using System;
using System.Collections.Generic;

namespace Swan {
  /// <summary>
  /// Functional programming extension methods.
  /// </summary>
  public static class FunctionalExtensions {
    /// <summary>
    /// Whens the specified condition.
    /// </summary>
    /// <typeparam name="T">The type of IEnumerable.</typeparam>
    /// <param name="list">The list.</param>
    /// <param name="condition">The condition.</param>
    /// <param name="fn">The function.</param>
    /// <returns>
    /// The IEnumerable.
    /// </returns>
    /// <exception cref="ArgumentNullException">
    /// this
    /// or
    /// condition
    /// or
    /// fn.
    /// </exception>
    public static IEnumerable<T> When<T>(this IEnumerable<T> list, Func<Boolean> condition, Func<IEnumerable<T>, IEnumerable<T>> fn) {
      if(list == null) {
        throw new ArgumentNullException(nameof(list));
      }

      if(condition == null) {
        throw new ArgumentNullException(nameof(condition));
      }

      if(fn == null) {
        throw new ArgumentNullException(nameof(fn));
      }

      return condition() ? fn(list) : list;
    }
  }
}