Support for C# collection expressions in F# lists and sets

[IsaacSchemm]

I propose we add support for C# collection expressions to F# lists and sets, so that a collection of this type can be initialized from C# code with a collection expression.

The use case for F# lists and sets in C# code, as I see it, is to extend the immutability and value equality of C# records to those with collections of equatable items (strings, integers, other records) as properties/fields. By combining a record with an immutable collection type that (like the record) implements value equality, a complex type can be constructed that is fully immutable and supports "deep" equality checks all the way down, as is often done in F#. This becomes very useful when writing unit tests that need to define and check all fields and sub-fields on objects,. It also opens up use cases for these complex types to be used in a HashSet or as keys in a Dictionary.

C# collection expressions don't have "automatic" support for immutable collections (the compiler looks for an Add method), but support can be added to a collection type by adding a CollectionBuilderAttribute to the type and implementing a static creation method.

This code can be used today in C#:

record Poem {
    public string Name { get; init; } = "Untitled";
    public string Content { get; init; } = "";
    public FSharpSet<string> Tags { get; init; } = SetModule.Empty<string>();
}

Poem originalPoem = repository.GetPoem(id);
Poem newPoem = originalPoem with { Tags = SetModule.FromArray(["sonnet", "moon", "sky"]) };
if (originalPoem != newPoem) {
    repository.UpdatePoem(id, newPoem);
}

With this change, the code could become:

record Poem {
    public string Name { get; init; } = "Untitled";
    public string Content { get; init; } = "";
    public FSharpSet<string> Tags { get; init; } = [];
}

Poem originalPoem = repository.GetPoem(id);
Poem newPoem = originalPoem with { Tags = ["sonnet", "moon", "sky"] };
if (originalPoem != newPoem) {
    repository.UpdatePoem(id, newPoem);
}

The existing ways of approaching this problem are to:

• use a non-F# collection type such as the ones in the namespace System.Collections.Immutable; these types do not implement value equality; so another approach must be used to compare two objects
• initialize another collection type (with or without collection expressions) and convert to an F# list using ListModule.OfSeq or ListModule.OfArray
• implement a custom collection type that supports value equality (with object.Equals and with IEquatable<T>) and collection expressions, as well as whatever other functions / features are needed

Pros and Cons

The advantages of making this adjustment to F# are:

• Better interoperability with C# code that uses F#-defined types
• Making the F# list type more useful for C# developers

The disadvantages of making this adjustment to F# are:

• Code must be added to the F# library (and maintained) to support a C# feature that does not (currently) exist in F#

Extra information

Estimated cost (XS, S, M, L, XL, XXL): XS

Related suggestions:

• Type-directed resolution of [ .. ] syntax #1086
• Indexer, collection, event and sublevel initialization #969

Affidavit (please submit!)

Please tick these items by placing a cross in the box:

• This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
• This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
• I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

[Happypig375]

Do you mean F# should define these methods for C# consumption? Should this relate to #969 for usage in F# too?

[dsyme]

Marked as approved in principle!

[IsaacSchemm]

@Happypig375 I'll add #969 as related - it looks like if F# adds the ability to consume CollectionBuilderAttribute, it would be tracked as part of that issue. Right now this would just be for C# consumption.

[xperiandri]

This is my favorite feature since C# added async and await. I would love to see it appear in F# ASAP

[abelbraaksma]

@xperiandri are you interested in writing an RFC for this and/or a PR to further the proposal?

[vzarytovskii]

To make sure everyone understands what it covers. It doesn't cover having "one syntax" for all collections in F#, but make C# "understand" out collections.

[brianrourkeboll]

• RFC: [RFC FS-1145] C# collection expression support for lists & sets fslang-design#776
• RFC discussion: [RFC FS-1145] C# collection expression support for lists & sets fslang-design#777
• Proposed implementation: C# collection expression support for F# lists & sets dotnet/fsharp#17359