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> basic interface that all entities conform to e.g. being able to ask an entity to provide a hash code, or to carry out a comparison check with some other entity

What do you do with values that can't be meaningfully hashed/compared? Java is the only place I've come across this odd idea that everything is hashable, and I've never thought of it as a positive thing.

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Well, the problem with a value not having a corresponding hash is that it can't be used in any code that relies upon it having a hash. E.g. in Java, there are many hash based data structures that work for any object as they all have hashes. Even if there is no meaningful hash, by default the memory address can be used. This may mean you don't gain the performance benefits of a hash based data structure, but at least it is compatible with the type system.

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