\end{grammar}
%%%--------------------------------------------------------------------------
+\section{Properties} \label{sec:syntax.prop}
+
+\begin{grammar}
+<properties> ::= "[" <list>$[\mbox{@<property>}]$ "]"
+
+<property> ::= <identifier> "=" <expression>
+
+<expression> ::= <term> | <expression> "+" <term> | <expression> "--" <term>
+
+<term> ::= <factor> | <term> "*" <factor> | <term> "/" <factor>
+
+<factor> ::= <primary> | "+" <factor> | "--" <factor>
+
+<primary> ::=
+ <integer-literal> | <string-literal> | <char-literal> | <identifier>
+\alt "<" <plain-type> ">"
+\alt "{" <c-fragment> "}"
+\alt "?" <s-expression>
+\alt "(" <expression> ")"
+\end{grammar}
+
+\emph{Property sets} are a means for associating miscellaneous information
+with compile-time metaobjects such as modules, classes, messages, methods,
+slots, and initializers. By using property sets, additional information can
+be passed to extensions without the need to introduce idiosyncratic syntax.
+(That said, extensions can add additional first-class syntax, if necessary.)
+
+An error is reported if an unrecognized property is associated with an
+object.
+
+
+\subsection{Property values} \label{sec:syntax.prop.value}
+
+A property has a name, given as an @<identifier>, and a value computed by
+evaluating an @<expression>. The value can be one of a number of types.
+
+\begin{itemize}
+
+\item An @<integer-literal> denotes a value of type @|int|.
+
+\item Similarly @<string-literal> and @<char-literal> denote @|string| and
+ @|char| values respectively. Note that, as properties, characters are
+ quite distinct from integers, whereas in C, a character literal denotes a
+ value of type @|int|.
+
+\item There are no variables in the property-value syntax. Rather, an
+ @<identifier> denotes that identifier, as a value of type @|id|.
+
+\item A C type (a @<plain-type>, as described in \xref{sec:syntax.type})
+ between angle brackets, e.g., @|<int>|, or @|<char *>|, or @|<void (*(int,
+ void (*)(int)))(int)>|, denotes that C type, as a value of type @|type|.
+
+\item A @<c-fragment> within braces denotes the tokens between (and not
+ including) the braces, as a value of type @|c-fragment|.
+
+\end{itemize}
+
+As shown in the grammar, there are four binary operators, @"+" (addition),
+@"--" (subtraction), @"*" (multiplication), and @"/" (division);
+multiplication and division have higher precedence than addition and
+subtraction, and operators of the same precedence associate left-to-right.
+There are also unary @"+" (no effect) and @"--" (negation) operators, with
+higher precedence. All of the above operators act only on integer operands
+and yield integer results. (Although the unary @"+" operator yields its
+operand unchanged, an error is still reported if it is applied to a
+non-integer value.) There are currently no bitwise, logical, or comparison
+operators.
+
+Finally, an S-expression preceded by @|?| causes the expression to be read in
+the current package (which is always @|sod-user| at the start of a module)
+and immediately evaluated (using @|eval|); the resulting value is converted
+into a property value using the \descref{decode-property}[generic
+function]{gf}.
+
+
+\subsection{Property output types and coercions}
+\label{sec:syntax.prop.coerce}
+
+When a property value is inspected by the Sod translator, or an extension, it
+is \emph{coerced} so as to conform to a requested output type. This coercion
+process is performed by the \descref{coerce-property-value}[generic
+function]{gf}, and additional output types and coercions can be defined by
+extensions. The built-in output types coercions, from the value types listed
+above, are as follows.
+
+\begin{itemize}
+
+\item The output types @|int|, @|string|, @|char|, @|id|, and @|c-fragment|
+ correspond to the like-named value types described above. No coercions to
+ these output types are defined for the described value types.\footnote{%
+ There is a coercion to @|id| from the value type @|symbol|, but it is
+ only possible to generate a property value of type @|symbol| using Lisp.}
+
+\item The output type @|type| denotes a C type, as does the value type
+ @|type|. In addition, a value of type @|id| can be coerced to a C type if
+ it is the name of a class, a type name explicitly declared by @|typename|,
+ or it is one of: @|bool|, @|_Bool|, @|void|, @|char|, @|short|, @|int|,
+ @|signed|, @|unsigned|, @|long|, @|size_t|, @|ptrdiff_t|, @|wchar_t|,
+ or @|va_list|.
+
+\item The @|boolean| output type denotes a boolean value, which may be either
+ true or false. A value of type @|id| is considered true if it is @|true|,
+ @|t|, @|yes|, @|on|, or @|verily|; or false if it is @|false|, @|nil|,
+ @|no|, @|off|, or @|nowise|; it is erroneous to provide any other
+ identifier where a boolean value is wanted. A value of type @|int| is
+ considered true if it is nonzero, or false if it is zero.
+
+\item The @|symbol| output type denotes a Lisp symbol.
+
+ A value of type @|id| is coerced to a symbol as follows. First, the
+ identifier name is subjected to \emph{case inversion}: if all of the
+ letters in the name have the same case, either upper or lower, then they
+ are replaced with the corresponding letters in the opposite case, lower or
+ upper; if the name contains letters of both cases, then it is not changed.
+ For example, @|foo45| becomes @|FOO45|, or \emph{vice-versa}; but @|Splat|
+ remains as it is. Second, the name is subjected to \emph{separator
+ switching}: all underscores in the name are replaced with hyphens (and
+ \emph{vice-versa}, though hyphens aren't permitted in identifiers in the
+ first place). Finally, the resulting name is interned in the current
+ package, which will usually be @|sod-user| unless changed explicitly by the
+ module.
+
+ A value of type @|string| is coerced to a symbol as follows. If the string
+ contains no colons, then it is case-inverted (but not separator-switched)
+ and interned in the current package. Otherwise, the string either has the
+ form $p @|:| q$, where $q$ does not begin with a colon (the
+ \emph{single-colon} case) or $p @|::| q$ (the \emph{double-colon} case);
+ where $p$ does not contain a colon. Both $p$ and $q$ are case-inverted
+ (but not separator-switched). If $p$ does not name a package, then an
+ error is reported; as a special case, if $p$ is empty, then it is
+ considered to name the @|keyword| package. Otherwise, $q$ is looked up as
+ a symbol name in package~$p$; in the single-colon case, if the symbol is
+ not an exported symbol in package~$p$, then an error is reported; in the
+ double-colon case, $q$ is interned in package~$p$ (and so there needn't be
+ an exported symbol -- or, indeed, and symbol at all -- named $q$
+ beforehand).
+
+\item The @|keyword| output type denotes symbols within the @|keyword|
+ package. Value of type @|id| or @|string| can be coerced to a @|keyword|
+ in the same way as to a @|symbol|, as described above, only the converted
+ name is looked up in the @|keyword| package rather than the current
+ package. (A @|string| can override this by specifying an explicit package
+ name, but this is unlikely to be very helpful.)
+
+\end{itemize}
+
+%%%--------------------------------------------------------------------------
\section{Module syntax} \label{sec:syntax.module}
\begin{grammar}
elsewhere in the generated output files.
-\subsection{Property sets} \label{sec:syntax.module.properties}
-\begin{grammar}
-<properties> ::= "[" <list>$[\mbox{@<property>}]$ "]"
-
-<property> ::= <identifier> "=" <expression>
-\end{grammar}
-
-Property sets are a means for associating miscellaneous information with
-classes and related items. By using property sets, additional information
-can be passed to extensions without the need to introduce idiosyncratic
-syntax.
-
-A property has a name, given as an @<identifier>, and a value computed by
-evaluating an @<expression>. The value can be one of a number of types,
-though the only operators currently defined act on integer values only.
-
-\subsubsection{The expression evaluator}
-\begin{grammar}
-<expression> ::= <term> | <expression> "+" <term> | <expression> "--" <term>
-
-<term> ::= <factor> | <term> "*" <factor> | <term> "/" <factor>
-
-<factor> ::= <primary> | "+" <factor> | "--" <factor>
-
-<primary> ::=
- <integer-literal> | <string-literal> | <char-literal> | <identifier>
-\alt "<" <plain-type> ">"
-\alt "?" <s-expression>
-\alt "(" <expression> ")"
-\end{grammar}
-
-The arithmetic expression syntax is simple and standard; there are currently
-no bitwise, logical, or comparison operators.
-
-A @<primary> expression may be a literal or an identifier. Note that
-identifiers stand for themselves: they \emph{do not} denote values. For more
-fancy expressions, the syntax
-\begin{quote}
- @"?" @<s-expression>
-\end{quote}
-causes the @<s-expression> to be evaluated using the Lisp \textsf{eval}
-function.
-%%% FIXME crossref to extension docs
-
-
\subsection{Class definitions} \label{sec:syntax.module.class}
\begin{grammar}
~mdw / sod / commitdiff