Funzo.SourceGenerators 3.1.0

Funzo

Contents

• Where is this coming from
• Description
• Usage (recommended way)
• Documentation
  • Unit
  • Option
  • Result
  • ResultBuilder
  • Union
• Serialization
• Example
• Further work
• Design Philosophy
• What's missing (updated after adding union types & source generators)

Where is this coming from

This package was previously called OptionTypes, but given that now more things are being added, I think a new name could fit better. Wanted to make it sound like something functional, so Funzo it is. Sounds fine. Apart from the name, the Maybe<T> class was renamed to Option<T>. It is more common than Maybe<T> so I thought it would be better for people to identify it. I copied a lot of things took inspiration of the work done by OneOf (specially source generators) but decided to adapt it to my way of doing things: making everything explicit.

Description

Funzo allows the developer to use some classes more commonly used in functional programming for error-proof programming and better type definition. It contains 4 classes:

• The Unit class represents an empty class. Because functions always return something, Unit is the equivalent to void

• The Option<T> class allows to create an item of type T that may have no value. This value cannot be accessed in an unsafe manner by design, making really easy to completely remove null references from your code and reducing the number of NullReferenceException exceptions thrown.

• The Result<TOk, TErr> class represents an operation that has been completed. This reduces the number of try/catch blocks needed to manage application flow, making error management explicit and ending with more reliable code.

• The Union<T1, T2> and all its siblings allows you to create union types. This is pretty similar to the OneOf library (in fact, heavily inspired in it), but here you will NOT be able to get values without checking first.

• The attributes ResultAttribute and UnionAttribute allow you to generate types and use them with ease.

Usage (recommended way)

The best way to approach this package is through source generators. This will allow you to define your types with ease and work on your problems straight away. You will need the Funzo and Funzo.SourceGenerators packages. For unions, decorate it with the Union attribute, supplying the generic parameters you want as possible options. For results, decorate it with the attribute Result, using generics for the Error and Ok variant, if needed. In both cases, the class should be partial so new code can be generated.

[Union<Error1, Error2, Error3>]
public partial class ApiError;

[Result<ApiError>]
public partial class ApiCallResult;

[Union<ItemWithNameAlreadyExists, InvalidItemName, EmptyItemName>]
public partial class ItemCreationError;

[Result<ItemId, ItemCreationError>]
public partial class ItemCreatedResult;

Documentation

Unit

Unit is a helper type to represent the absence of a return value (think of it as void). Because in functional programming every function returns a value, it is added here for compatibility.

Option

Create a new Option using one of the helper methods (Option.FromValue, Option.Some, and Option.None):

var optionInt = Option.Some(1);
var optionFloat = Option.FromValue(12);
string? nullableString = null;
var optionString = Option.FromValue(nullableString);

Map its content using the Map method:

var optionText = await ReadTextFromFile(filePath);

var uppercaseText = optionText.Map(text => text.ToUpper());

You can also map to another Option and it will be flatten:

var number = Option.FromValue(1);

// double type is Option<int>
var double = number.Map(x => Option.FromValue(x * 2));

If you want to check both options, use the Match method:

let user = await GetUser();


var userName = user.Match(x => x.Name, () => "User not found");

In case you want to provide a fallback value, you can use ValueOr:

var optionUserName = await GetUserName();
var userName = optionUserName.ValueOr("Unknown user");

In case you want to do something if there is a value present, you can use the IsSome method:

Option<User> optionUser = await GetUser();

if(!optionUser.IsSome(out User user))
{
    return Results.NotFound();
}

var posts = postService.GetPostsByUserId(user.Id);

You can force the value out using the Unwrap method. This approach is not recommended:

var optionValue = Option.Some(1);

var value = optionValue.Unwrap() // 1

var optionString = Option<string>.None;
optionString.Unwrap(); // throws NullReferenceException

There are also extension methods for Task<Option<T>> so you can chain Map, Match, ValueOr, and Unwrap to your tasks.

var userBalance = GetUser() // GetUser returns a Task<Option<User>>
                    .Map(user => bankService.GetAccounts(user.Id))
                    .Map(accounts => accounts.Sum(a => a.Balance))
                    .ValueOr(0m);

In case you want to do something with the value first, you can use the Inspect function:

Option<User> maybeUser = GetUser();

user.Inspect(user => Console.WriteLine($"Retrieved user {user.Name}");

Result

To use results, you can use the already existing classes Result<TOk, TErr> or Result<TErr>, depending on whether you need an Ok value, or use the source generators (much recommended, see above).

You can create an instance using the Ok/Err static methods:

var okResult = Result<ProcessError>.Ok();
var errorResult = Result<Unit, ProcessError>.Err(ProcessError.DatabaseConnection);

You can map the ok value or err value using Map and MapErr methods:

var okResult = Result<int, Exception>.Ok(3).Map(x => x*2)); // Ok(6)
var errResult = Result<string>.Err("failure").MapErr(x => x.ToUpper()); // Err("FAILURE")

To provide handlers for both cases, which should be the normal usage, use the Match method:

var result = await CreateUser();

result.Match(
    user => Results.Created("/user", { id: user.Id }),
    err => err switch {
        CreateUserError.EmailExists => Results.Conflict(),
        _ => Results.BadRequest()
    });

Sometimes you only want to know if an operation has completed successfully to get the ok value. You can use the AsOk method:

var parsingResult = ParseLines(path);

var linesParsed = parsingResult.AsOk().ValueOr(0);

Console.WriteLine($"Parsed {linesParsed} lines");

In order to get early returns when needed, there is an IsErr method:

Result<User, string> userCreationResult = await CreateUser(userPayload);

if (userCreationResult.IsErr(out User user, out string error))
{
    return Result<Unit, UserCreationError>.Err(UserCreationError.CannotCreateUser);
}

var userRoleResult = await AssignRoles(user, Roles.Admin);

if (userRoleResult.IsErr(out var roleError)) 
{
    return Result<Unit, UserCreationError>.Err(UserCreationError.CannotAssignRole);
}

emailService.NotifyUser(user.Email);

As with Option, there are some extensions in Task to be able to chain methods:

public async IResult Post([FromBody] UserPayload payload)
    => await CreateUser(payload).Match<IResult>(
        user => Results.Created("/user", { id: user.Id }),
        err => err switch {
            CreateUserError.EmailExists => Results.Conflict(),
            _ => Results.BadRequest()
        });

Same as Option, we have 2 methods: Inspect and InspectErr (and their async variants) in case you want to do something with the values without actually changing anything.

ResultBuilder

Created by using the static method For, lets you map different exceptions to errors, allowing you to migrate from an exception based approach to a result based approach.

So you can move from this:

public async Task Main()
{
    try
    {
        await ReserveTable(userId, reservationDate);
    }
    catch(Exception e)
    {
        // Manage
    }
}

public Task<int> ReserveTable(int userId, DateTime reservationDate)
{
    if(reservationDate.TimeOfDay > TimeSpan.FromHours(22))
    {
        throw new RestaurantClosedException("Restaurant is closed at 22:00h");
    }

    var freeTables = await GetFreeTables(reservationDate);

    if(freeTables is null || freeTables.Count == 0)
    {
        throw new RoomFullException("There are no free tables");
    }

    var reservation = new Reservation(userId, reservationDate);
    context.Reservations.Add(reservation);

    await context.SaveChangesAsync();

    return reservation.Id;
}

To this:

public async Task Main()
{
    ReservationResult.For()
        .MapException<RestaurantClosedException>(e => new RestaurantClosedError())
        .MapException<RoomFullException>(e => new RoomFullError())
        .MapElse(e => new UnknownError())
        .TryAsync(async () => await ReserveTable(userId, reservationDate));
}

[Result<int, ReservationError>]
public partial class ReservationResult;

[Union<RestaurantClosed, RoomFullError, UnknownError>]
public partial class ReservationError;

In this way, you can refactor safely and remove exceptions one by one, making all your methods explicit about how they can fail so the developer can handle it.

Union

Unions represent a variable that can be of several different types. At the moment, there are only unions for 5 generic types max. This was on purpose, as it usually more than 5 means you need a refactor to group some of them (at least in my opinion). If you need more, feel free to use the Funzo.Generator project and change the ordinality to whatever you need.

You can create an instance by using the constructor or by implicitly converting from it:

var union = new Union<int, string, DateTime>("text");

Union<int, string, DateTime> newUnion = 44;

To check if the union is from an specific type, you can use the Is method:

Union<int, string, DateTime> union = "text";

if (union.Is<string>(out var text))
{
    Console.WriteLine("Union is a string");
}

If you want to do different actions depending on the inner value, you can use the Switch or the Match methods:

Union<int, string, DateTime> union = DateTime.UtcNow;

var typeOfDate = union.Match(
    i => "Unix timestamp",
    s => "ISO string",
    d => "DateTime");

union.Switch(
    i => Console.WriteLine("Unix timestamp"),
    s => Console.WriteLine("ISO string"),
    d => Console.WriteLine("DateTime"))

Unions in this package don't have a .Value property and they will never have.

Serialization

The package Funzo.Serialization holds the JsonConverterFactory classes needed to work with System.Text.Json. At the moment, Option serializes to:

{ "HasValue": bool, "Value": *whatever* }

and Result to:

{ "IsOk": true, "Ok": *whatever* }
or
{ "IsOk": false, "Err": *whatever* }

Union serializes as a property, with the name of the property taking the value of the type used:

[Union<int, string, DateTime>]
public partial class MyUnion ;

public class MyClass
{
    public MyUnion Union { get; set; } = new MyUnion(DateTime.UtcNow);
}

{"Union":{"DateTime":"2025-07-12T13:34:57.6941483Z"}}

Example

There is a working example of a payment system under Funzo.Example. Feel free to take a look at how the code will look after using it.

Design philosophy

The idea behind this small package was to provide Option/Result monads that work idiomatically with C#, whithout losing the essence of them.

In order to achieve this, an approach of Explicit better than implicit was used:

• When working with Option, minimize the posibility of NullReferenceException by limiting the options to get the value out, enforcing the developer to handle all the cases.
• When working with Result, minimize the risk of unforseen consequences (λ) by encouraging to use the Match statement.
• Unions don't have the possibility of getting the value explicitly, forcing the developer to use the Is method or Switch/Match
• Encourage the usage of Error values, let it be records with some payload or enums, that provide useful information and force the developer to take action for each one of them. By being explicit in what kind of errors can pop out, the developer is forced to handle all the cases than can go wrong and not rely on catch blocks.