Add a Map#merge function

[DartBot]

This issue was originally filed by @MarkBennett

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What steps will reproduce the problem?

1. Create a map called A. (Such as one of request parameter defaults.)
2. Use Map#forEach() to merge values from another map, B, into A.
3. Get annoyed when doing this many times.

What is the expected output? What do you see instead?

The Map interface should define a merge function which merges together two Maps.

What version of the product are you using? On what operating system?

Version 0.1.0.201204121423, Build 6479
Dart SDK version 6478, Dartium version

[madsager]

Added Area-Library, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

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I think both mutative and non-mutative versions would be useful:

// mutative
void putAll(Map<K, V> other);
// non-mutative
Map<K, V> merge(Map<K, V> other);

However, an Object#clone would make non-mutative versions of APIs slightly less necessary:

var merged = map.clone().putAll(otherMap);

Though I'm not sure whether cloning an Map/List/etc. should produce another immutable object in which case this wouldn't work.

[DartBot]

This comment was originally written by @seaneagan

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Nevermind about Object#clone, looks like someone with much more experience has discoverered that copy constructors are superior:

http://www.artima.com/intv/bloch13.html

So my example would instead be:

var merged = new Map.from(map).putAll(otherMap);

which isn't so bad.

[lrhn]

Issue #6415 has been merged into this issue.

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cc @nex3.

[justinfagnani]

FYI: HashMap and LinkedHashMap both have an addAll() method, but Map does not. The analyzer just starting reporting warnings for objects typed Map, and it looked like to a lot of people that Map had just removed addAll()

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Set owner to @floitschG.
Added C1 label.

[lrhn]

I must admit I thought Map had an addAll method, which is why I implemented it in HashMap.

If we add addAll, we should also consider having addMissing, where addAll uses the argument map's value if both have the key, and addMissing uses the original value.

[DartBot]

This comment was originally written by @seaneagan

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instead of:

map.addMissing(otherMap);

in many cases one could mutate otherMap:

otherMap.addAll(map);

or create a new Map:

var merged = new Map.from(otherMap)..addAll(map);

[floitschG]

I agree. No need to implement addMissing. If we really need it, we can add it later.

I don't like the addAll name, since All usually stands for Iterables, but it's too late to change this now.

Ok to add addAll to the interface, but at the same time we need to adapt SplayTreeMap (which doesn't have addAll yet.

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Added Ready-to-implement, Accepted labels.

[lrhn]

Added Fixed label.