Question about Iterable.reduce signature

[Brian Goetz]

> One streaming analog to the left-fold operation is called /scan/ or
> /prefix sum/[1], and there is a natural parallel analog called /parallel
> prefix/[2].  A surprising number of problems can be reduced naturally to
> an application of this primitive.  It is natural to consider this for
> the set of operations common between the lazy and parallel collection APIs.

Agreed, and this is already part of the design (and partially 
implemented) -- see cumulate(BinaryOperator).

> Cheers,
> Neal
>
> [1] http://en.wikipedia.org/wiki/Prefix_sum
> [2]
> http://ocw.mit.edu/courses/mathematics/18-337j-applied-parallel-computing-sma-5505-spring-2005/lecture-notes/chapter_3.pdf
>
> On Mon, May 21, 2012 at 5:15 AM, Brian Goetz <brian.goetz at oracle.com
> <mailto:brian.goetz at oracle.com>> wrote:
>
>     What you are suggesting is to fuse mapping and reducing into a single
>     operation.  While this often makes sense, the approach you suggest gets
>     more difficult when you want to execute in parallel.
>
>     If you define your reducer over T as a (T,T) -> T (assuming
>     associativity), then to compute the reduction in parallel, you can
>     decompose your collection into chunks, reduce each chunk to a T, and
>     then reduce the reduced values up the tree until you have a single T.
>
>     If you define your reducer as a (T,U) -> U, you can reduce each chunk to
>     a U, and then you have no way to combine the per-chunk Us into a single
>     answer.
>
>     On 5/21/2012 2:25 AM, François Sarradin wrote:
>      > Hi all,
>      >
>      > I'm looking for an answer to a question about the method reduce.
>     But, I
>      > still haven't found it in archives of the mailing list. So here is my
>      > question:
>      >
>      > The current signature of Iterable<T>.reduce is:
>      >
>      >      T reduce(T base, BinaryOperator<T>  reducer)
>      >
>      > I would like to know why the signature doesn't look like this one?
>      >
>      > <U>  U reduce (U base, BiOp<T, U>  reducer)
>      >
>      > where BiOp<T, U>  is supposed to represent a function of two
>     arguments of
>      > respective types T and U, that returns a value of type U.
>      >
>      > I really think that this other signature can lead to more readable
>      > code. Below is an example with, say, a set of products that you
>     want to buy
>      > in whatever store you want. And you want to know the total.
>      >
>      > With the current version of reduce:
>      >
>      >      double total = products.map(p ->  p.getPrice()).reduce(0.0,
>     (price,
>      > subTotal) ->  price + subTotal);
>      >
>      > With the other version of reduce in this mail:
>      >
>      >      double total = products.reduce(0.0, (p, subTotal) ->
>       p.getPrice() +
>      > subTotal);
>      >
>      > I know that there is a method named mapReduce. But I'm still
>     uncomfortable
>      > with it:
>      >
>      >      double total = products.mapReduce(p ->  p.getPrice(), 0.0,
>     (price,
>      > subTotal) ->  price + subTotal);
>      >
>      >
>      > Regards,
>      >
>      > Francois
>      >
>
>