{\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
+underlying {\RDF} database, and the infrastructure to post-process the
query results. The components of this infrastructure are listed below:
\begin{itemize}
\item
-Explicit sets of attributed values.
-
+\textbf{Explicit sets of attributed values.}
An explicit {\av} set can be placed in a query in two forms:
-as a single 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 sorrounded by
-square brackets, like \verb+["head" attr {"attribute-name" = "contents"}]+.
-\newline
-In the second form, the contents of an attribute can be the result of a query,
-like
-\verb+["head" attr {"attribute-name" = property /"metadata" of "resource"}]+.
+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 this case the contents of the attribute are the head strings of the query
-result, whose attributes (if any) are discarded.
+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}
-\item
-Variable assignment.
+\begin{center}
+\verb+["head" attr {/"attribute" = property /"metadata" of "resource"}]+
+\end{center}
-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.
-\newline
+\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-name" = $var}]+.
+\verb+let $var = "contents" in ["head" attr {/"attribute" = $var}]+.
-The scope rules of {\MathQL} variables are tipical for an imperative
-programming language and any case of assignment propagation will be indicated.
+The scope of {\MathQL} variables is typical for an imperative programming
+language and any case of assignment propagation will be indicated.
\item
-Sequential composition.
-
+\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
-Bounded iteration.
+\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.
-\end{itemize}
+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.
-\xcomment {
+\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.
-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.
+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.
-} % \xcomment
+\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}
projects / helm.git / blobdiff