Proposed change to [[strawman:data_parallelism]]
The ParallelArray methods map, combine, reduce, scan, scatter, and filter, each
take a kernel function as an argument. Within this kernel function |this| is
currently bound to the ParallelArray. This was natural and non-controversial as
long as we used the function(){..} form which did not restrict how |this| was
bound and explaining the semantics in terms of call or apply was perfectly
reasonable. => is likely to change that. =>, as proposed, enforces a lexical
|this| and is semantically different than the function () {..} form. Going
forward we expect folks to use the => form more than the function form. For the
most part we will be leaving the kernel signatures as they are except that we
will no longer bind |this| to the ParalleArray is inside a kernel function.
Instead |this| will be bound in accordance to existing JavaScript
specifications and [[strawman:data_parallelism]] will no longer refer to
|this|. For the combine method which currently gets only an index as an
argument we will now also pass the ParallelArray in. If the programmer wants
the ParallelArray in methods like reduce or map then it will have to be passed
in as a free variable.
We considered and decided not to mimic Array's function (element, index, array)
{ ... } form. We felt that it causes intellectual confusion about parallel
programming since passing in index and array force the programmer to think
about things like order and location when using methods like map. It is even
more intellectually confusing when using reduce since in a parallel world both
of the values passed in may be the results of previous kernel invocations and
as such not have an index in any reasonable sense. For combine and filter we
pass the ParallelArray in as the second argument. For the other methods one can
pass the ParallelArray in using a free variable.
- Rick
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