Bogoware Monads

Yet another functional library for C#

Quickstart

Install from Nuget and enjoy it!

dotnet add package Bogoware.Monads

Introduction to Monads

Monads are powerful tools for modeling operations in a functional way, making them a cornerstone of functional programming. While we won't delve into a detailed explanation of monads and their inner workings, there are numerous resources available online that approach the topic from different perspectives.

For the purpose of this introduction, we can consider monads as am abstraction of safe container that encapsulates the result of an operation. They provide methods that enable manipulation of the result in a safe manner, ensuring that the execution flow follows the "happy" path in case of success and the "unhappy" path in case of failure. This model is also known as railway-oriented programming.

By employing monads, code can be protected from further processing in case of errors or missing data. Adopting a functional approach offers benefits such as increased readability, improved reasoning capabilities, and more robust and error-resistant code.

Bogoware Monads

This library provides two well-known monads: Result and Maybe monads (also referred to as Either, Optional, Option in other contexts):

The Result<T> monad is used to model operations that can fail.

The Maybe<T> monad is used to model operations that can optionally return a value.

Additionally, the library provides the Error abstract class, which complements the Result<T> monad and offers an ergonomic approach to error management at an application-wide scale.

Design Goals for Result<T>

The Result<T> monad is designed for modeling operations that can either fail or return a value. It is a generic type, with T representing the type of the value returned by the successful operation.

Result<T> provides a set of methods that facilitate chaining operations in a functional way:

• Map: Allows transformation of the value returned by the operation, representing the "happy" flow.
  • Map to void functor will map to Result<Unit>
  • MapToUnit() is just a shortcut for Map(_ => { })
• MapError: Allows transformation of the error returned by the operation, representing the "unhappy" flow.
• Bind: Enables chaining of operations providing a fluent syntax that allows to capture the values on the "happy" path and use them in subsequent steps.
• Match: Facilitates handling of the operation's result by providing separate paths for the "happy" and "unhappy" flows.
• RecoverWith: Provides a way to recover from an error by returning a Result<T>
• Ensure: Allows asserting a condition on the value returned by the operation.
• ExecuteIfSuccess: Executes if the operation succeeds. It is typically used to generate side effects.
• ExecuteIfFailure: Executes if the operation fails. It is typically used to generate side effects.

There are also some unsafe methods intended to support developers who are less familiar with the functional approach and may need to resort to a procedural style to achieve their goals. These methods should be used sparingly, as they deviate from the functional paradigm and make the code less robust, potentially leading to unexpected exceptions:

• ThrowIfFailure: Throws an exception if the operation fails. It is typically used to terminate the execution of the pipeline discarding the result of the operation.
• GetValueOrThrow: Extracts the value from the Result<T> monad.
• GetErrorOrThrow: Extracts the error from the Result<T> monad.

By adhering to the Result<T> monad, code can be modeled in a more readable and reasoned manner. It also contributes to writing more robust code with reduced error-proneness.

Result Helper Methods

The Result class provides a set of helper methods that facilitate the creation of Result<T> instances or make the code more readable.

• Result.Success: Creates a successful Result<T> instance with the specified value.
• Result.Failure: Creates a failed Result<T> instance with the specified error.
• Result.Ensure: Creates a successful Result<Unit> instance if the specified condition is true, otherwise creates a failed instance with the specified error.
• Result.Bind: Creates a Result<T> instance from a delegate. This method is particularly useful when you need to start a chain of operations with a Result<T> instance and you like to have a consistent syntax for all the steps of the chain.

For example, instead of writing:

/// Publishes the project
public Result<Unit> Publish() {
    if (PublishingStatus == PublishingStatus.Published)
        return new InvalidOperationError("Already published");
    
    return ValidateCostComponents() // Note the explicit invocation of the method
        .Bind(ValidateTimingComponents)
        // ... more binding to validation methods
        .ExecuteIfSuccess(() => PublishingStatus = PublishingStatus.Published);
}

You can write:

/// Publishes the project
public Result<Unit> Publish() => Result
    .Ensure(PublishingStatus != PublishingStatus.Published, () => new InvalidOperationError("Already published")
    .Bind(ValidateCostComponents)
    .Bind(ValidateTimingComponents)
    // ... more binding to validation methods
    .ExecuteIfSuccess(() => PublishingStatus = PublishingStatus.Published);

Manipulating IEnumerable<Maybe<T>>

The library provides a set of extension methods that enable manipulation of sequences of Maybe<T> instances.

• MapEach: Maps each Maybe in the sequence, preserving the None values
• BindEach: Binds each Maybe in the sequence, preserving the None values
• MatchEach: Matches each Maybe in the sequence

Manipulating IEnumerable<Result<T>>

The library provide a set of extension methods that enable manipulation of sequences of Result<T> instances.

• MapEach: Maps each Result in the sequence, preserving the failed Results
• BindEach: Binds each Result in the sequence, preserving the failed Results
• MatchEach: Matches each Result in the sequence
• AggregateResults: Transforms a sequence of Results into a single Result that contains a sequence of the successful values. If the original sequence contains any Error then will return a failed Result with an AggregateError containing all the errors found.

Design Goals for Error

The Error class is used for modeling errors and works in conjunction with the Result<T> monad.

There are two types of errors:

• LogicError: These errors are caused by application logic and should be programmatically handled. Examples include InvalidEmailError, InvalidPasswordError, InvalidUsernameError, etc.
• RuntimeError: These errors are caused by external sources and are unrelated to domain logic. Examples include DatabaseError, NetworkError, FileSystemError, etc.

Distinguishing between LogicErrors and RuntimeErrors is important, as they require different handling approaches:

• LogicErrors should be programmatically handled and can be safely reported to the user in case of a malformed request.
• RuntimeErrors should be handled by the infrastructure and should not be reported to the user.

For example, in a typical ASP.NET Core application, LogicErrors can be handled by returning a BadRequest response to the client, while RuntimeErrors can be handled by returning an InternalServerError response.

Error Hierarchy Best Practices

Each application should model its own logic errors by deriving from a root class that represents the base class for all logic errors. The root class should derive from the LogicError class.

For different kinds of logic errors that can occur, the application should derive specific classes, each modeling a particular logic error and providing the necessary properties to describe the error.

In the following example, we model two logic errors: NotFoundError and InvalidOperationError:

public abstract class ApplicationError: LogicError
{
	
	public int ErrorCode { get; }

	protected ApplicationError(string message, int errorCode)
		: base(message)
	{
		ErrorCode = errorCode;
	}
}

public class NotFoundError : ApplicationError
{
	
	public string ResourceName { get; }
	public string ResourceId { get; }
	public NotFoundError(string message, int errorCode, string resourceName, string resourceId)
		: base(message, errorCode)
	{
		ResourceName = resourceName;
		ResourceId = resourceId;
	}
}

public class InvalidOperationError : ApplicationError
{
	
	public string OperationName { get; }
	public string Reason { get; }
	public InvalidOperationError(string message, int errorCode, string operationName, string reason)
		: base(message, errorCode)
	{
		OperationName = operationName;
		Reason = reason;
	}
}

As demonstrated in the project FluentValidationSample the FluentValidation library can be used to model validation errors.

In contrast to LogicErrors, RuntimeErrors are generated by the Result.Execute() methods to encapsulate exceptions thrown by the application.

Design Goals for Maybe<T>

Before discussing what can be achieved with the Maybe<T> monad, let's clarify that it is not intended as a replacement for Nullable<T>. This is mainly due to fundamental libraries, such as Entity Framework, relying on Nullable<T> to model class attributes, while support for structural types remains limited.

A pragmatic approach involves using Nullable<T> for modeling class attributes and Maybe<T> for modeling return values and method parameters.

The advantage of using Maybe<T> over Nullable<T> is that Maybe<T> provides a set of methods that enable chaining operations in a functional manner. This becomes particularly useful when dealing with operations that can optionally return a value, such as querying a database.

The implicit conversion from Nullable<T> to Maybe<T> allows for lifting Nullable<T> values to Maybe<T> values and utilizing Maybe<T> methods for chaining operations.

Practical rule: Use Nullable<T> to model class attributes and Maybe<T> to model return values and method paramethers.

Converting Maybe<T> to Result<T>

It is common to implement a pipeline of operations where an empty Maybe<T> instance should be interpreted as a failure, in this case the Maybe<T> instance can be converted to a Result<T> instance by using the ToResult method.

The ToResult method accepts an error as a parameter and returns a Result<T> instance with the specified error in case the Maybe<T> instance is empty.