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Showing content from https://github.com/tc39/proposal-composites/issues/15 below:

Should composites be eagerly interned (`===` equal) · Issue #15 · tc39/proposal-composites · GitHub

A question raised during yesterday's plenary was if the creation of Composite's should eagerly intern/canonicalize, such that Composite({ x: 1 }) === Composite({ x:1 }).

Upsides:

• "pay to play"
  • not requiring any changes to existing equality checks avoiding adding new code branches which can have a cost even if the application is not using Composites
• No new equality
  • Existing libraries/APIs would 'just work'
• Checking if two composites are equal would be one pointer equality check

Downsides:

• Composite creation becomes more expensive
• This would mean that unique symbol keys would not be possible
  • Only sortable values are usable as keys
  • Symbol.iterator could not be a key
• Could not do SameValueZero
  • Would either be SameValue or -0 is canonicalized to 0.
• Some objects would need to be banned from Weak{Map,Set}
  • e.g. Composite({ x: 1, y: 4 }) has no constituents that provide a non-infinite lifetime
• Potential for more garbage collection time due to added complexity
  • e.g. A WeakMap where Composite({ a: objectA, b: objectB, c: 42 }) is a key should remain in the WeakMap until either objectA or objectB becomes unreachable - this applies deeply for composites made from other composites.
• Prototype equality checks:
  • This may close the door for non-null prototypes with cross-realm equality
  • In a world where there are built in ordinal Composites (Tuples) with a Tuple.prototype to provide Symbol.iterator and list methods such as .map, .some for these to still be equal across realms they would need to have a realm scoped [[prototype]] lookup similar to primitives and the one being discussed for Shared Structs.

Important

The quantity of the above lists should not be interpreted as weights on different sides of a scale. 10 downsides could still be trumped by 1 upside.

One good next step here IMO is to get some empirical data from a prototype implementation to measure some of performance impacts of different designs with different application use cases. Maybe this will be a good time for me to learn how to build v8 and get a custom binary on to some mobile phones!

cc: @syg

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