Have Seq.seq(Map) provide a mapValues

[stephenh]

Mapping values is a very common operation; right now it requires:

    Map<Long, Long> m = new HashMap<>();
    m.put(1L, 2L);
    Map<Long, Long> m2 = Seq.toMap(Seq.seq(m).map(e -> Tuple.tuple(e.v1, e.v2 * 2)));

In theory I'd like to do:

    Map<Long, Long> m2 = seq(m).mapValues(v -> v * 2);

If the Seq.seq(Map) method returned some sort of MapSeq subclass of Seq, it could provide Map/tuple-aware methods like mapValues.

[stephenh]

I guess technically mapValues would probably result a Seq<Tuple<K, V>> or MapSeq<K, V>, which means mapValues in my example might need another toMap (which granted is already taken) to get it back to a Map type:

    Map<Long, Long> m2 = seq(m).mapValues(v -> v * 2).toMap();

[stephenh]

A coworker pointed out I'm abusing Seq when I really have a map, so probably want something like:

https://github.com/javaslang/javaslang/blob/master/javaslang/src/main/java/javaslang/collection/Map.java#L183

[bendem]

So basically Map#replaceAll(BiFunction)?

map.replaceAll((k, v) -> v * 2);

[stephenh]

Haha, yes! Today I learned. :-)

That said, I still wouldn't mind a Seq.mapValues-style way of doing it.

[lukaseder]

Note, there's Tuple2.map2() which might be a bit more useful in your case, although I absolutely agree with @bendem that a replaceAll() call is optimal in this particular situation.

I'm not sure if a new MapSeq type would do the trick. I feel that it is too specific to be really useful. I much prefer a Seq2 type as suggested here: https://github.com/jOOQ/jOOL/pull/172. The only difference being that a MapSeq would have unique keys. With Seq2, your mapValues() method would simply become a map2() method, or a map((k, v) ->tuple(k, v * 2)) call.

Unfortunately, both MapSeq and Seq2 are not trivial to introduce into the type system in a backwards-compatible way, but with the additional use-case in mind, I will think that Seq2 will really add value to jOOλ. I'm closing this issue as a duplicate of #172