public void EitherMapLeftWhenLeft()
{
double expected = 10;
Either <string, int> either = "10";
Either <double, int> eitherResult =
EitherModule.MapLeft(
left => Convert.ToDouble(left),
either);
double result = eitherResult.Match(right => 0, left => left);
Assert.AreEqual(expected, result);
}
/// <summary>
/// Creates a new <see cref="Either{TLeft, TRight}"/> whose value is the result of applying the given <paramref name="mapping"/> function when <see cref="Either{TLeft, TRight}.IsLeft"/>.
/// Otherwise, returns itself.
/// </summary>
/// <typeparam name="TLeft">The type of the left value.</typeparam>
/// <typeparam name="TRight">The type of the right value.</typeparam>
/// <typeparam name="TLeftResult">The type of the left value returned by mapping functions.</typeparam>
/// <param name="mapping">The function to transform either value when <see cref="Either{TLeft, TRight}.IsLeft"/>.</param>
/// <param name="either">the input either.</param>
/// <returns>
/// Returns a new <see cref="Either{TLeft, TRight}"/> whose value is the result of applying the given mapping functions.
/// </returns>
public static Either <TLeftResult, TRight> MapLeft <TLeft, TRight, TLeftResult>(
this Either <TLeft, TRight> either,
Func <TLeft, TLeftResult> mapping)
=> EitherModule.MapLeft(mapping, either);