public void EitherFoldBackLeftWhenLeft()
{
string state = "Hello";
string expected = "Hello World";
Either <string, int> either = " World";
string result =
EitherModule.FoldBackLeft(
(left, _state) => string.Concat(_state, left),
either, state);
Assert.AreEqual(expected, result);
}
/// <summary>
/// Creates a new <typeparamref name="TState"/> value by applying the given <paramref name="folder"/> function
/// to <see cref="Either{TLeft, TRight}"/> value and <paramref name="state"/> when <paramref name="either"/> <see cref="Either{TLeft, TRight}.IsLeft"/>.
/// Otherwise, returns the <paramref name="state"/>.
/// </summary>
/// <typeparam name="TLeft">The type of the left value.</typeparam>
/// <typeparam name="TRight">The type of the right value.</typeparam>
/// <typeparam name="TState">The type of initial and final states</typeparam>
/// <param name="folder">The function to transform either value from the input <paramref name="either"/> to a new state value.</param>
/// <param name="state">The initial state.</param>
/// <param name="either">the input either.</param>
/// <returns>
/// Returns a new <typeparamref name="TState"/> value by applying the given <paramref name="folder"/> function
/// to <see cref="Either{TLeft, TRight}"/> value and <paramref name="state"/> when <paramref name="either"/> <see cref="Either{TLeft, TRight}.IsLeft"/>.
/// Otherwise, returns the input <paramref name="either"/>.
/// </returns>
public static TState FoldBackLeft <TLeft, TRight, TState>(this Either <TLeft, TRight> either, Func <TLeft, TState, TState> folder, TState state)
=> EitherModule.FoldBackLeft(folder, either, state);