fixes and adjustments

Railway/Error.cs

@@ -9,6 +9,12 @@ public abstract class Error : IEquatable<Error>, IComparable<Error>
 {
     protected internal Error(){}
 
+    /// <summary>
+    /// A reusable singleton representing the absence of an error (null-object pattern).
+    /// </summary>
+    [Pure]
+    public static Error Empty => EmptyError.Instance;
+
     /// <summary>
     /// Create an <see cref="ExceptionalError"/>
     /// </summary>
@@ -99,11 +105,33 @@ public abstract class Error : IEquatable<Error>, IComparable<Error>
     [Pure] public abstract bool IsExpected { get; }
     [Pure] public abstract bool IsExceptional { get; }
 
-    /// <summary>
-    /// A reusable singleton representing the absence of an error (null-object pattern).
-    /// </summary>
-    [Pure]
-    public static Error Empty => EmptyError.Instance;
+    [Pure] public Error WithExtensionData(IEnumerable<KeyValuePair<string, string>> extensionData)
+    {
+        return this switch
+        {
+            { IsEmpty: true } => Empty,
+            ExpectedError expected => new ExpectedError(expected.Type, expected.Message)
+            {
+                ExtensionData = extensionData
+                    .UnionBy(expected.ExtensionData, x => x.Key)
+                    .ToImmutableDictionary(),
+            },
+            ExceptionalError exceptional => new ExceptionalError(exceptional.Type, exceptional.Message, exceptional.Exception)
+            {
+                ExtensionData = extensionData
+                    .UnionBy(exceptional.ExtensionData, x => x.Key)
+                    .ToImmutableDictionary(),
+            },
+            ManyErrors manyErrors => new ManyErrors(manyErrors.AccessUnsafe())
+            {
+                ExtensionData = extensionData
+                    .UnionBy(manyErrors.ExtensionData, x => x.Key)
+                    .ToImmutableDictionary(),
+            },
+
+            _ => throw new NotImplementedException(), // should not reach this path
+        };
+    }
 
     [Pure] public Error Append(Error? next)
     {
@@ -179,7 +207,7 @@ public sealed class EmptyError : Error
     [Pure] public override bool IsExceptional => false;
 
     [Pure]
-    public override IEnumerable<Error> ToEnumerable() { yield break; }
+    public override IEnumerable<Error> ToEnumerable() => Enumerable.Empty<Error>();
 
     [Pure]
     public override Exception ToException() =>
@@ -250,6 +278,11 @@ public sealed class ExceptionalError : Error
         Exception = exception;
         ExtensionData = ExtractExtensionData(exception);
     }
+    internal ExceptionalError(string type, string message, Exception? exception)
+        : this(type, message)
+    {
+        Exception = exception;
+    }
 
     public ExceptionalError(string type, string message)
     {
@@ -457,6 +490,7 @@ public sealed class ManyErrors : Error, IEnumerable<Error>, IReadOnlyList<Error>
             errors.Add(error);
     }
 
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)] internal ImmutableArray<Error> AccessUnsafe() => _errors;
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)] internal override Error AccessUnsafe(int position) => _errors[position];
 }
 

Railway/ReflectionHelper.cs

@@ -4,9 +4,22 @@ namespace Just.Railway;
 
 internal static class ReflectionHelper
 {
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static R ObjectCast<R>(object value) where R: class => (R)value;
+
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool IsEqual<T>(T? left, T? right) => TypeReflectionCache<T>.IsEqualFunc(left, right);
 
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    /// <summary>
+    /// Test for potential nullability
+    /// </summary>
+    /// <returns>true if T is reference type or Nullable value type.</returns>
+    public static bool IsNullable<T>() => TypeReflectionCache<T>.IsNullable;
+
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static bool IsNullableStruct<T>() => TypeReflectionCache<T>.IsNullableStruct;
+
     private static class TypeReflectionCache<T>
     {
         public static readonly Func<T?, T?, bool> IsEqualFunc;
@@ -30,8 +43,14 @@ internal static class ReflectionHelper
             {
                 IsEqualFunc = static (left, right) => left is null ? right is null : left.Equals(right);
             }
+
+            IsNullableStruct = isNullableStruct;
+            IsNullable = isNullableStruct || !type.IsValueType;
         }
 
+        public static bool IsNullable { get; }
+        public static bool IsNullableStruct { get; }
+
     #pragma warning disable CS8604 // Possible null reference argument.
         [Pure] public static bool IsEqual<R>(R? left, R? right) where R : notnull, IEquatable<R>, T => left is null ? right is null : left.Equals(right);
         [Pure] public static bool IsEqualNullable<R>(R? left, R? right) where R : struct, IEquatable<R> => left is null ? right is null : right is not null && left.Value.Equals(right.Value);

Railway/Result.cs

@@ -45,6 +45,9 @@ public readonly partial struct Result : IEquatable<Result>
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static Result Failure(string error) => Error.New(error ?? throw new ArgumentNullException(nameof(error)));
 
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Result Failure(string type, string error) => Error.New(type, error ?? throw new ArgumentNullException(nameof(error)));
+
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static Result Failure(Error error) => new(error ?? throw new ArgumentNullException(nameof(error)));
 
@@ -54,6 +57,9 @@ public readonly partial struct Result : IEquatable<Result>
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static Result<T> Failure<T>(string error) => Error.New(error ?? throw new ArgumentNullException(nameof(error)));
 
+    [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Result<T> Failure<T>(string type, string error) => Error.New(type, error ?? throw new ArgumentNullException(nameof(error)));
+
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static Result<T> Failure<T>(Error error) => new(error ?? throw new ArgumentNullException(nameof(error)));
 
@@ -76,7 +82,10 @@ public readonly partial struct Result : IEquatable<Result>
     public static explicit operator Result(SuccessUnit _) => new(null);
 
     [Pure] public bool IsSuccess => State == ResultState.Success;
-    [Pure] public bool IsFailure => State == ResultState.Error;
+
+    [Pure]
+    [MemberNotNullWhen(true, nameof(Error))]
+    public bool IsFailure => State == ResultState.Error;
 
     [Pure] public bool TryGetValue([MaybeNullWhen(false)]out SuccessUnit? u, [MaybeNullWhen(true), NotNullWhen(false)]out Error? error)
     {
@@ -97,17 +106,18 @@ public readonly partial struct Result : IEquatable<Result>
     }
     [Pure] public bool TryGetError([MaybeNullWhen(false)]out Error error)
     {
-        if (IsSuccess)
+        switch (State)
         {
-            error = default;
-            return false;
+            case ResultState.Success:
+                error = default;
+                return false;
+
+            case ResultState.Error:
+                error = Error!;
+                return true;
+
+            default: throw new ResultNotInitializedException();
         }
-        if (IsFailure)
-        {
-            error = Error!;
-            return true;
-        }
-        throw new ResultNotInitializedException();
     }
 
     [Pure] public override string ToString() => State switch
@@ -127,9 +137,12 @@ public readonly partial struct Result : IEquatable<Result>
     [Pure] public override bool Equals(object? obj) => obj is Result other && Equals(other);
     [Pure] public bool Equals(Result other)
     {
-        if (State == ResultState.Bottom || other.State == ResultState.Bottom)
+        if (State == ResultState.Bottom)
             throw new ResultNotInitializedException();
 
+        if (other.State == ResultState.Bottom)
+            throw new ResultNotInitializedException(nameof(other));
+
         return Error == other.Error;
     }
     [Pure] public static bool operator ==(Result left, Result right) => left.Equals(right);

Railway/ResultExtensions.cs

@@ -4,6 +4,83 @@ namespace Just.Railway;
 
 public static partial class ResultExtensions
 {
+    #region MapError
+
+    public static Result MapError(this in Result result, Func<Error, Error> func) => result.State switch
+    {
+        ResultState.Error => func(result.Error!),
+        ResultState.Success => result,
+        _ => throw new ResultNotInitializedException(nameof(result)),
+    };
+    public static Result<T> MapError<T>(this in Result<T> result, Func<Error, Error> func) => result.State switch
+    {
+        ResultState.Error => func(result.Error!),
+        ResultState.Success => result,
+        _ => throw new ResultNotInitializedException(nameof(result)),
+    };
+
+    public static async Task<Result> MapError(this Task<Result> resultTask, Func<Error, Error> func)
+    {
+        var result = await resultTask.ConfigureAwait(false);
+        return result.State switch
+        {
+            ResultState.Error => func(result.Error!),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+    public static async Task<Result> MapError(this Result result, Func<Error, Task<Error>> func)
+    {
+        return result.State switch
+        {
+            ResultState.Error => await func(result.Error!).ConfigureAwait(false),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+    public static async Task<Result> MapError(this Task<Result> resultTask, Func<Error, Task<Error>> func)
+    {
+        var result = await resultTask.ConfigureAwait(false);
+        return result.State switch
+        {
+            ResultState.Error => await func(result.Error!),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+
+    public static async Task<Result<T>> MapError<T>(this Task<Result<T>> resultTask, Func<Error, Error> func)
+    {
+        var result = await resultTask.ConfigureAwait(false);
+        return result.State switch
+        {
+            ResultState.Error => func(result.Error!),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+    public static async Task<Result<T>> MapError<T>(this Result<T> result, Func<Error, Task<Error>> func)
+    {
+        return result.State switch
+        {
+            ResultState.Error => await func(result.Error!).ConfigureAwait(false),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+    public static async Task<Result<T>> MapError<T>(this Task<Result<T>> resultTask, Func<Error, Task<Error>> func)
+    {
+        var result = await resultTask.ConfigureAwait(false);
+        return result.State switch
+        {
+            ResultState.Error => await func(result.Error!),
+            ResultState.Success => result,
+            _ => throw new ResultNotInitializedException(nameof(result)),
+        };
+    }
+
+    #endregion
+
     #region Finally
 
     public static Result Finally(this in Result result, Action action)
@@ -142,20 +219,23 @@ public static partial class ResultExtensions
             switch (result.State)
             {
                 case ResultState.Success:
-                    break;
+                    continue;
 
                 case ResultState.Error:
                     errors ??= ImmutableArray.CreateBuilder<Error>();
                     ManyErrors.AppendSanitized(errors, result.Error!);
-                    break;
+                    continue;
 
                 default: throw new ResultNotInitializedException(nameof(results));
             }
         }
-
-        return errors is null
-            ? new(null)
-            : new(new ManyErrors(errors.ToImmutable()));
+    
+        return errors switch
+        {
+            null => new Result(null),
+            { Count: 1 } => errors[0],
+            _ => new(new ManyErrors(errors.ToImmutable()))
+        };
     }
     public static async Task<Result> Merge(this IEnumerable<Task<Result>> tasks)
     {
@@ -192,20 +272,23 @@ public static partial class ResultExtensions
                 case ResultState.Success:
                     values ??= ImmutableArray.CreateBuilder<T>();
                     values.Add(result.Value);
-                    break;
+                    continue;
 
                 case ResultState.Error:
                     errors ??= ImmutableArray.CreateBuilder<Error>();
                     ManyErrors.AppendSanitized(errors, result.Error!);
-                    break;
+                    continue;
 
                 default: throw new ResultNotInitializedException(nameof(results));
             }
         }
 
-        return errors is null
-            ? new(values?.ToImmutable() ?? Enumerable.Empty<T>())
-            : new(new ManyErrors(errors.ToImmutable()));
+        return errors switch
+        {
+            null => new Result<IEnumerable<T>>(values?.ToImmutable() ?? Enumerable.Empty<T>()),
+            { Count: 1 } => errors[0],
+            _ => new(new ManyErrors(errors.ToImmutable()))
+        };
     }
     public static async Task<Result<IEnumerable<T>>> Merge<T>(this IEnumerable<Task<Result<T>>> tasks)
     {