top.combinators.tex

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