public void OptionFoldWhenSome()
{
int initial = 25;
int state = 10;
int expected = state + initial;
Option <int> optionValue = initial;
Option <int> optionResult = OptionModule.Fold((_state, value) => _state + value, state, optionValue);
int result = optionResult.Match(value => value, () => 0);
Assert.AreEqual(expected, result);
}
/// <summary>
/// Creates a new <typeparamref name="TState"/> value by applying the given <paramref name="folder"/> function
/// to <paramref name="state"/> and <see cref="Option{T}.Some(T)"/> option value.
/// Otherwise returns the <paramref name="state"/> itself.
/// </summary>
/// <typeparam name="T">The type of the option value.</typeparam>
/// <typeparam name="TState">The type of initial and final states</typeparam>
/// <param name="folder">The function to transform option value from the input option to a new state value.</param>
/// <param name="state">The initial state.</param>
/// <param name="option">The input option.</param>
/// <returns>
/// Returns a new <typeparamref name="TState"/> value by applying the given <paramref name="folder"/> function
/// to <paramref name="state"/> and <see cref="Option{T}.Some(T)"/> option value.
/// Otherwise returns the <paramref name="state"/> itself.
/// </returns>
public static TState Fold <T, TState>(this Option <T> option, TState state, Func <TState, T, TState> folder)
=> OptionModule.Fold(folder, state, option);