W4k.Either.CodeGeneration 0.2.0-preview

W4k.Either.CodeGeneration

⚠️ Code generator - despite targeting netstandard2.0 - requires project to use at least net6.0 (due to features used in generated code).

All types distributed with W4k.Either package are generated using this source generator. To see how to use those types and what is being generated, please refer to W4k.Either package.

The W4k.Either.CodeGeneration package enables you to generate your own types with custom logic. It generates types in two ways:

Generic type

Whenever you need another type with different properties representing several different things, you can declare generic type using EitherAttribute. Compiler will generate rest of type for you:

[Either]
public readonly partial struct Gelf<TCamille, TCat, TCrichton>
{
}

Predefined types

In case you know what types you want to use, you can specify them using EitherAttribute following way:

// use `EitherAttribute` and provide types in its constructor
[Either(typeof(string), typeof(int))]
public readonly partial struct StringOrInt
{
}

// use generic `EitherAttribute<,>`
[Either<string, int>]
public readonly partial struct StringOrInt
{
}

Installation

Reference W4k.Either.CodeGeneration package in your project and use generated types:

  <ItemGroup>
    <PackageReference Include="W4k.Either.CodeGeneration" Version="0.0.0" PrivateAssets="All" />
  </ItemGroup>

Usage

Decorating your types

Declare your type as partial and decorate it with EitherAttribute. Remember to mark the containing type as partial as well if the type is nested.

Please adhere to these rules:

• The type itself MUST be partial.
• If the type is nested, the containing type MUST be partial as well.
• There must be type defined either as generic type or as predefined type using attribute.

Other properties and behaviors include:

• When declaring a generic type, it is up to you to define the type parameter constraint.
• When implementing IEquatable<T>, the Equals and GetHashCode methods are automatically generated for you, unless you have implemented them yourself.
• When implementing ISerializable, the serializable constructor and the GetObjectData method are automatically generated for you, unless you have implemented them yourself.
• You are allowed to declare constructors matching those which would be generated. In that case, the constructors are not generated to avoid causing conflicts.

[Either]
[Serializable] // <- make type serializable
public readonly partial struct Polymorph<T1, T2> : IEquatable<Polymorph<T1, T2>>, ISerializable // <- `IEquatable<>` and `ISerializable` are implemented by code generator for you 
    where T1 : struct // <- generator reflects constraints in generated code
    where T2 : notnull, ICrewMember
{
    // you can declare constructor with same signature as generator would normaly produce (generator will skip it)
    public Polymorph(T2 value)
    {
        // NB! you are responsible for checking input - if you care about it ¯\_(ツ)_/¯
        ArgumentNullException.ThrowIfNull(value);
    
        // your custom logic
        if (value is Lister)
        {
            throw new ArgumentException("...");
        }
        
        // NB! you are responsible for proper initialization of the instance;
        _idx = 2;
        _v1 = default;
        _v2 = value;
    }
    
    // custom property
    public bool IsRimmer => _idx == 2 && _v2 is Rimmer;
}

Notice that value fields starts at 1, as well as state index value:

• State is stored in field _idx:
  • 0 is reserved for invalid state (as this is default value)
  • 1, 2, ..., n are used as state index (up to 255)
• Values are indexed from 1 as well corresponding to state index (_idx = 1 means that value is kept in field _v1)

You can declare any constructor you want, but keep in mind you need to initialize the monad properly - always set _idx and _v# fields.

Skip generating (some) members

When decorating your type by [Either] attribute, it's possible to let generator know that you want to skip (some) members normally generated. This is done by specifying Skip property of the attribute:

[Either(Skip = new[] { "Case", "Switch*" })]
public partial readonly struct ThisOrThat<TLeft, TRight>
{
}

Code above generates type while skipping generating Case property and all Switch methods.

Members you can omit from being generated:

Switch method implementation relies on Match, thus skipping generation of Match will also skip Switch method.

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