Saturday, April 5, 2014

A simple and useful iterator pattern in ParaSail

ParaSail has three basic iterator formats, with some sub-formats (unbolded "[ ... ]" indicate optional parts, unbolded "|" indicate alternatives, bold indicates reserved words or symbols that are part of the syntax itself):

    1. Set iterator:
    for I in Set [forward | reverse | concurrent] loop ...

    2. Element iterator:
    for each E of Container [forward | reverse | concurrent] loop ...
    for each [K => V] of Container 
                         [forward | reverse | concurrent] loop ...

    3. Value iterator:
    for V := Initial [then Next_Value(V)] [while Predicate(V)] loop ...
    for T => Root [then T.Left | T.Right] [while T not null] loop ...

These iterators can be combined into a single loop by parenthesizing them;  the loop quits as soon as any of the iterators completes:

   for (each E of Container; I in 1..100) forward loop ...
   //  Display up to 100 elements of Container

As we have written more ParaSail, one pattern that has come up multiple times (particularly when doing output) is the case where you have a special value to be used for the first iteration, and then the same value thereafter.  For example, presume we want to display each of the values of a Vector, separated by commas, enclosed in brackets.  A common way of doing this is to have an "Is_First" boolean value which is tested, and then set to #false, to decide whether to display the opening "[" or the separating ", ".  By using a combination of two iterators, this becomes simpler, with no extra conditionals:

    for (each V of Vec; Sep := "[" then ", ") forward loop
       Print(Sep | V);
    end loop

Note that the value iterator has no "while" part, so it will continue forever to have the loop variable "Sep" bound to the value ", "; when combined with another iterator as it is in this case, the loop will end once the container iterator ends.

Nothing too magical here, but it is sometimes interesting to see a pattern like this that keeps cropping up.  The ParaSail reference manual has more details on these iterators.  The most recent manual may be found at .

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