\item
Variable assignment.
-Variables for {\av} sets (called \emph{set variables}) can be assigned using
-a standard \emph{let-in} construction and may appear wherever an {\av} set
-({\ie} a query result) is allowed.
+Variables for {\av} sets (preceded by a \TT{\$} sign and called
+\emph{set variables}) can be assigned using a standard \emph{let-in}
+construction and may appear wherever an {\av} set ({\ie} a query result) is
+allowed.
\newline
The assignment has the form:
\TT{let \$}\EM{variable} \TT{=} \EM{av-set} \TT{in} \EM{av-set}
one is discarded but the variables assigned in the first {\av} set are
available to the second one.
+\item
+Unbounded iteration.
+\newline
+This construction comes in two forms:
+
+\TT{while} \EM{av-set} \TT{sup} \EM{av-set}:
+\newline
+iterates the evaluation of the second {\av} set until the first {\av set} is
+empty and returns the {\MathQL} set-theoretic union of all the evaluiations
+of the second {\av set}.
+
+\TT{while} \EM{av-set} \TT{inf} \EM{av-set}:
+\newline
+like the former but set-theoretic intersection is used instead of
+set-theoretic union.
+
+In order for \TT{while} to work as expected, both {\av} sets are evaluaed in
+a common context during the iteration ({\ie} the variables defined in both
+are alailable to both) and this context is also propagated outside the
+\TT{while} for convenience.
+
\item
Bounded iteration.
+\newline
+Also this construction comes in two forms:
+\TT{for @}\EM{variable} \TT{in} \EM{av-set} \TT{sup} \EM{av-set}:
+\newline
+iterates the evaluation of the second \EM{av-set} assigning the \EM{variable}
+to each element in the first \EM{av-set} and builds the {\MathQL}
+set-theoretic union of the obtained results.
-\end{itemize}
+\TT{for @}\EM{variable} \TT{in} \EM{av-set} \TT{inf} \EM{av-set}:
+\newline
+like the former but set-theoretic intersection is used instead of
+set-theoretic union.
-\xcomment {
+The variables for attributed values (preceded by a \TT{@} sign and called
+\emph{element variables}) may appear wherever an {\av} set is allowed and
+and in some additional places.
+\newline
+The element variables are kept distinct from the set variables (therefore
+\TT{\$variable} and \TT{@variable} may appear in the same query without
+abiguity).
+
+Concerning the scope rules used in these constructions, the variables
+assigned by the first {\av} set are available to the second {\av} set during
+the iteration and the variables assigned by both {\av} sets are available
+outside the \TT{for} as in the previous case.
-Attributed values can be used to store any auxiliary information needed during
-query execution.
-In particular, {\MathQL} provides variables for {\av}'s which, in its textual
-syntax, are identifiers%
-\footnote{To be understood as in programming languages.}
-preceded by the @ sign, as in \TT{@variable}, and that are introduced by the
-\TT{for} and \TT{select} constructions to be explained below.
+\item
+Addition of groups.
+
+\TT{add} \EM{optional-flag} \EM{attribute-groups} \TT{in} \EM{av-set}
+\newline
+builds an {\av} set adding the specified \EM{attribute-groups} to each element
+of the given {\av} set.
+If no \EM{flag} is specified the addition is set-theoretic, whereas with the
+\TT{distr} flag the addition is distributive.
+The \EM{attribute-groups} can be given explicitly (in the same syntax used for
+explicit {\av} sets) or they can be replaced by an element variable. In the
+latter case the attribute groups of the {\av} stored in the variable are
+considered.
-} % \xcomment
+\figref{Add} shows how to build a one-element {\av} set using \TT{add}.
+
+\begin{figure}[ht]
+\begin{footnotesize} \begin{verbatim}
+The set of attributed values given explicitly:
+["head" attr {"attribute-name" = property /"metadata" of "resource"}]
+
+The same set built with the add operator
+add {"attribute-name" = property /"metadata" of "resource"} in "head"
+\end{verbatim} \end{footnotesize}
+\vskip-1pc
+\caption{A simple use of the add operator}
+\label{Add}
+\end{figure}
+
+\item
+Existential test.
+
+The existential test has the form \TT{ex} \EM{av-set} where the
+specification of the {\av} set contains some instances of the construction
+\TT{@}\EM{variable}\TT{.}\EM{attribute-name}, and runs as follows:
+the given {\av} set is evaluated replacing each
+\TT{@}\EM{variable}\TT{.}\EM{attribute-name}
+with the contents of \EM{attribute-name} in an attribute group of the {\av}
+stored in \TT{@}\EM{variable} and the evaluation is repeated for every
+possible choice of the groups (recall that different groups are allowed to
+contain attributes with the same name). If one evaluation gives a non empty
+result, the defaut representation of \emph{true} is returned (the test
+succeded) in the other case the empty {\av} set, {\ie} \emph{false}, is
+returned (the test failed).
+
+\item
+Function invocation:
+
+The core language allows to invoke two kinds of external functions (with
+which a language extension may be provided): the functions of the first kind
+return an {\av} set, the functions of the second kind return a piece of
+{\MathQL} code representing an {\av} set ({\ie} they interface a {\MathQL}
+code generator). In particular:
+
+\EM{function-name} \verb+{+
+\EM{name} \TT{,} $\cdots$ \TT{,} \EM{name} \verb+} {+
+\EM{av-set} \TT{,} $\cdots$ \TT{,} \EM{av-set} \verb+}+
+\newline
+invokes the specified function of the firs kind on the given arguments and
+returns it result. The \EM{name} argument are {\MathQL} paths and usually
+represent attribute names.
+
+\TT{gen} \EM{function-name} \verb+{+
+\EM{av-set} \TT{,} $\cdots$ \TT{,} \EM{av-set} \verb+}+
+\newline
+invokes the specified function of the second kind on the given arguments and
+replaces the function invocation with its result.
+
+The function names are {\MathQL} paths exactly as the attribute nanes and the
+graph paths used by the \TT{property} operator. The names of the two kinds of
+functions are kept in distinct environments so they do not clash.
+
+{\MathQL}.4 comes with a basic library of functions of the first kind
+(see \subsecref{IBasic}) that integrate the core language providing several
+facilities to the user.
+
+\end{itemize}
{\MathQL} character escaping syntax aims at complying with W3C character model
for the World Wide Web \cite{W3Ca} which recommends a support for standard
-Unicode characters (U+0000 to U+FFFF) and escape sequences with
-start/end delimiters.
+Unicode characters (U+0000 to U+FFFF) and escape sequences with start/end
+delimiters.
In particular {\MathQL} escape delimiters (backslash and caret) are chosen
among the {\em unwise} characters for URI references (see \cite{URI}) because
URI references are the natural content of constant strings and these
Query expressions can contain variables for {\av}'s (production \GP{avar})
and variables for {\av} sets, {\ie} for query results (production \GP{svar})
-according to the syntax of \figref{VarTS}.
+according to the syntax of \figref{VarTS}.%
+\footnote{This syntax resembles the one of programming languages identifiers.}
\begin{figure}[ht]
\begin{footnotesize} \begin{verbatim}
\caption{Textual syntax of results} \label{ResultTS}
\end{figure}
-\xcomment {
+The textual syntax of the language extension provided by the basic library
+is in \figref{BasicTS}.
\begin{figure}[ht]
\begin{footnotesize} \begin{verbatim}
- | "select" <avar> "from" <query> "where" <query>
+<query> ::= "empty" | "false" | "true"
+ | "not" <query>
+ | <query> [ "and" | "or" | "xor"| "sub" | "meet" | "eq" | "le"| "lt" ] <query>
+ | <query> [ "union" | "intersect" ] <query> | { <queries> }
+ | "if" <query> "then" <query> "else" <query>
+ | "select" <avar> "from" <query> "where" <query>
\end{verbatim} \end{footnotesize}
\vskip-1pc
-\caption{Textual syntax of basic query extensions} \label{BasicTS}
+\caption{Textual syntax of basic extension} \label{BasicTS}
\end{figure}
-}
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