Another option (and shameless self promotion) is the rollup family of
functions in https://github.com/sbelak/huri
Your case would be:
(rollup first identity second foos)
=> {:a ("foo" "baz"), :b ("bar")}
On Friday, August 12, 2016 at 7:10:37 PM UTC+2, Erik Assum wrote:
>
> I’ve been working on a new project in java 8 at a new client where I’ve
> been able to play with streams.
> In doing so I’ve come across Collectors.groupingBy which has some
> additional features that group-by doesn’t seem to have.
>
> Example:
> Given
> (def foos [[:a "foo"] [:b "bar"] [:a "baz"]])
>
> (group-by first foos)
> ;=> {:a [[:a "foo"] [:a "baz"]], :b [[:b "bar"]]}
>
> but I’d like the result to be
> ;=> {:a ["FOO" "BAZ"] :b ["BAR”]}
>
> This is solved in javas stream api by something like this:
>
> List<List<String>> = [["a", "foo"], ["b" "bar"], ["a", "baz"]];
>
> foos
> .stream()
> .collect(Collectors.groupingBy(e -> e.get(0),
> Collectors.mapping(e -> e.get(1).toUpperCase(),
> Collectors.toList())));
>
> Where the key point is that Collectors.groupingBy accepts a mapping
> function in which to map the values being grouped.
>
> So how would one go about writing this in Clojure? I could of-course write
> something that mapped over the grouped map,
> but I’d really like to write something like
>
> (defn mapping-group-by grouping-fn mapping-fn coll)
>
> but I have a suspicion that this already exists in some form already?
>
> Erik.
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