fix: Using covariant interface for IParser and IParseResult.

Rename CanonSharp.Common to CanonSharp.Pascal.

CanonSharp.Combinator/Abstractions/IFailedResult.cs

@@ -0,0 +1,47 @@
+namespace CanonSharp.Combinator.Abstractions;
+
+/// <summary>
+/// 失败解析结果基类
+/// </summary>
+/// <typeparam name="TToken">输入流类型</typeparam>
+/// <typeparam name="T">解析结果类型</typeparam>
+public interface IFailedResult<TToken, out T> : IParseResult<TToken, T>
+{
+    T IParseResult<TToken, T>.Value => throw Exception;
+
+    /// <summary>
+    /// 当前读取到的状态
+    /// </summary>
+    public IReadState<TToken> State { get; }
+
+    /// <summary>
+    /// 解析失败的消息
+    /// </summary>
+    public string Message { get; }
+
+    /// <summary>
+    /// 解析失败的异常
+    /// </summary>
+    public ParseException Exception => new(ToString());
+
+    /// <summary>
+    /// 转换该失败结果的类型
+    /// </summary>
+    /// <typeparam name="TNext">转换之后的结果类型</typeparam>
+    /// <returns>转换之后的失败解析类型</returns>
+    public IFailedResult<TToken, TNext> Convert<TNext>();
+
+    IParseResult<TToken, TResult> IParseResult<TToken, T>.Next<TNext, TResult>(
+        Func<T, IParser<TToken, TNext>> nextParser,
+        Func<IParseResult<TToken, TNext>, IParseResult<TToken, TResult>> continuation)
+        => continuation(Convert<TNext>());
+
+    IParseResult<TToken, TResult> IParseResult<TToken, T>.Map<TResult>(Func<T, TResult> map)
+        => Convert<TResult>();
+
+    TResult IParseResult<TToken, T>.CaseOf<TResult>(Func<ISuccessfulResult<TToken, T>, TResult> successfulHandler,
+        Func<IFailedResult<TToken, T>, TResult> failedHandler)
+        => failedHandler(this);
+
+    string IParseResult<TToken, T>.ToString() => $"Parse Failed: {Message}";
+}