Funcky Discriminated Unions

A source generator that generates Match methods for all your discriminated unions needs. ✨

Installation

Add <ProjectReference Include="Funcky.DiscriminatedUnion" Version="..." PrivateAssets="all" /> to your project file.

Usage

Apply the [DiscriminatedUnion] to an abstract class (or record) with nested types representing the variants.

Example

using Funcky;

var result = Result<int>.Ok(42);
var resultOrFallback = result.Match(ok: ok => ok.Value, error: _ => 0);

[DiscriminatedUnion]
public abstract partial record Result<T>
    where T : notnull
{
    public sealed partial record Ok(T Value) : Result<T>;

    public sealed partial record Error(Exception Exception) : Result<T>;
}

Minimum Required Versions

• Visual Studio 2022
• Roslyn 4.0.0
• .NET 6

Settings

The attribute allows configuration of some aspects of source generation.

NonExhaustive

The auto-generated Match and Switch methods are public by default. When NonExhaustive is set to true, these methods are generated with internal visibility instead.

MatchResultTypeName

The auto-generated Match method uses a generic type for the result. This type is named TResult by default. This can cause conflict with generic types on the discriminated union itself. Use MatchResultTypeName to set a custom name for this type.

using Funcky;

[DiscriminatedUnion(MatchResultTypeName = "TMatchResult")]
public abstract partial record Result<TResult> { ... }

// Generated code
partial record Result<TResult>
{
    public abstract TMatchResult Match<TMatchResult>(...);

    ...
}

Flatten

The auto-generated Match and Switch methods only accept one level of inheritance by default. Set Flatten to true to include arbitrarily deep inherited types in these methods.

using Funcky;

SyntaxNode node = ...;
var nodeAsString = node.Match(
    keyword: keyword => keyword.Value,
    integer: integer => integer.Value.ToString(),
    double: @double => @double.Value.ToString());

[DiscriminatedUnion(Flatten = true)]
public abstract partial record SyntaxNode
{
    public sealed partial record Keyword(string Value) : SyntaxNode;

    public abstract partial record Literal : SyntaxNode;

    public abstract partial record Number : Literal;

    public sealed partial record Integer(int Value) : Number;

    public sealed partial record Double(double Value) : Number;
}

[JsonPolymorphic]

System.Text.Json adds support for serializing derived classes starting with .NET 7. This generator supports this feature by generating the required [JsonDerivedType] attributes for you.

All missing [JsonDerivedType] attributes are generated if at least one [JsonDerivedType] or [JsonPolymorphic] attribute is specified.

using Funcky;
using System.Text.Serialization;

[DiscriminatedUnion]
[JsonPolymorphic]
public abstract partial record Shape
{
    public sealed partial record Rectangle(double Width, double Length) : Shape;

    public sealed partial record Circle(double Radius) : Shape;

    public sealed partial record EquilateralTriangle(double SideLength) : Shape;
}

Generated code

using System.Text.Serialization;

[JsonDerivedType(typeof(Rectangle), typeDiscriminator: nameof(Rectangle))]
[JsonDerivedType(typeof(Circle), typeDiscriminator: nameof(Circle))]
[JsonDerivedType(typeof(EquilateralTriangle), typeDiscriminator: nameof(EquilateralTriangle))]
partial record Shape
{
    // ...
}