[Rd] parallel::mclapply list coercion limits opportunities for code re-use?

[Martin Morgan]

The attached diff address the following issues in mclapply

mclapply coerces non-lists or objects (S3 or S4) to lists, but a list may not be 
an efficient representation and is not required if the object implements length, 
[, and [[ methods (lapply must also work on the object, either through coercion 
to a list at the 'inner.do' level or through other means, e.g., promoting lapply 
to a generic and writing a method specialized for the object). As written 
someone wishing to implement mclapply on an object not efficiently represented 
as a list would need to promote mclapply to a generic, and then re-implement an 
mclapply method for their object, rather than re-using the existing code.

mcparallel is not consistently invoked with a 'name' argument; a name seems to 
be superfluous to the code.

Creating the variable 'schedule' makes a full copy of a potentially large object 
in the master process; delaying until required in the inner.do function may (?) 
result in less copying.

Avoiding coercion to a list is similar to a suggestion for pvec.

   https://stat.ethz.ch/pipermail/r-devel/2012-October/065097.html

Martin
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