All objects should be Hashable

[DartBot]

This issue was originally filed by renggl...@gmail.com

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What steps will reproduce the problem?

Commonly the following rule is known regarding the implementation of == and hashChode:

• Whenever a == b, then a.hashCode() must be same as b.hashCode().
• If you override one, then you should override the other.

This is kind of tricky to achieve in Dart (please correct me if I am wrong), because Object implements only ==. To make it possible to put an object into a Set/Map based on its identity it needs to implement Hashable. I figured out the following code:

class A implements Hashable {

  // kind of complicated code to make an object hashable by its identity
  static int _nextHashCode = 0;
  final int _hashCode;
  A() : _hashCode = _nextHashCode++;
  int hashCode() => _hashCode;

  // already implemented in superclass, duplicated for consistency
  operator == (other) => this === other;

}

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

1. Object introduces an imbalance between == and hashCode.
2. There is no easy way to get an identityHash of an object.

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

Dart SDK version 7904

[madsager]

Added Area-Language, Triaged labels.

[gbracha]

Recategorizing as a request for a universal hash function.

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Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Accepted labels.
Changed the title to: "All objects should be Hashable".

[gbracha]

We expect to add a hashCode method to Object.

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Added this to the M1 milestone.

[lrhn]

When we do this, can we also add
  int identityHash(Object object);
as a top-level function, to match the toplevel
  bool identical(Object a, Object b);
and to make access to the identity hash of an object available even to objects that have overwritten Object's hashCode() implementation?

[gbracha]

Your in charge of libraries :-) I don't see why not. But other's mileage may vary.

[DartBot]

This comment was originally written by @seaneagan

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Regarding the identical and identityHash top-level methods, these already exist as instance methods on Object (== and hashCode respectively), there is just no way to get at them. It might be nice to have a syntax to extract such instance methods from classes, whose "this" parameter would be passed as the first parameter. A syntax for this might be:

ClassName#methodName

Also, it may already possible to extract instance methods from classes via mirrors ? (but you do lose type information doing it that way).

[sethladd]

Issue #3218 has been merged into this issue.

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cc @dgrove.
cc @kwalrath.
cc @kasperl.
cc @rakudrama.

[sethladd]

Issue #167 has been merged into this issue.

[sethladd]

VM issue #5267
dart2js issue #5268
library issue #5266

[gbracha]

Added Done label.