Map#updateOrInsert

Bradley Farias - GoDaddy

Map#setIfMissing(key, () => value);

• Add if a generated value to map if map does not have something at key

~=

if (!map.has(key))� map.set(key, (() => value)());

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• Impact of creating new value is interesting bit
• May want to update if it isn’t missing?

2 lookups

Other Langs

• C++ map emplace
• Python dict setdefault
• Rust Entry or_insert
• CSharp TryAdd
• Java map computeIfAbsent
• SQL INSERT IF NOT EXISTS
• Ruby hash default_proc

Map#update(key, old => updated)

~=

old = map.get(key);�updated = (old => updated)(old);�map.set(key, updated);

• Do we care if it was missing already?
• Could add 3rd arg as an emplace
• could throw if missing

2 lookups

Other Langs

• C++ map[] assignment ops
• Rust Entry and_modify
• Python dict[] assignment ops
• Java Map computeIfPresent
• Lots of things allowing map[key] += style assignment ops

Example code with both

if (!map.has(key)) {� map.set(key, defaultValue);�}�map.get(key).doThing();

3 lookups

Map#updateOrInsert(key, old => updated, () => value) => updated | value

• Combined to single method?
• When you are inserting missing you often then do an update…
• When you do an update you don’t want it to be missing…
• Identity if callback for update handler missing?
• Error if insert handler missing and missing entry? Error on update if missing entry?
• Should it be insertOrUpdate? (which is more popular)

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Why does this matter?

• Collection normalization can make the cost of lookup much higher
• Collection equality hook can also increase lookup costs
• Convenience while allowing error avoidance
• We are expanding stdlib