How to implement equality in Ruby

Superseded by The complete guide to implementing equality in Ruby.

Topics not touched on (yet):

Transitivity and typed values. If 15 USD = 15, and 15 EUR = 15, then it must follow that 15 USD = 15 EUR, which is certainly not right.
#deconstruct and #deconstruct_keys

Note last edited November 2023.

Incoming links: Equality in programming languages.