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% -*- latex -*- \section{Export lists} \label{sec:exportcalculus} In the simplest case, the export list is given as a sequence of SML symbol names such as shown in earlier examples. But for more complicated scenarios, CM provides a little ``set calculus'' for writing export lists. Formally, the export section specifies the union of a sequence of {\em symbol sets}. Each individual set in this sequence can be one of the following: \begin{itemize}\setlength{\itemsep}{0pt} \item a singleton set given by the name of its sole member \item the set difference of two other sets $s_1$ and $s_2$, written as $s_1 \mbox{\tt -} s_2$ \item the intersection of two sets $s_1$ and $s_2$, written as $s_1 \mbox{\tt *} s_2$ \item the union of sets $s_1, \ldots, s_n$ for $n \ge 0$, written as $\mbox{\tt (} s_1 \ldots s_n \mbox{)}$ \item the top-level defined symbols of one of the SML source files $f$ that the given library or group consists of, written as $\mbox{\tt source(}f\mbox{)}$ \item the top-level defined symbols of all constituent SML source files that are not marked {\em local} (see section~\ref{sec:toolparam:sml}), written as {\tt source(-)} \item the exported symbols of one of the subgroups $g$, written as $\mbox{\tt group(}g\mbox{)}$ \item the exported symbols of {\em all} subgroups, written as {\tt group(-)} \item the exported symbols of another library $l$ \\ In most situations, the notation for this is $\mbox{\tt library(}l\mbox{)}$. However, notice that unlike in the case of subgroups and SML source files, $l$ may or may not be listed as one of the members in the current description. For the case that $l$ is not a member, its elaboration---determining its set of exported symbols---may require {\em tool parameters} to be specified (see section~\ref{sec:toolparam}). When tool parameters are necessary, they are given within an extra pair of parentheses following the name $l$. Example: \begin{verbatim} library(foo/bar.cm (bind:(anchor:x value:y))) \end{verbatim} \noindent If $l$ is a member of the current description, then CM will re-use the earlier elaboration result. Tool parameters within the export set specification are neither necessary nor permitted in this case. \end{itemize} \noindent Notes: \begin{itemize}\setlength{\itemsep}{0pt} \item Intersection has higher precedence than set difference. \item Conditional compilation constructs (see section~\ref{sec:preproc}) may be used within set union costructs---both the parenthesized variety and at top level. \end{itemize} Using the export list calculus, it is easy to set up ``proxy'' libraries. A proxy library $A$ is a library with a single member which is another library $B$ so that the export lists of $A$ and $B$ coincide. Proxy libraries are mainly used for the purpose of managing anchor names and anchor environments (see Section~\ref{sec:anchor:env}). Using the $\mbox{\tt library(}l\mbox{)}$ construct one can avoid the cumbersome repetition of lengthy explicit export lists and, thus, improve maintainability.

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