Difference between `IOEither` and `Either` about fpdart

[SandroMaglione]

Hi @haashem

The difference between IOEither and Either is that IOEither is designed for handling synchronous side effects, while Either is not supposed to run any side effect at all.

An example would be reading and validating a user input:

/// No side effects, [Either]
Either<String, String> validateUserInput(String? userInput) => userInput
    .toEither(
      () => "Missing user input",
    )
    .flatMap(
      (str) => str.isEmpty ? Either.left("Empty input") : Either.of(str),
    );

/// IO side effect, [IOEither]
IOEither<String, String> getAndValidateUserInput = IOEither(() {
  final line = stdin.readLineSync(); /// 👈 Side effect
  return validateUserInput(line);
});

• As you can see, the validation itself does not include any side effect: it only takes a String? and returns an Either, without any IO event or external request (no side effects)
• On the other hand, getting the user input requires an IO event, which is a side effect. Therefore this operation requires to convert Either to IOEither

Let me know if this example is clear to understand the difference.