Simple syntax for defining value objects

[DartBot]

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

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Dart should provide syntatic sugar for defining value objects, automatically defining the == operator and hashCode property based upon class member variables. The common pattern for defining such an object is tedious and error prone. For instance:

class ValueObject {
  final dynamic prop1;
  final dynamic prop2;
  //.etc

  final int hashCode;

  
  ValueObject(var prop1, var prop2) :
    this.prop1 = prop1,
    this.prop2 = prop2,
    this.hashCode = generateHashCode([prop1, prop2]); // Similar to guava's Objects.hashcode()
  
  bool operator==(other){
    if (identical(this, other)) {
      return true;
    } else if (other is ValueObject) {
      ValueObject that = other;
      return (this.prop1 == that.prop1) && (this.prop2 == that.prop2);
    } else {
      return false;
    }
  }
}

This pattern requires a diligent developer to write a complete set of tests to verify they've implemented both hashCode and == correctly. Instead dart should provided syntax for defining such classes and automatically implement the == and hashCode methods during compilation.

[dgrove]

Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

I think this is a dup of issue #501, though it is not all clear from the description. Please have a look and see if value type support is what you are really after. This of course is about much more than syntax or ==/hashCode.

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Added NeedsInfo label.

[DartBot]

This comment was originally written by bordol...@gmail.com

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I think that this issue is related to 501 but not strictly a dupe. 501 is seeking to add strong guarantees of deep immutability, which while desirable, seems like it may be complicated to implement in dart. With this RFE, I'm looking for an easier way to implement equality reliably and correctly, assuming the developer is not actively trying to subvert the equals contract by using mutable member variables.

[DartBot]

This comment was originally written by tobias.r...@gmail.com

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Another way to simplify implementation of value types is to add a syntax to generate methods such as get hashCode, equals, compare, toString, ... Just as constructors can be generated using the Contstuctor(this.s, this.b) syntax. Some annotations would cover most cases. Annotate the class to be a value type, and annotate the members that should or should not be included in the comparisons, etc.

[davidmorgan]

FYI https://github.com/google/built_value.dart is a new library that uses codegen to provide value types.