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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