MonadicSharp

Railway-Oriented Programming for C#. Replace exception-driven control flow with composable, explicit error handling.

dotnet add package MonadicSharp

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The problem

// You never know what this can throw, or when, or why public User CreateUser(string name, string email) { if (string.IsNullOrWhiteSpace(name)) throw new ValidationException("Name is required"); if (!email.Contains('@')) throw new ValidationException("Invalid email"); var user = _db.Users.FirstOrDefault(u => u.Email == email); if (user != null) throw new ConflictException("Email already exists"); return _db.Save(new User(name, email)); }

Every caller needs a try/catch. Errors are invisible in the signature. Composing multiple operations that can fail is painful.

The solution

public Result<User> CreateUser(string name, string email) { return ValidateName(name) .Bind(_ => ValidateEmail(email)) .Bind(_ => CheckEmailNotTaken(email)) .Map(_ => new User(name, email)) .Bind(user => _db.Users.AddAsync(user)); }

The signature tells the truth. Errors propagate automatically. No exceptions, no hidden branches.

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Core types

Result<T> — success or failure

Result<int> Parse(string input) => int.TryParse(input, out var n) ? Result<int>.Success(n) : Result<int>.Failure(Error.Validation("Not a number", field: "input")); var result = Parse("42") .Map(n => n * 2) .Where(n => n < 200, "Value too large") .Match( onSuccess: n => $"Result: {n}", onFailure: err => $"Error: {err.Message}" ); // "Result: 84"

Option<T> — value or nothing

Option<User> FindUser(int id) => _db.TryGetValue(id, out var user) ? Option<User>.Some(user) : Option<User>.None; string name = FindUser(42) .Map(u => u.Name.ToUpper()) .GetValueOrDefault("Anonymous");

Error — structured, composable errors

// Semantic factory methods Error.Validation("Name is required", field: "name") Error.NotFound("User", identifier: id.ToString()) Error.Forbidden("Admin access required") Error.Conflict("Email already registered", resource: "email") Error.FromException(ex) // Enrich with context Error.Create("Payment failed") .WithMetadata("orderId", order.Id) .WithMetadata("amount", order.Total) .WithInnerError(gatewayError) // Combine multiple errors Error.Combine(nameError, emailError, ageError)

Either<TLeft, TRight> — two explicit tracks

Either<Error, User> Authenticate(string token) => _tokenService.Validate(token) ? Either<Error, User>.FromRight(_users.GetByToken(token)) : Either<Error, User>.FromLeft(Error.Forbidden("Invalid token")); var message = Authenticate(token).Match( onLeft: err => $"Denied: {err.Message}", onRight: user => $"Welcome, {user.Name}" );

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Async pipelines

Chain async operations with automatic short-circuiting on failure:

var result = await GetOrderAsync(orderId) // Task<Result<Order>> .Then(ValidateInventoryAsync) // stops here if invalid .Then(ReserveStockAsync) .Then(SendConfirmationEmailAsync) .ExecuteAsync();

Conditional steps

await ProcessOrder(order) .ThenIf(o => o.Total > 1000, ApplyDiscountAsync) .ThenIf(o => o.IsInternational, CalculateShippingAsync) .ExecuteAsync();

Built-in retry

await GetOrder(id) .ThenWithRetry( operation: CallExternalPaymentApiAsync, maxAttempts: 3, delay: TimeSpan.FromSeconds(2)) .ExecuteAsync();

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Entity Framework Core integration

DbSetExtensions wraps EF Core operations to return Result<T> and Option<T> instead of throwing:

// FindAsync returns Option<T> — no null checks needed Option<User> user = await _db.Users.FindAsync(id); // AddAsync returns Result<T> Result<User> created = await _db.Users.AddAsync(newUser); // Composable pipeline with EF var result = await _db.Users.FindAsync(id) .ToResult(Error.NotFound("User", id.ToString())) .BindAsync(user => _db.Users.Update(updatedUser)) .BindAsync(_ => _db.SaveChangesAsync());

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Collecting multiple errors

Sequence and Traverse let you run all validations and collect every failure — not just the first one:

var errors = new[] { ValidateName(name), ValidateEmail(email), ValidateAge(age) }.Sequence(); errors.Match( onSuccess: values => Save(values), onFailure: err => ReturnAllErrors(err.GetAllErrors()) );

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Project templates

Scaffold a full project pre-wired with MonadicSharp:

dotnet new install MonadicSharp.Templates dotnet new monadic-api -n MyApi # Minimal API + Result pattern + EF Core dotnet new monadic-clean -n MyApp # Clean Architecture + CQRS + MediatR

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Requirements

• .NET 8.0+
• C# 10.0+

Contributing

Issues and pull requests are welcome. See CHANGELOG for version history.

License

MIT — see LICENSE.