\section{iTasks}
\gls{TOP} is a modern recent programming paradigm implemented as
\gls{iTasks}\cite{achten_introduction_2015} in the pure lazy functional
language \gls{Clean}\cite{brus_cleanlanguage_1987}. \gls{iTasks} is a
\gls{EDSL} to model workflow tasks in the broadest sense. A \gls{Task} is just
a function that --- given some state --- returns the observable \CI{TaskValue}. The
\CI{TaskValue} of a \CI{Task} can have different states. Not all state
transitions are possible as shown in Figure~\ref{fig:taskvalue}. Once a value
is stable it can never become unstable again. Stability is often reached
by pressing a confirmation button. \glspl{Task} yielding a constant value are
immediately stable.

A simple \gls{iTasks} example illustrating the route to stability of a
\gls{Task} in which the user has to enter a full name is shown in
Listing~\ref{lst:taskex}. The code is accompanied by screenshots showing the
user interface in Figure~\ref{fig:taskex1},~\ref{fig:taskex2}
and~\ref{fig:taskex3}. The \CI{TaskValue} of the \gls{Task} is in the first
image in the \CI{NoValue} state, the second image does not have all the fields
filled in and therefore the \CI{TaskValue} remains \CI{Unstable}. In the third
image all fields are entered and the \CI{TaskValue} transitions to the
\CI{Unstable} state. When the user presses \emph{Continue} the value becomes
\CI{Stable} and cannot be changed any further.

\begin{figure}[H]
	\centering
	\includegraphics[width=.5\linewidth]{fig-taskvalue}
	\caption{The states of a \CI{TaskValue}}\label{fig:taskvalue}
\end{figure}

\begin{lstlisting}[label={lst:taskex},%
	caption={An example \gls{Task} for entering a name}]
:: Name = { firstname :: String
          , lastname  :: String
          }

derive class iTask Name

enterInformation :: String [EnterOption m] -> (Task m) | iTask m

enterName :: Task Name
enterName = enterInformation "Enter your name" []
\end{lstlisting}

\begin{figure}[H]
	\centering
	\begin{subfigure}{.25\textwidth}
		\centering
		\includegraphics[width=.9\linewidth]{taskex1}
		\caption{Initial interface}\label{fig:taskex1}
	\end{subfigure}
	\begin{subfigure}{.25\textwidth}
		\centering
		\includegraphics[width=.9\linewidth]{taskex2}
		\caption{Incomplete entrance}\label{fig:taskex2}
	\end{subfigure}
	\begin{subfigure}{.25\textwidth}
		\centering
		\includegraphics[width=.9\linewidth]{taskex3}
		\caption{Complete entry}\label{fig:taskex3}
	\end{subfigure}
	\caption{Example of a generated user interface}
\end{figure}

For a type to be suitable, it must have instances for a collection of generic
functions that is captured in the class \CI{iTask}. Basic types have
specialization instances for these functions and show an according interface.
Generated interfaces can be modified with decoration operators.

\section{Combinators}
\Glspl{Task} can be combined using so called \gls{Task}-combinators.
Combinators describe relations between \glspl{Task}. \Glspl{Task} can be
combined in parallel, sequenced and their result values can be converted to
\glspl{SDS}. Moreover, a very important combinator is the step combinator which
starts a new task according to specified predicates on the \CI{TaskValue}.
Type signatures of the basic combinators are shown in
Listing~\ref{lst:combinators}.

\begin{itemize}
	\item Step:

		The step combinator is used to start \glspl{Task} when a predicate on
		the \CI{TaskValue} holds or an action has taken place. The bind
		operator can be written as a step combinator.
		\begin{lstlisting}[language=Clean]
(>>=) infixl 1 :: (Task a) (a -> (Task b)) -> (Task b) | iTask a & iTask b
(>>=) ta f = ta >>* [OnAction "Continue" onValue, OnValue onStable]
    where
        onValue (Value a _)     = Just (f a)
        onValue _               = Nothing

        onStable (Value a True) = Just (f a)
        onStable _              = Nothing
		\end{lstlisting}
	\item Parallel:

		The parallel combinator allows for concurrent \glspl{Task}. The
		\glspl{Task} combined with these operators will appear at the same time
		in the web browser of the user and the results are combined as the type
		dictates.
\end{itemize}

\begin{lstlisting}[%
	caption={\Gls{Task}-combinators},label={lst:combinators}]
//Step combinator
(>>*)  infixl 1 :: (Task a) [TaskCont a (Task b)] -> Task b     | iTask a & iTask b
(>>=)  infixl 1 :: (Task a) (a -> Task b)         -> Task b   | iTask a & iTask b
:: TaskCont a b
	=     OnValue             ((TaskValue a)  -> Maybe b)
	|     OnAction    Action  ((TaskValue a)  -> Maybe b)
	| E.e: OnException         (e              -> b)       & iTask e
	|     OnAllExceptions     (String         -> b)
:: Action = Action String

//Parallel combinators
(-||-) infixr 3 :: (Task a) (Task a)              -> Task a     | iTask a
(||-)  infixr 3 :: (Task a) (Task b)              -> Task b     | iTask a & iTask b
(-||)  infixl 3 :: (Task a) (Task b)              -> Task a     | iTask a & iTask b
(-&&-) infixr 4 :: (Task a) (Task b)              -> Task (a,b) | iTask a & iTask b
\end{lstlisting}

\section{Shared Data Sources}
\Glspl{SDS} are an abstraction over resources that are available in the world
or in the \gls{iTasks} system. The shared data can be a file on disk, the
system time, a random integer or just some data stored in memory. The actual
\gls{SDS} is just a record containing functions on how to read and write the
source. In these functions the \CI{*IWorld} --- which in turn contains the real
program \CI{*World} --- is available. Accessing the outside world is required
for interacting with it and thus the functions can access files on disk, raw
memory, other shares and hardware.

The basic operations for \glspl{SDS} are get, set and update. The signatures
for these functions are shown in Listing~\ref{lst:shares}. By default, all
shares are files containing a \gls{JSON} encoded version of the object and thus
are persistent between restarts of the program. Library functions for shares
residing in memory are available as well. The three main operations on shares
are atomic in the sense that during reading no other tasks are executed. The
system provides useful functions to transform, map and combine \glspl{SDS}
using combinators. The system also provides functionality to inspect the value
of a \gls{SDS} and act upon a change. \Glspl{Task} waiting on a \gls{SDS} to
change are notified when needed. This results in low resource usage because
\glspl{Task} are never constantly inspecting \gls{SDS} values but are notified.

\begin{lstlisting}[%
	label={lst:shares},caption={\Gls{SDS} functions}]
:: RWShared p r w = ... 
:: ReadWriteShared r w :== RWShared () r w
:: ROShared p r :== RWShared p () r
:: ReadOnlyShared r :== ROShared () r

:: Shared r :== ReadWriteShared r r

get ::          (ReadWriteShared r w)           -> Task r | iTask r
set :: w        (ReadWriteShared r w)           -> Task w | iTask w
upd :: (r -> w) (ReadWriteShared r w)           -> Task w | iTask r & iTask w

sharedStore :: String a -> Shared a | JSONEncode{|*|}, JSONDecode{|*|}
\end{lstlisting}

\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} were 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 the void type (i.e. \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 specific value.

\begin{lstlisting}[label={lst:focus},
	caption={Parametric lens functions}]
sdsFocus :: p (RWShared p r w) -> RWShared p` r w | iTask p
\end{lstlisting}
