Just.Railway/Railway/ResultExtensions.cs

using System.Collections.Immutable;

namespace Just.Railway;

public static partial class ResultExtensions
{
    #region Finally

    public static Result Finally(this in Result result, Action action)
    {
        switch (result.State)
        {
            case ResultState.Success: 
            case ResultState.Error:
                action.Invoke();
                break;

            default: throw new ResultNotInitializedException(nameof(result));
        }

        return result;
    }

    public static async Task<Result> Finally(this Task<Result> resultTask, Action action)
    {
        var result = await resultTask;
        switch (result.State)
        {
            case ResultState.Success: 
            case ResultState.Error:
                action.Invoke();
                break;

            default: throw new ResultNotInitializedException(nameof(resultTask));
        }

        return result;
    }
    public static async Task<Result> Finally(this Result result, Func<Task> action)
    {
        switch (result.State)
        {
            case ResultState.Success: 
            case ResultState.Error:
                await action.Invoke();
                break;

            default: throw new ResultNotInitializedException(nameof(result));
        }

        return result;
    }
    public static async Task<Result> Finally(this Task<Result> resultTask, Func<Task> action)
    {
        var result = await resultTask;
        switch (result.State)
        {
            case ResultState.Success: 
            case ResultState.Error:
                await action.Invoke();
                break;

            default: throw new ResultNotInitializedException(nameof(resultTask));
        }

        return result;
    }

    #endregion

    #region ValueOr

    public static T ValueOr<T>(this in Result<T> result, T fallback)
    {
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => fallback,
            _ => throw new ResultNotInitializedException(nameof(result))
        };
    }
    public static T ValueOr<T>(this in Result<T> result, Func<Error, T> fallback)
    {
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => fallback(result.Error!),
            _ => throw new ResultNotInitializedException(nameof(result))
        };
    }

    public static async Task<T> ValueOr<T>(this Task<Result<T>> resultTask, T fallback)
    {
        var result = await resultTask.ConfigureAwait(false);
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => fallback,
            _ => throw new ResultNotInitializedException(nameof(resultTask))
        };
    }
    public static async Task<T> ValueOr<T>(this Result<T> result, Func<Error, Task<T>> fallback)
    {
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => await fallback(result.Error!).ConfigureAwait(false),
            _ => throw new ResultNotInitializedException(nameof(result))
        };
    }
    public static async Task<T> ValueOr<T>(this Task<Result<T>> resultTask, Func<Error, T> fallback)
    {
        var result = await resultTask.ConfigureAwait(false);
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => fallback(result.Error!),
            _ => throw new ResultNotInitializedException(nameof(resultTask))
        };
    }
    public static async Task<T> ValueOr<T>(this Task<Result<T>> resultTask, Func<Error, Task<T>> fallback)
    {
        var result = await resultTask.ConfigureAwait(false);
        return result.State switch
        {
            ResultState.Success => result.Value,
            ResultState.Error => await fallback(result.Error!).ConfigureAwait(false),
            _ => throw new ResultNotInitializedException(nameof(resultTask))
        };
    }

    #endregion

    #region Merge

    public static Result Merge(this IEnumerable<Result> results)
    {
        ImmutableArray<Error>.Builder? errors = null;

        foreach (var result in results)
        {
            switch (result.State)
            {
                case ResultState.Success:
                    break;

                case ResultState.Error:
                    errors ??= ImmutableArray.CreateBuilder<Error>();
                    ManyErrors.AppendSanitized(errors, result.Error!);
                    break;

                default: throw new ResultNotInitializedException(nameof(results));
            }
        }

        return errors is null
            ? new(null)
            : new(new ManyErrors(errors.ToImmutable()));
    }
    public static async Task<Result> Merge(this IEnumerable<Task<Result>> tasks)
    {
        var taskList = tasks.ToArray();
        var results = new Result[taskList.Length];

        // Wait for all tasks to complete (suppress AggregateException — we'll check each individually)
        try { await Task.WhenAll(taskList).ConfigureAwait(false); } catch { /* handled per-task below */ }

        for (int i = 0; i < taskList.Length; i++)
        {
            var task = taskList[i];
            results[i] = task.Status switch
            {
                TaskStatus.RanToCompletion => task.Result,
                TaskStatus.Faulted => Result.Failure(task.Exception!),
                TaskStatus.Canceled => Result.Failure(Error.New("task_canceled", "Task was canceled")),
                _ => Result.Failure(Error.New("task_unexpected", $"Task in unexpected state: {task.Status}"))
            };
        }

        return results.Merge();
    }

    public static Result<IEnumerable<T>> Merge<T>(this IEnumerable<Result<T>> results)
    {
        ImmutableArray<Error>.Builder? errors = null;
        ImmutableArray<T>.Builder? values = null;

        foreach (var result in results)
        {
            switch (result.State)
            {
                case ResultState.Success:
                    values ??= ImmutableArray.CreateBuilder<T>();
                    values.Add(result.Value);
                    break;

                case ResultState.Error:
                    errors ??= ImmutableArray.CreateBuilder<Error>();
                    ManyErrors.AppendSanitized(errors, result.Error!);
                    break;

                default: throw new ResultNotInitializedException(nameof(results));
            }
        }

        return errors is null
            ? new(values?.ToImmutable() ?? Enumerable.Empty<T>())
            : new(new ManyErrors(errors.ToImmutable()));
    }
    public static async Task<Result<IEnumerable<T>>> Merge<T>(this IEnumerable<Task<Result<T>>> tasks)
    {
        var taskList = tasks.ToArray();
        var results = new Result<T>[taskList.Length];

        // Wait for all tasks to complete (suppress AggregateException — we'll check each individually)
        try { await Task.WhenAll(taskList).ConfigureAwait(false); } catch { /* handled per-task below */ }

        for (int i = 0; i < taskList.Length; i++)
        {
            var task = taskList[i];
            results[i] = task.Status switch
            {
                TaskStatus.RanToCompletion => task.Result,
                TaskStatus.Faulted => Result.Failure<T>(task.Exception!),
                TaskStatus.Canceled => Result.Failure<T>(Error.New("task_canceled", "Task was canceled")),
                _ => Result.Failure<T>(Error.New("task_unexpected", $"Task in unexpected state: {task.Status}"))
            };
        }

        return results.Merge();
    }

    #endregion
}