ap<A, B, C> function

Applies a function inside an Either to a value inside another Either.

This function takes an Either that may contain a function of type B Function(C) and applies it to a value inside another Either of type Either<A, C>. If the first Either is a Left<A>, the resultant Either will also be a Left<A>. If the second Either is a Left<A>, the resultant Either will also be a Left<A>, irrespective of the first Either's value.

Example:

Given an Either that might contain a function:

Either<int, int Function(String)> fnEither = Right((s) => s.length);

And another Either that might contain a string:

Either<int, String> valueEither = Right("Bob");

You can use the ap function to apply the function inside fnEither to the value inside valueEither:

final result = ap(fnEither)(valueEither);

if (result is Right<int>) {
  print("String length is ${result.value}");
} else if (result is Left<int>) {
  print("Error code: ${result.value}");
}

In this example, the output will be:

String length is 3

If either fnEither or valueEither were a Left<int>, the output would display the error code.

@param fEither The Either that may contain a function of type B Function(C). @return A function that takes an Either<A, C> and returns an Either<A, B>.