Hi Myron, I understand the convenience of a `MapSet.intersection/1` but there is also `Enum.reduce/2`, which is very suitable for this use case - when the accumulator is the same type as the elements of the enumerable and the first element would become the accumulator.
James On 26 June 2017 at 23:50, Myron Marston <[email protected]> wrote: > I have a list of MapSets, and I want to find the intersection of all of > them. When I’ve wanted to find the union of a list of MapSets, it’s felt > natural to use Enum.reduce: > > Enum.reduce(list_of_sets, MapSet.new, &MapSet.union/2) > > To find the intersection of a list of MapSets, I wanted to use the same > technique: > > Enum.reduce(list_of_sets, MapSet.new, &MapSet.intersection/2) > > …but that does not work, because the intersection of an empty set with > another set is the empty set — so this reduction will always produce the > empty set, regardless of what list_of_sets is. In fact, the starting > accumulator you need is the union of all the given sets. But of course, > that would be an inefficient way to go about it, as you’d reduce the list > twice (once to compute the union, then again to compute the intersection, > using the union as the initial accumulator). Instead, you should use a > different technique: > > [initial_set | rest] = list_of_setsEnum.reduce(rest, initial_set, > &MapSet.intersection/2) > > While that’s not too bad, it took me some time to figure out why my > initial reduction attempt did not work, and there’s no simple way to > provide an initial accumulator that lets you reduce over the whole list. > > One solution would be to introduce MapSet.intersection/1. Given an > enumerable of sets, it would compute the intersection of all of them. This > is similar to the fact that we have both Enum.concat/2 (which concats two > lists), and Enum.concat/1 (which, given an enumerable of enumerables, > concats them in to a single list). If we introduce MapSet.intersection/1, > we may also want MapSet.union/1 for parity. > > Thoughts? > Myron > > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit https://groups.google.com/d/ > msgid/elixir-lang-core/924446b6-9a96-4eb0-8ba4- > 2d47509190f3%40googlegroups.com > <https://groups.google.com/d/msgid/elixir-lang-core/924446b6-9a96-4eb0-8ba4-2d47509190f3%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/CA%2BibZ9-P7%3D3Ee1Cinex9UHj8hy3gNGWW4n2J9QgfjKX7g1DbWA%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
