Dartz - get left of Either

Question

var newRight;
if(either.isRight()) {
  newRight = either.getOrElse(() => throw UnimplementedError());
} else {
  return Left(either.fold((f) => f, (r) => throw UnimplementedError()))
}

How can I get left side of either and return it as in the example above? Any other, maybe cleaner solutions?

Taha · Accepted Answer · 2021-09-08 10:17:15Z

5

You can use swap(). It swaps the Left and Right part of an Either

either.swap().getOrElse(() => throw UnimplementedError());

or you can use fold if you can reduce left and right to same type.

myEither.fold(fToApplyLeft, gToApplyRight)

answered Sep 8, 2021 at 10:17

Taha

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1 Comment

Chris Over a year ago

I cannot reduce left and right to the same type. For now I went for fold return Left(either.fold( (f) => f, (r) => throw UnimplementedError()));. I hoped to achieve cleaner solution, but swap seems to be similarly messy. Nevertheless, thank you for your answer.

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