ResultBoxUnion (Preview)

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Preview Notice: This library is built on C# 15 union types, which are currently in preview (.NET 11 Preview 3). The union syntax and behavior may change before the official release. Do not use in production until unions are stable.

Railway-oriented programming library for C# using C# 15 Union Types (.NET 11).

A reimagination of ResultBoxes built entirely on the union keyword.

Core Concept

// ResultBox<TValue> is a union: either a TValue or an Exception
public union ResultBox<TValue>(TValue, Exception) where TValue : notnull;

No more IsSuccess flag checks with internal nullable fields — the type itself is the value or the error.

Quick Start

using ResultBoxUnion;

// Create results
ResultBox<int> success = 42;                              // implicit from value
ResultBox<int> failure = new Exception("something wrong"); // implicit from exception

// Pattern matching (exhaustive!)
var message = success switch
{
    int value    => $"Got {value}",
    Exception ex => $"Error: {ex.Message}",
};

// Railway chaining
var result = ResultBox.FromValue(10)
    .Remap(x => x * 2)                    // 20
    .Conveyor(x => ResultBox.Ok(x + 5))   // 25
    .Verify(x => x > 0
        ? ExceptionOrNone.None
        : new ArgumentException("must be positive"));

Railway Pipeline Example

// Combine multiple values
var result = ResultBox.FromValue("hello")
    .Combine(greeting => ResultBox.Ok(greeting.Length))  // TwoValues<string, int>
    .Remap((greeting, len) => $"{greeting} has {len} chars");

// Async pipeline
var asyncResult = await ResultBox.Start
    .Conveyor(_ => FetchUserAsync(userId))
    .Combine(user => LoadOrdersAsync(user.Id))
    .Remap((user, orders) => new UserSummary(user.Name, orders.Count));

// Error recovery with Rescue
var rescued = ResultBox<int>.Error(new TimeoutException())
    .Rescue(ex => ex is TimeoutException
        ? ValueOrException<int>.FromValue(0)   // recover with default
        : ValueOrException<int>.Exception);     // keep the error

// Side effects with Scan/Do
var logged = ResultBox.FromValue(42)
    .Scan(v => Console.WriteLine($"Value: {v}"))
    .Do(v => Console.WriteLine($"Processing: {v}"));

Union Types Used

Type	Declaration	Purpose
ResultBox<T>	union(T, Exception)	Success or failure
ExceptionOrNone	union(Exception, UnitValue)	Optional exception
OptionalValue<T>	union(T, NoneValue)	Optional value
ValueOrException<T>	union(T, ExceptionMarker)	Recovery helper

Requirements

• .NET 11 Preview 3+
• C# 15 (LangVersion preview)

License

MIT