FS-1039-struct-representation-for-active-patterns.md

F# RFC FS-1039 - Struct representations for active patterns

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

This RFC covers the detailed proposal starting from this suggestion

• Implementation

Summary

It should be possible to compile partial (|A|_|) active patterns to use struct options

Motivation

Powerful and extensible features such as active patterns and optional arguments should be near-zero-cost abstractions.

We should be able to provide better performance just via a simple attribute addition!

Detailed design

How to use - add the attribute and put the ValueSome/ValueNone instead of Some/None cases.

[<return: Struct>]
let (|Int|_|) str =
   match System.Int32.TryParse(str) with
   | (true,int) -> ValueSome(int)
   | _ -> ValueNone

Drawbacks

• More tricks for F# programmers to learn

Alternatives

• Don't do any of this

• Require programmers to code complex matching by hands without expressiveness of active patterns

• Provide better inlining and optimization for active patterns. It can be hard to achieve.

• Add a modality for "use structness for things declared in this scope", e.g.

  • use struct representations for return results for all active pattern declared in this scope
  • use struct representations for all syntactic tuples in this assembly in this scope
  • use struct representations for all optional arguments declared in this scope

Compatibility

TBD

Unresolved questions

TBD