Dunet 1.6.0-preview1

Dunet

Dunet is a simple source generator for discriminated unions in C#.

Install

• NuGet: dotnet add package dunet

Usage

// 1. Import the namespace.
using Dunet;

// 2. Add the `Union` attribute to a partial record.
[Union]
partial record Shape
{
    // 3. Define the union members as inner partial records.
    partial record Circle(double Radius);
    partial record Rectangle(double Length, double Width);
    partial record Triangle(double Base, double Height);
}

// 4. Use the union members.
var shape = new Shape.Rectangle(3, 4);
var area = shape.Match(
    circle => 3.14 * circle.Radius * circle.Radius,
    rectangle => rectangle.Length * rectangle.Width,
    triangle => triangle.Base * triangle.Height / 2
);
Console.WriteLine(area); // "12"

Generics Support

Use generics for more advanced union types. For example, an option monad:

// 1. Import the namespace.
using Dunet;
// Optional: use aliasing for more terse code.
using static Option<int>;

// 2. Add the `Union` attribute to a partial record.
// 3. Add one or more type arguments to the union record.
[Union]
partial record Option<T>
{
    partial record Some(T Value);
    partial record None();
}

// 4. Use the union members.
Option<int> ParseInt(string? value) =>
    int.TryParse(value, out var number)
        ? new Some(number)
        : new None();

string GetOutput(Option<int> number) =>
    number.Match(
        some => some.Value.ToString(),
        none => "Invalid input!"
    );

var input = Console.ReadLine(); // User inputs "not a number".
var result = ParseInt(input);
var output = GetOutput(result);
Console.WriteLine(output); // "Invalid input!"

input = Console.ReadLine(); // User inputs "12345".
result = ParseInt(input);
output = GetOutput(result);
Console.WriteLine(output); // "12345".

Implicit Conversion Support

Dunet generates implicit conversions between union member values and the union type if your union meets all of the following conditions:

• All members contain only a single parameter.
• All parameters are a different type.
• No parameters are an interface type.

For example, consider a Result union type that represents success as a double and failure as an Exception:

// 1. Import the namespace.
using Dunet;

// 2. Define a union type with single unique member values:
[Union]
partial record Result
{
    partial record Success(double Value);
    partial record Failure(Exception Error);
}

// 3. Return union member values directly.
Result Divide(double numerator, double denominator)
{
    if (denominator is 0d)
    {
        // No need for `new Result.Failure(new InvalidOperationException("..."));`
        return new InvalidOperationException("Cannot divide by zero!");
    }

    // No need for `new Result.Success(...);`
    return numerator / denominator;
}

var result = Divide(42, 0);
var output = result.Match(
    success => success.Value.ToString(),
    failure => failure.Error.Message
);

Console.WriteLine(output); // "Cannot divide by zero!"

Async Match Support

Dunet generates a MatchAsync() extension method for all Task<T> and ValueTask<T> where T is a union type. For example:

// Choice.cs

using Dunet;

namespace Core;

// 1. Define a union type within a namespace.
[Union]
partial record Choice
{
    partial record Yes;
    partial record No(string Reason);
}

// Program.cs

using Core;
using static Core.Choice;

// 2. Define async methods like you would for any other type.
static async Task<Choice> AskAsync()
{
    // Simulating network call.
    await Task.Delay(1000);

    // 3. Return unions from async methods like any other type.
    return new No("because I don't wanna!");
}

// 4. Asynchronously match any union `Task` or `ValueTask`.
var response = await AskAsync().MatchAsync(yes => "Yes!!!", no => $"No, {no.Reason}");

Console.WriteLine(response); // Prints "No, because I don't wanna!" after 1 second.

Note: MatchAsync() can only be generated for namespaced unions.

Nested Union Support

To declare a union nested within a class or record, the class or record must be partial. For example:

// This type declaration must be partial.
public partial class Parent1
{
    // So must this one.
    public partial class Parent2
    {
        // Unions must always be partial.
        [Union]
        public partial record Nested
        {
            public partial record Member1;
            public partial record Member2;
        }
    }
}

// Access union members like any other nested type.
var member1 = new Parent1.Parent2.Nested.Member1();

Samples

• Area Calculator
• Serialization/Deserialization
• Option Monad
• Web Client
• Recursive Expressions