallocation.
It is the programmer's responsibility to decide and implement an appropriate
protocol.  Note that to free an instance allocated from the heap, one must
correctly find its base address: the \descref{SOD_INSTBASE}[macro]{mac} will
do this for you.

The following simple mixin class is suggested.
\begin{prog}
  [nick = disposable] \\
  class DisposableObject : SodObject \{ \\- \ind
    void release() \{ ; \} \\
    \quad /* Release resources held by the receiver. */ \- \\-
  \}
  \\+
  code c : user \{ \\- \ind
    /\=\+* Free object p's instance storage.  If p is a DisposableObject \\
       {}* then release its resources beforehand. \\
       {}*/ \- \\
    void free_instance(void *p) \\
    \{ \\ \ind
      DisposableObject *d = SOD_CONVERT(DisposableObject, p); \\
      if (d) DisposableObject_release(d); \\
      free(d); \- \\
    \} \- \\
  \}
\end{prog}

\subsubsection{Conversions}
Suppose one has a value of type pointer to class type of some class~$C$, and
wants to convert it to a pointer to class type of some other class~$B$.
There are three main cases to distinguish.
\begin{itemize}
\item If $B$ is a superclass of~$C$, in the same chain, then the conversion
  is an \emph{in-chain upcast}.  The conversion can be performed using the
  appropriate generated upcast macro (see below), or by simply casting the
  pointer, using C's usual cast operator (or the \Cplusplus\ @|static_cast<>|
  operator).
\item If $B$ is a superclass of~$C$, in a different chain, then the
  conversion is a \emph{cross-chain upcast}.  The conversion is more than a
  simple type change: the pointer value must be adjusted.  If the direct
  class of the instance in question is not known, the conversion will require
  a lookup at runtime to find the appropriate offset by which to adjust the
  pointer.  The conversion can be performed using the appropriate generated
  upcast macro (see below); the general case is handled by the macro
  \descref{SOD_XCHAIN}{mac}.
\item If $B$ is a subclass of~$C$ then the conversion is an \emph{upcast};
  otherwise the conversion is a~\emph{cross-cast}.  In either case, the
  conversion can fail: the object in question might not be an instance of~$B$
  at all.  The macro \descref{SOD_CONVERT}{mac} and the function
  \descref{sod_convert}{fun} perform general conversions.  They return a null
  pointer if the conversion fails.  (There are therefore your analogue to the
  \Cplusplus @|dynamic_cast<>| operator.)
\end{itemize}
The Sod translator generates macros for performing both in-chain and
cross-chain upcasts.  For each class~$C$, and each proper superclass~$B$
of~$C$, a macro is defined: given an argument of type pointer to class type
of~$C$, it returns a pointer to the same instance, only with type pointer to
class type of~$B$, adjusted as necessary in the case of a cross-chain
conversion.  The macro is named by concatenating
\begin{itemize}
\item the name of class~$C$, in upper case,
\item the characters `@|__CONV_|', and
\item the nickname of class~$B$, in upper case;
\end{itemize}
e.g., if $C$ is named @|MyClass|, and $B$'s name is @|SuperClass| with
nickname @|super|, then the macro @|MYCLASS__CONV_SUPER| converts a
@|MyClass~*| to a @|SuperClass~*|.  See
\xref{sec:structures.layout.additional} for the formal description.

%%%--------------------------------------------------------------------------
\section{Keyword arguments} \label{sec:concepts.keywords}

In standard C, the actual arguments provided to a function are matched up
with the formal arguments given in the function definition according to their
ordering in a list.  Unless the (rather cumbersome) machinery for dealing
with variable-length argument tails (@|<stdarg.h>|) is used, exactly the
correct number of arguments must be supplied, and in the correct order.

A \emph{keyword argument} is matched by its distinctive \emph{name}, rather
than by its position in a list.  Keyword arguments may be \emph{omitted},
causing some default behaviour by the function.  A function can detect
whether a particular keyword argument was supplied: so the default behaviour
need not be the same as that caused by any specific value of the argument.

Keyword arguments can be provided in three ways.
\begin{enumerate}
\item Directly, as a variable-length argument tail, consisting (for the most
  part) of alternating keyword names, as pointers to null-terminated strings,
  and argument values, and terminated by a null pointer.  This is somewhat
  error-prone, and the support library defines some macros which help ensure
  that keyword argument lists are well formed.
\item Indirectly, through a @|va_list| object capturing a variable-length
  argument tail passed to some other function.  Such indirect argument tails
  have the same structure as the direct argument tails described above.
  Because @|va_list| objects are hard to copy, the keyword-argument support
  library consistently passes @|va_list| objects \emph{by reference}
  throughout its programming interface.
\item Indirectly, through a vector of @|struct kwval| objects, each of which
  contains a keyword name, as a pointer to a null-terminated string, and the
  \emph{address} of a corresponding argument value.  (This indirection is
  necessary so that the items in the vector can be of uniform size.)
  Argument vectors are rather inconvenient to use, but are the only practical
  way in which a caller can decide at runtime which arguments to include in a
  call, which is useful when writing wrapper functions.
\end{enumerate}

Keyword arguments are provided as a general feature for C functions.
However, Sod has special support for messages which accept keyword arguments
(\xref{sec:concepts.methods.keywords}).

%%%--------------------------------------------------------------------------
\section{Messages and methods} \label{sec:concepts.methods}

Objects can be sent \emph{messages}.  A message has a \emph{name}, and
carries a number of \emph{arguments}.  When an object is sent a message, a
function, determined by the receiving object's class, is invoked, passing it
the receiver and the message arguments.  This function is called the
class's \emph{effective method} for the message.  The effective method can do
anything a C function can do, including reading or updating program state or
object slots, sending more messages, calling other functions, issuing system
calls, or performing I/O; if it finishes, it may return a value, which is
returned in turn to the message sender.

The set of messages an object can receive, characterized by their names,
argument types, and return type, is determined by the object's class.  Each
class can define new messages, which can be received by any instance of that
class.  The messages defined by a single class must have distinct names:
there is no `function overloading'.  As with slots
(\xref{sec:concepts.classes.slots}), messages defined by distinct classes are
always distinct, even if they have the same names: references to messages are
always qualified by the defining class's name or nickname.

Messages may take any number of arguments, of any non-array value type.
Since message sends are effectively function calls, arguments of array type
are implicitly converted to values of the corresponding pointer type.  While
message definitions may ascribe an array type to an argument, the formal
argument will have pointer type, as is usual for C functions.  A message may
accept a variable-length argument suffix, denoted @|\dots|.

A class definition may include \emph{direct methods} for messages defined by
it or any of its superclasses.

Like messages, direct methods define argument lists and return types, but
they may also have a \emph{body}, and a \emph{role}.

A direct method need not have the same argument list or return type as its
message.  The acceptable argument lists and return types for a method depend
on the message, in particular its method combination
(\xref{sec:concepts.methods.combination}), and the method's role.

A direct method body is a block of C code, and the Sod translator usually
defines, for each direct method, a function with external linkage, whose body
contains a copy of the direct method body.  Within the body of a direct
method defined for a class $C$, the variable @|me|, of type pointer to class
type of $C$, refers to the receiving object.


\subsection{Effective methods and method combinations}
\label{sec:concepts.methods.combination}

For each message a direct instance of a class might receive, there is a set
of \emph{applicable methods}, which are exactly the direct methods defined on
the object's class and its superclasses.  These direct methods are combined
together to form the \emph{effective method} for that particular class and
message.  Direct methods can be combined into an effective method in
different ways, according to the \emph{method combination} specified by the
message.  The method combination determines which direct method roles are
acceptable, and, for each role, the appropriate argument lists and return
types.

One direct method, $M$, is said to be more (resp.\ less) \emph{specific} than
another, $N$, with respect to a receiving class~$C$, if the class defining
$M$ is a more (resp.\ less) specific superclass of~$C$ than the class
defining $N$.

\subsubsection{The standard method combination}
The default method combination is called the \emph{standard method
combination}; other method combinations are useful occasionally for special
effects.  The standard method combination accepts four direct method roles,
called `primary' (the default), @|before|, @|after|, and @|around|.

All direct methods subject to the standard method combination must have
argument lists which \emph{match} the message's argument list:
\begin{itemize}
\item the method's arguments must have the same types as the message, though
  the arguments may have different names; and
\item if the message accepts a variable-length argument suffix then the
  direct method must instead have a final argument of type @|va_list|.
\end{itemize}
Primary and @|around| methods must have the same return type as the message;
@|before| and @|after| methods must return @|void| regardless of the
message's return type.

If there are no applicable primary methods then no effective method is
constructed: the vtables contain null pointers in place of pointers to method
entry functions.

The effective method for a message with standard method combination works as
follows.
\begin{enumerate}

\item If any applicable methods have the @|around| role, then the most
  specific such method, with respect to the class of the receiving object, is
  invoked.

  Within the body of an @|around| method, the variable @|next_method| is
  defined, having pointer-to-function type.  The method may call this
  function, as described below, any number of times.

  If there any remaining @|around| methods, then @|next_method| invokes the
  next most specific such method, returning whichever value that method
  returns; otherwise the behaviour of @|next_method| is to invoke the before
  methods (if any), followed by the most specific primary method, followed by
  the @|around| methods (if any), and to return whichever value was returned
  by the most specific primary method, as described in the following items.
  That is, the behaviour of the least specific @|around| method's
  @|next_method| function is exactly the behaviour that the effective method
  would have if there were no @|around| methods.  Note that if the
  least-specific @|around| method calls its @|next_method| more than once
  then the whole sequence of @|before|, primary, and @|after| methods occurs
  multiple times.

  The value returned by the most specific @|around| method is the value
  returned by the effective method.

\item If any applicable methods have the @|before| role, then they are all
  invoked, starting with the most specific.

\item The most specific applicable primary method is invoked.

  Within the body of a primary method, the variable @|next_method| is
  defined, having pointer-to-function type.  If there are no remaining less
  specific primary methods, then @|next_method| is a null pointer.
  Otherwise, the method may call the @|next_method| function any number of
  times.

  The behaviour of the @|next_method| function, if it is not null, is to
  invoke the next most specific applicable primary method, and to return
  whichever value that method returns.

  If there are no applicable @|around| methods, then the value returned by
  the most specific primary method is the value returned by the effective
  method; otherwise the value returned by the most specific primary method is
  returned to the least specific @|around| method, which called it via its
  own @|next_method| function.

\item If any applicable methods have the @|after| role, then they are all
  invoked, starting with the \emph{least} specific.  (Hence, the most
  specific @|after| method is invoked with the most `afterness'.)

\end{enumerate}

A typical use for @|around| methods is to allow a base class to set up the
dynamic environment appropriately for the primary methods of its subclasses,
e.g., by claiming a lock, and restore it afterwards.

The @|next_method| function provided to methods with the primary and
@|around| roles accepts the same arguments, and returns the same type, as the
message, except that one or two additional arguments are inserted at the
front of the argument list.  The first additional argument is always the
receiving object, @|me|.  If the message accepts a variable argument suffix,
then the second addition argument is a @|va_list|; otherwise there is no
second additional argument; otherwise, In the former case, a variable
@|sod__master_ap| of type @|va_list| is defined, containing a separate copy
of the argument pointer (so the method body can process the variable argument
suffix itself, and still pass a fresh copy on to the next method).

A method with the primary or @|around| role may use the convenience macro
@|CALL_NEXT_METHOD|, which takes no arguments itself, and simply calls
@|next_method| with appropriate arguments: the receiver @|me| pointer, the
argument pointer @|sod__master_ap| (if applicable), and the method's
arguments.  If the method body has overwritten its formal arguments, then
@|CALL_NEXT_METHOD| will pass along the updated values, rather than the
original ones.

A primary or @|around| method which invokes its @|next_method| function is
said to \emph{extend} the message behaviour; a method which does not invoke
its @|next_method| is said to \emph{override} the behaviour.  Note that a
method may make a decision to override or extend at runtime.

\subsubsection{Aggregating method combinations}
A number of other method combinations are provided.  They are called
`aggregating' method combinations because, instead of invoking just the most
specific primary method, as the standard method combination does, they invoke
the applicable primary methods in turn and aggregate the return values from
each.

The aggregating method combinations accept the same four roles as the
standard method combination, and @|around|, @|before|, and @|after| methods
work in the same way.

The aggregating method combinations provided are as follows.
\begin{description} \let\makelabel\code
\item[progn] The message must return @|void|.  The applicable primary methods
  are simply invoked in turn, most specific first.
\item[sum] The message must return a numeric type.\footnote{%
    The Sod translator does not check this, since it doesn't have enough
    insight into @|typedef| names.} %
  The applicable primary methods are invoked in turn, and their return values
  added up.  The final result is the sum of the individual values.
\item[product] The message must return a numeric type.  The applicable
  primary methods are invoked in turn, and their return values multiplied
  together.  The final result is the product of the individual values.
\item[min] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  The final result is the smallest of the
  individual values.
\item[max] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  The final result is the largest of the
  individual values.
\item[and] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  If any method returns zero then the final
  result is zero and no further methods are invoked.  If all of the
  applicable primary methods return nonzero, then the final result is the
  result of the last primary method.
\item[or] The message must return a scalar type.  The applicable primary
  methods are invoked in turn.  If any method returns nonzero then the final
  result is that nonzero value and no further methods are invoked.  If all of
  the applicable primary methods return zero, then the final result is zero.
\end{description}

There is also a @|custom| aggregating method combination, which is described
in \xref{sec:fixme.custom-aggregating-method-combination}.


\subsection{Messages with keyword arguments}
\label{sec:concepts.methods.keywords}

A message or a direct method may declare that it accepts keyword arguments.
A message which accepts keyword arguments is called a \emph{keyword message};
a direct method which accepts keyword arguments is called a \emph{keyword
method}.

While method combinations may set their own rules, usually keyword methods
can only be defined on keyword messages, and all methods defined on a keyword
message must be keyword methods.  The direct methods defined on a keyword
message may differ in the keywords they accept, both from each other, and
from the message.  If two superclasses of some common class both define
keyword methods on the same message, and the methods both accept a keyword
argument with the same name, then these two keyword arguments must also have
the same type.  Different applicable methods may declare keyword arguments
with the same name but different defaults; see below.

The keyword arguments acceptable in a message sent to an object are the
keywords listed in the message definition, together with all of the keywords
accepted by any applicable method.  There is no easy way to determine at
runtime whether a particular keyword is acceptable in a message to a given
instance.

At runtime, a direct method which accepts one or more keyword arguments
receives an additional argument named @|suppliedp|.  This argument is a small
structure.  For each keyword argument named $k$ accepted by the direct
method, @|suppliedp| contains a one-bit-wide bitfield member of type
@|unsigned|, also named $k$.  If a keyword argument named $k$ was passed in
the message, then @|suppliedp.$k$| is one, and $k$ contains the argument
value; otherwise @|suppliedp.$k$| is zero, and $k$ contains the default value
from the direct method definition if there was one, or an unspecified value
otherwise.

%%%--------------------------------------------------------------------------
\section{Metaclasses} \label{sec:concepts.metaclasses}

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