更新IEnumerable的Tail扩展方法

Sanchime.Functional/Core/Extensions/IEnumerable.cs

@@ -11,21 +11,77 @@ public static class IEnumerableExtension
 
     public static Func<T, IEnumerable<T>> Return<T>() => t => List(t);
 
-    public static Option<T> Head<T>(this IEnumerable<T> list)
+    /// <summary>
+    /// 将序列内第一条元素转换为<see cref="Option"/>
+    /// </summary>
+    /// <param name="source"></param>
+    /// <typeparam name="T"></typeparam>
+    /// <returns></returns>
+    public static Option<T> Head<T>(this IEnumerable<T> source)
     {
-        if (list is null)
+        if (source is null)
         {
             return Option.None;
         }
-        var enumerator = list.GetEnumerator();
+        var first = source.FirstOrDefault();
-        return enumerator.MoveNext() ? Option.Some(enumerator.Current) : Option.None;
+        return first is not null ? Option.Some(first) : Option.None;
     }
 
-    public static R Match<T, R>(this IEnumerable<T> list, Func<R> Empty, Func<T, IEnumerable<T>, R> Otherwise)
+    /// <summary>
-        => list.Head().Match(Empty, (head) => Otherwise(head, list.Skip(1)));
+    /// 给定谓词，查询序列内第一条数据并将其转换为<see cref="Option"/>
+    /// </summary>
+    /// <param name="source">数据源</param>
+    /// <param name="predicate">谓词</param>
+    /// <typeparam name="T"></typeparam>
+    /// <returns></returns>
+    public static Option<T> Head<T>(this IEnumerable<T> source, Func<T, bool> predicate)
+    {
+        if (source is null)
+        {
+            return Option.None;
+        }
+        var first = source.Where(predicate).FirstOrDefault();
+        return first is not null ? Option.Some(first) : Option.None;
+    }
 
-    public static IEnumerable<R> Map<T, R>(this IEnumerable<T> list, Func<T, R> func)
+    /// <summary>
-        => list.Select(func);
+    /// 将序列内最后一个元素转换为<see cref="Option"/>
+    /// </summary>
+    /// <param name="source"></param>
+    /// <typeparam name="T"></typeparam>
+    /// <returns></returns>
+    public static Option<T> Tail<T>(this IEnumerable<T> source)
+    {
+        if (source is null)
+        {
+            return Option.None; 
+        }
+        var tail = source.LastOrDefault();
+        return tail is not null ? Option.Some(tail) : Option.None;
+    }
+
+    /// <summary>
+    /// 给定谓词，查询序列内最后一条数据并将其转换为<see cref="Option"/>
+    /// </summary>
+    /// <param name="source">数据源</param>
+    /// <param name="predicate">谓词</param>
+    /// <typeparam name="T"></typeparam>
+    /// <returns></returns>
+    public static Option<T> Tail<T>(this IEnumerable<T> source, Func<T, bool> predicate)
+    {
+        if (source is null)
+        {
+            return Option.None;
+        }
+        var last = source.Where(predicate).LastOrDefault();
+        return last is not null ? Option.Some(last) : Option.None;
+    }
+
+    public static R Match<T, R>(this IEnumerable<T> source, Func<R> Empty, Func<T, IEnumerable<T>, R> Otherwise)
+        => source.Head().Match(Empty, (head) => Otherwise(head, source.Skip(1)));
+
+    public static IEnumerable<R> Map<T, R>(this IEnumerable<T> source, Func<T, R> func)
+        => source.Select(func);
 
     /// <summary>
     /// 给集合内每个元素执行一个无返回值函数
@@ -37,11 +93,11 @@ public static class IEnumerableExtension
     public static IEnumerable<Unit> ForEach<T>(this IEnumerable<T> source, Action<T> action)
         => source.Map(action.ToFunc()).ToImmutableList();
 
-    public static IEnumerable<R> Bind<T, R>(this IEnumerable<T> list, Func<T, IEnumerable<R>> func)
+    public static IEnumerable<R> Bind<T, R>(this IEnumerable<T> source, Func<T, IEnumerable<R>> func)
-        => list.SelectMany(func);
+        => source.SelectMany(func);
 
-    public static IEnumerable<R> Bind<T, R>(this IEnumerable<T> list, Func<T, Option<R>> func)
+    public static IEnumerable<R> Bind<T, R>(this IEnumerable<T> source, Func<T, Option<R>> func)
-        => list.Bind(t => func(t).AsEnumerable());
+        => source.Bind(t => func(t).AsEnumerable());
 
 
     private static IEnumerable<T> List<T>(T t)

Sanchime.Functional/Core/Extensions/Option.cs

@@ -107,6 +107,10 @@ public static class OptionExtension
     public static Either<L, R> ToEither<L, R>(this Option<L> option, R error)
         => option.IsSome() ? Either.Right(error) : Either.Left(option.Value);
 
+    public static Either<L, string> ToEither<L, R>(this Option<L> option, string error)
+        => option.IsSome() ? Either.Right(error) : Either.Left(option.Value);
+
+
     #region Linq式
 
     public static Option<R> Select<T, R>(this Option<T> source, Func<T, R> mapping)

Sanchime.Functional/Core/Products/Either.cs

@@ -49,7 +49,6 @@ public readonly struct Either<TLeft, TRight>
         => Match(l => $"Left({l})", r => $"Right({r})");
 }
 
-
 public readonly struct Left<TLeft>
 {
     internal TLeft Value { get; }