Question 348904 in Flutter Question posted in
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Flutter – Union type for primitive types data in dart

How can I define a variable with only two possible primitive types, such as (String or num) for example, in Dart? Is there a way to do this?
like union types in TypeScript.

const x:string|number;
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2

Answers


  1. 0

    Related Info/Resources:

    This question is a possible duplicate of this question here.

    There WAS a union package that has been discontinued. It was replaced by the freezed package, which includes the functionality you are looking for but with a bunch of other stuff as well.

    I think you may be interested in dart’s sealed classes or the either package.

    Ideas for implementing from scratch:

    To achieve the functionality you are looking for, you could create your own class that takes a dynamic type in its constructor, and throws an error if the type does not evaluate to one of the types that you want to enforce.

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  2. 0

    I’ll add another recommendation for Freezed. It’s a good package that will solve your problem and can also be used for things like data classes w/ JSON support.

    If you don’t want to add dependencies / use code generation / whatever, you can write your own sealed class:

    sealed class MyUnion {
  const MyUnion();
}

class MyInt extends MyUnion {
  const MyInt(this.val);
  final int val;
}

class MyString extends MyUnion {
  const MyString(this.val);
  final String val;
}

void printUnion(MyUnion arg) {
  switch (arg) {
    case MyInt(:final val):
      print('Found int: $val');
    case MyString(:final val):
      print('Found String: $val');
  }
}

    As an aside: Dart does not have primitive types. It has built in types, but every type except Null is a subtype of Object.

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