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FS-1057-valueoption.md

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F# RFC FS-1039 - ValueOption

The design suggestion Struct representation for active patterns has been marked "approved in principle".

Likewise, the idea of a struct option type is implied by this suggestion and has also been approved.

Summary

This RFC covers Part 1 of RFC FS-1039 Struct representation for active patterns, but factors out the specific concern of a struct-based optional type, ValueOption.

Motivation

Powerful and extensible features such as Active Patterns and Optional Arguments should be near-zero-cost abstractions. A ValueOption type is a first step in this direction.

Additionally, in situations where a struct can provide a benefit over a reference type, ValueOption may allow developers to squeeze out more performance independently of their usage of Active Patterns and Optional Arguments.

Detailed design

An unboxed (struct) version of the Option type:

/// <summary>The type of optional values, represented as structs.</summary>
///
/// <remarks>Use the constructors <c>ValueSome</c> and <c>ValueNone</c> to create values of this type.
/// Use the values in the <c>ValueOption</c> module to manipulate values of this type,
/// or pattern match against the values directly.
[<StructuralEquality; StructuralComparison>]
[<CompiledName("FSharpValueOption`1")>]
[<Struct>]
type ValueOption<'T> =
    /// <summary>The representation of "No value"</summary>
    | ValueNone: 'T voption

    /// <summary>The representation of "Value of type 'T"</summary>
    /// <param name="Value">The input value.</param>
    /// <returns>An option representing the value.</returns>
    | ValueSome: 'T -> 'T voption

    /// <summary>Get the value of a 'ValueSome' option. An InvalidOperationException is raised if the option is 'ValueNone'.</summary>
    member Value : 'T

/// <summary>The type of optional values, represented as structs.</summary>
///
/// <remarks>Use the constructors <c>ValueSome</c> and <c>ValueNone</c> to create values of this type.
/// Use the values in the <c>ValueOption</c> module to manipulate values of this type,
/// or pattern match against the values directly.
and 'T voption = ValueOption<'T>

With a defaultValueArg function:

[<CompiledName("DefaultValueArg")>]
val defaultValueArg : arg:'T voption -> defaultValue:'T -> 'T

Drawbacks

  • Another thing F# programmers must learn (i.e., Option types can come in two forms based on how you want the code to run).
  • Potentially confusing people who are not familiar with how performance vis-a-vis structs and reference types work on .NET, thus allowing them to think that this is merely a "faster" version of Option.

Alternatives

Not adding anything.

Compatibility

It's not breaking change due to it not requiring new syntax at all, just addition to FSharp.Core and changes in code generation.

Unresolved questions

None.