methods.top.tex

   1 \section{iTasks}
   2 \gls{TOP} is a recent programming paradigm implemented as
   3 \gls{iTasks}\cite{achten_introduction_2015} in the pure lazy functional
   4 language \gls{Clean}\cite{brus_cleanlanguage_1987}. \gls{iTasks} is a
   5 \gls{EDSL} to model workflow tasks in the broadest sense. A \CI{Task} is just
   6 a function that --- given some state --- returns the observable \CI{TaskValue}. The
   7 \CI{TaskValue} of a \CI{Task} can have different states. Not all state
   8 transitions are possible as shown in Figure~\ref{fig:taskvalue}. Once a value
   9 is stable it can never become unstable again. Stability is often reached
  10 by pressing a confirmation button. \glspl{Task} yielding a constant value are
  11 immediately stable.
  12
  13 A simple \gls{iTasks} example illustrating the route to stability of a
  14 \gls{Task} in which the user has to enter a full name is shown in
  15 Listing~\ref{lst:taskex}. The code is accompanied by screenshots showing the
  16 user interface in Figure~\ref{fig:taskex1},~\ref{fig:taskex2}
  17 and~\ref{fig:taskex3}. The \CI{TaskValue} of the \gls{Task} is in the first
  18 image in the \CI{NoValue} state, the second image does not have all the fields
  19 filled in and therefore the \CI{TaskValue} remains \CI{Unstable}. In the third
  20 image all fields are entered and the \CI{TaskValue} transitions to the
  21 \CI{Unstable} state. When the user presses \emph{Continue} the value becomes
  22 \CI{Stable} and cannot be changed any further.
  23
  24 \begin{figure}[H]
  25         \centering
  26         \includegraphics[width=.5\linewidth]{fig-taskvalue}
  27         \caption{The states of a \CI{TaskValue}}\label{fig:taskvalue}
  28 \end{figure}
  29
  30 \begin{lstlisting}[language=Clean,label={lst:taskex},%
  31         caption={An example \gls{Task} for entering a name}]
  32 :: Name = { firstname :: String
  33           , lastname  :: String
  34           }
  35
  36 derive class iTask Name
  37
  38 enterInformation :: String [EnterOption m] -> (Task m) | iTask m
  39
  40 enterName :: Task Name
  41 enterName = enterInformation "Enter your name" []
  42 \end{lstlisting}
  43
  44 \begin{figure}[H]
  45         \begin{subfigure}{.25\textwidth}
  46                 \centering
  47                 \includegraphics[width=.9\linewidth]{taskex1}
  48                 \caption{Initial interface}\label{fig:taskex1}
  49         \end{subfigure}
  50         \begin{subfigure}{.25\textwidth}
  51                 \centering
  52                 \includegraphics[width=.9\linewidth]{taskex2}
  53                 \caption{Incomplete entrance}\label{fig:taskex2}
  54         \end{subfigure}
  55         \begin{subfigure}{.25\textwidth}
  56                 \centering
  57                 \includegraphics[width=.9\linewidth]{taskex3}
  58                 \caption{Complete entry}\label{fig:taskex3}
  59         \end{subfigure}
  60         \caption{Example of a generated user interface}
  61 \end{figure}
  62
  63 For a type to be suitable it must have instances for a collection of generic
  64 functions that are captured in the class \CI{iTask}. Basic types have
  65 specialization instances for these functions and show an according interface.
  66 Generated interfaces can be modified with decoration operators.
  67
  68 \section{Combinators}
  69 \Glspl{Task} can be combined using so called \gls{Task}-combinators.
  70 Combinators describe relations between \glspl{Task}. \Glspl{Task} can be
  71 combined in parallel, sequenced and their result values can be converted to
  72 \glspl{SDS}. Moreover, a very important combinator is the step combinator which
  73 starts a new task according to specified predicates on the \CI{TaskValue}.
  74 Type signatures of the basic combinators are shown in
  75 Listing~\ref{lst:combinators}.
  76
  77 \begin{itemize}
  78         \item Step:
  79
  80                 The step combinator is used to start \glspl{Task} when a predicate on
  81                 the \CI{TaskValue} holds or an action has taken place. The bind
  82                 operator can be written as a step combinator.
  83                 \begin{lstlisting}[language=Clean]
  84 (>>=) infixl 1 :: (Task a) (a -> (Task b)) -> (Task b) | iTask a & iTask b
  85 (>>=) ta f = ta >>* [OnAction "Continue" onValue, OnValue onStable]
  86     where
  87         onValue (Value a _)     = Just (f a)
  88         onValue _               = Nothing
  89
  90         onStable (Value a True) = Just (f a)
  91         onStable _              = Nothing
  92                 \end{lstlisting}
  93         \item Parallel:
  94
  95                 The parallel combinator allows for concurrent \glspl{Task}. The
  96                 \glspl{Task} combined with these operators will appear at the same time
  97                 in the web browser of the user and the results are combined as the type
  98                 dictates.
  99 \end{itemize}
 100
 101 \begin{lstlisting}[language=Clean,%
 102         caption={\Gls{Task}-combinators},label={lst:combinators}]
 103 //Step combinator
 104 (>>*)  infixl 1 :: (Task a) [TaskCont a (Task b)] -> Task b     | iTask a & iTask b
 105 (>>=)  infixl 1 :: (Task a) (a -> Task b)         -> Task b   | iTask a & iTask b
 106 :: TaskCont a b
 107         =     OnValue             ((TaskValue a)  -> Maybe b)
 108         |     OnAction    Action  ((TaskValue a)  -> Maybe b)
 109         | E.e: OnException         (e              -> b)       & iTask e
 110         |     OnAllExceptions     (String         -> b)
 111 :: Action = Action String
 112
 113 //Parallel combinators
 114 (-||-) infixr 3 :: (Task a) (Task a)              -> Task a     | iTask a
 115 (||-)  infixr 3 :: (Task a) (Task b)              -> Task b     | iTask a & iTask b
 116 (-||)  infixl 3 :: (Task a) (Task b)              -> Task a     | iTask a & iTask b
 117 (-&&-) infixr 4 :: (Task a) (Task b)              -> Task (a,b) | iTask a & iTask b
 118 \end{lstlisting}
 119
 120 \section{\acrlongpl{SDS}}
 121 \Glspl{SDS} are an abstraction over resources that are available in the world
 122 or in the \gls{iTasks} system. The shared data can be a file on disk, it can be
 123 the time, a random integer or just some data stored in memory. The actual
 124 \gls{SDS} is just a record containing functions on how to read and write the
 125 source. In these functions the \CI{*World} is available and therefore it can
 126 interact with the outside world. The \CI{*IWorld} is also available and
 127 therefore the functions can also access other shares, possibly combining them.
 128
 129 The basic operations for \glspl{SDS} are get, set and update. The signatures
 130 for these functions are shown in Listing~\ref{lst:shares}. All of the
 131 operations are atomic in the sense that during reading no other tasks are
 132 executed.
 133
 134 \begin{lstlisting}[%
 135         language=Clean,label={lst:shares},caption={\Gls{SDS} functions}]
 136 get ::          (ReadWriteShared r w)           -> Task r | iTask r
 137 set :: w        (ReadWriteShared r w)           -> Task w | iTask w
 138 upd :: (r -> w) (ReadWriteShared r w)           -> Task w | iTask r & iTask w
 139
 140 \end{lstlisting}