+
+\section{Parametric Lenses}
+\Glspl{SDS} can contain complex data structures such as lists, trees and even
+resources in the outside world. Sometimes, an update action only updates a part
+of the resource. When this happens, all waiting \glspl{Task} looking at the
+resource are notified of the update. However, it may be the case that
+\glspl{Task} where only looking at parts of the structure that was not updated.
+To solve this problem, parametric lenses were
+introduced~\cite{domoszlai_parametric_2014}.
+
+Parametric lenses add a type variable to the \gls{SDS} that is in the current
+library functions fixed to \CI{()}. When a \gls{SDS} executes a write
+operation it also provides the system with a notification predicate. This
+notification predicate is a function \CI{p -> Bool} where \CI{p} is the
+parametric lens type. This allows programmers to create a big share, and have
+\glspl{Task} only look at parts of the big share. This technique is used in the
+current system in memory shares. The \CI{IWorld} contains a map that is
+accessible through an \gls{SDS}. While all data is stored in the map, only
+\glspl{Task} looking at a specific entry are notified when the structure is
+updated. The type of the parametric lens is the key in the map.
+
+Functionality for setting parameters is added in the system. The most important
+function is the \CI{sdsFocus} function. This function is listed in
+Listing~\ref{lst:focus} and allows the programmer to fix the parametric lens to
+a value.
+
+\begin{lstlisting}[label={lst:focus},
+ caption={Parametric lens functions}]
+sdsFocus :: p (RWShared p r w) -> RWShared p` r w | iTask p
+\end{lstlisting}