Singleton Types

A singleton type is a refined type that can only be inhabited by a particular instance of an underlying value.

Let's see an example,


/// <summary>
/// This is a singleton type, it's an int that can only hold the value 42
/// </summary>
public readonly partial struct FortyTwoInt
{
    [Refinement("The value must be '42', instead found '{value}'")]
    private static bool FortyTwo(int value) => value == 42;

    /// <summary>
    /// The singleton instance of 42
    /// </summary>
    public static readonly FortyTwoInt Instance = Parse(42);
}

Which can be used like this,


[Fact(DisplayName = "The value 42 can be refined to a singleton type")]
public void Case1()
{
    const int value = 42;
    var refined = (FortyTwoInt)value;
    Assert.Equal(42, refined.Value);
    Assert.Equal(42, FortyTwoInt.Instance.Value);
}

[Fact(
    DisplayName = "Any other int value that is not 42 cannot be refined to the singleton type of 42"
)]
public void Case2()
{
    const int value = 43;
    var ex = Assert.Throws<ArgumentOutOfRangeException>(() => (FortyTwoInt)value);
    Assert.StartsWith("The value must be '42', instead found '43'", ex.Message);
    Assert.False(FortyTwoInt.TryParse(value, out _, out _));
}

Singleton types are useful in places that require a particular special value.

For example, zero or empty or Pi.

Table of Contents

Singleton Types