top.combinators.tex

   1 \Glspl{Task} in \gls{iTasks} can be combined using so called
   2 \gls{Task}-combinators.  Combinators describe relations between \glspl{Task}.
   3 There are only two basic types of combinators; parallel and sequence. All other
   4 combinators are derived from the basic combinators. Type signatures of
   5 simplified versions of the basic combinators and their derivations are given in
   6 Listing~\ref{lst:combinators}
   7
   8 \begin{lstlisting}[language=Clean,%
   9         caption={\Gls{Task}-combinators},label={lst:combinators}]
  10 //Step combinator
  11 (>>=)  infixl 1 :: (Task a) (a -> Task b)         -> Task b   | iTask a & iTask b
  12 (>>*)  infixl 1 :: (Task a) [TaskCont a (Task b)] -> Task b     | iTask a & iTask b
  13 :: TaskCont a b
  14         =     OnValue             ((TaskValue a)  -> Maybe b)
  15         |     OnAction    Action  ((TaskValue a)  -> Maybe b)
  16         | E.e: OnException         (e              -> b)       & iTask e
  17         |     OnAllExceptions     (String         -> b)
  18 :: Action = Action String
  19
  20 //Parallel combinators
  21 (-||-) infixr 3 :: (Task a) (Task a)              -> Task a     | iTask a
  22 (||-)  infixr 3 :: (Task a) (Task b)              -> Task b     | iTask a & iTask b
  23 (-||)  infixl 3 :: (Task a) (Task b)              -> Task a     | iTask a & iTask b
  24 (-&&-) infixr 4 :: (Task a) (Task b)              -> Task (a,b) | iTask a & iTask b
  25 \end{lstlisting}
  26
  27 \paragraph{Sequence:}
  28 The implementation for the sequence combinator is called the \CI{step}
  29 (\CI{>>*}). This combinator runs the left-hand \gls{Task} and starts the
  30 right-hand side when a certain predicate holds. Predicates can be propositions
  31 about the \CI{TaskValue}, user actions from within the web browser or a thrown
  32 exception. The familiar bind-combinator is an example of a sequence combinator.
  33 This combinator runs the left-hand side and continues to the right-hand
  34 \gls{Task} if there is an \CI{UnStable} value and the user presses continue or
  35 when the value is \CI{Stable}. The combinator could have been implemented as
  36 follows:
  37 \begin{lstlisting}[language=Clean]
  38 (>>=) infixl 1 :: (Task a) (a -> (Task b)) -> (Task b) | iTask a & iTask b
  39 (>>=) ta f = ta >>* [OnAction "Continue" onValue, OnValue onStable]
  40     where
  41         onValue (Value a _)     = Just (f a)
  42         onValue _               = Nothing
  43
  44         onStable (Value a True) = Just (f a)
  45         onStable _              = Nothing
  46 \end{lstlisting}
  47
  48 \paragraph{Parallel:}
  49 The parallel combinator allows for concurrent \glspl{Task}. The \glspl{Task}
  50 combined with these operators will appear at the same time in the web browser
  51 of the user and the results are combined as the type dictates. All parallel
  52 combinators used are derived from the basic parallel combinator that is very
  53 complex and only used internally.