+++ /dev/null
-\subsection{The core language}
-
-{\MathQL}.4 consists of a core language and of a basic library. Other
-user-defined libraries can be added at will. The core language includes the
-\TT{property} operator mentioned in \subsecref{HighAccess} that queries the
-underlying {\RDF} database, and the infrastructure to post-process the
-query results. The components of this infrastructure are listed below:
-
-\begin{itemize}
-
-\item
-\textbf{Explicit sets of attributed values.}
-An explicit {\av} set can be placed in a query in two forms:
-as a quoted string, like \verb+"this is a query result"+, that evaluates in a
-single {\av} with that value and no attributes, or as a full {\av} set in the
-syntax shown in the previous sections but surrounded by square brackets, like
-\verb+["head" attr {/"attribute" = "contents"}]+.
-\newline
-In the second form, the contents of an attribute can be the result of a query
-and in this case the contents of the attribute are the head strings of the
-query result, whose attributes (if any) are discarded.
-\end{itemize}
-
-\begin{center}
-\verb+["head" attr {/"attribute" = property /"metadata" of "resource"}]+
-\end{center}
-
-\begin{itemize}
-
-\item
-\textbf{Variable assignment.}
-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.
-The assignment has the form:
-\TT{let \$}\EM{variable} \TT{=} \EM{av-set} \TT{in} \EM{av-set}
-so we can write:
-\newline
-\verb+let $var = "contents" in ["head" attr {/"attribute" = $var}]+.
-
-The scope of {\MathQL} variables is typical for an imperative programming
-language and any case of assignment propagation will be indicated.
-
-\item
-\textbf{Sequential composition.}
-This construction has the form: \EM{av-set} \TT{;;} \EM{av-set} and works as
-follows: the two {\av} sets are evaluated one after the other and the first
-one is discarded but the variables assigned in the first {\av} set are
-available to the second one.
-
-\item
-\textbf{Unbounded iteration.}
-This construction comes in two forms:
-
-\TT{while} \EM{av-set-1} \TT{sup} \EM{av-set-2}:
-iterates the evaluation of \EM{av-set-2} until \EM{av-set-1} is empty and
-returns the {\MathQL} set-theoretic union of all the evaluations of
-\EM{av-set-2}.
-\TT{while} \EM{av-set-1} \TT{inf} \EM{av-set-2}:
-like the former but the set-theoretic intersection is used instead of the
-set-theoretic union.
-
-In order for \TT{while} to work as expected, both {\av} sets are evaluated in
-a common context during the iteration ({\ie} the variables defined in both
-are available to both) and this context is also propagated outside the
-\TT{while}.
-
-\item
-\textbf{Bounded iteration.}
-Also this construction comes in two forms:
-
-\TT{for @}\EM{variable} \TT{in} \EM{av-set} \TT{sup} \EM{av-set}:
-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.
-
-\TT{for @}\EM{variable} \TT{in} \EM{av-set} \TT{inf} \EM{av-set}:
-like the former but the set-theoretic intersection is used instead of the
-set-theoretic union.
-
-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.
-The element variables are kept distinct from the set variables (therefore
-\TT{\$variable} and \TT{@variable} may appear in the same query without
-ambiguity).
-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.
-
-\item
-\textbf{Addition of groups.}
-\TT{add} \EM{optional-flag} \EM{attribute-groups} \TT{in} \EM{av-set}
-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.
-
-\figref{Add} shows how to build a one-element {\av} set using \TT{add}.
-
-\begin{figure}
-\begin{footnotesize} \begin{verbatim}
-The set of attributed values given explicitly:
- ["head" attr {/"attribute" = property /"metadata" of "resource"}]
-
-The same set built with the add operator:
- add {/"attribute" = property /"metadata" of "resource"} in "head"
-\end{verbatim} \end{footnotesize}
-\vspace{-1pc}
-\caption{A simple use of the add operator}
-\label{Add}
-\end{figure}
-
-\item
-\textbf{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 these groups (recall that different groups are allowed to
-contain attributes with the same name). If one evaluation gives a non empty
-result, the default representation of \emph{true} is returned, in the other
-case the empty {\av} set, {\ie} \emph{false}, is returned.
-
-\item
-\textbf{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+}+
-invokes a function of the first kind on the given arguments and returns its
-result. The \EM{name} arguments are {\MathQL} paths and usually
-represent attribute names.
-
-\TT{gen} \EM{function-name} \verb+{+
-\EM{av-set} \TT{,} $\cdots$ \TT{,} \EM{av-set} \verb+}+
-invokes a function of the second kind on the given arguments and replaces
-itself with the function result.
-
-The function names are {\MathQL} paths exactly as the attribute names 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{Basic}) that integrate the core language providing several
-facilities.
-
-\end{itemize}
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