Add a refactoring between fun x -> x.Prop and _.Prop

[xperiandri]

Fixes #16234

Offers Ctrl+. to turn a lambda that only reads a member chain of its parameter (fun x -> x.Name, fun x -> x.M(y), fun x -> x.Items[0]) into the _. shorthand, and a shorthand back into a fun lambda. It is a refactoring rather than a code fix because both forms are correct.

Not offered for lambdas with several or annotated parameters, bodies that do more than read the chain, lambdas applied directly, or inside quotations.

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[github-actions]

❗ Release notes required

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Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.200.md	No release notes found or release notes format is not correct

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md

[T-Gro]

Maybe in general, could you run e.g. 2-3 rounds of the "code compaction" skill from this repo which will check any shared patterns between added code and code which already exists in the compiler (like various Ops, helpers or other pieces of logic) - the skill covers some common patterns of logic reuse.

[xperiandri]

Covered cases

Converts

• A lambda passed as an argument loses its parentheses: xs |> List.map (fun x -> x.Prop) → xs |> List.map _.Prop, List.map (fun x -> x.A.B) xs → List.map _.A.B xs
• Member chains on the parameter: method call Seq.filter (fun x -> x.M(y)) → Seq.filter _.M(y), indexers x.Xs[0] and x.Xs.[0], generic method x.M<int>(), member of a call result x.M().P
• Spaces inside the parentheses ( fun x -> x.P ) and a backticked parameter fun ``x`` -> ``x``.P both → _.P
• A method argument keeps the call's parentheses: x.M(fun y -> y.P) → x.M(_.P); a bare fun x -> x.P → _.P; a lone (fun x -> x.P) → (_.P)
• Multi-line lambdas: List.map (fun x -> with x.Prop) xs on the next line → List.map _.Prop xs; a binding body fun x -> with x.P on its own line → _.P
• Shorthand to lambda: _.P → fun x -> x.P, _.A.B → fun x -> x.A.B, _.Xs[0] → fun x -> x.Xs[0]
• Shorthand as an argument gets parentheses: List.map _.P xs → List.map (fun x -> x.P) xs; inside method parentheses they are not added: x.M(_.P) → x.M(fun x -> x.P)
• The parameter name avoids names used in the body: xs |> List.map _.M(x) → xs |> List.map (fun x1 -> x1.M(x))
• Nested shorthands convert one at a time: caret on the outer one of _.M(_.P) → fun x -> x.M(_.P), on the inner one → _.M(fun x -> x.P)
• Round trip: List.map (fun x -> x.P) xs converted to shorthand and back gives the original text

Not offered

• Several parameters or a non-plain parameter pattern: fun x y -> x.P, fun (x: T) -> x.P, fun (x) -> x.P
• The body is more than a member chain: fun x -> (x.P), fun x -> f x.P, fun x -> x.M y, fun x -> x.P + 1
• The parameter is used more than once: fun x -> x.M(x), fun x -> x.M(fun x -> x)
• The chain doesn't start at the parameter, or there is no member access: fun x -> y.P, fun x -> x, fun x -> x[0], fun _ -> 1
• A lambda applied directly: (fun x -> x.P) y
• A lambda inside a quotation: <@ fun x -> x.P @>
• Caret on neither a lambda nor a shorthand: 1 + 2

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Changes IDE integration executed during design-time.

Generated by PR Tooling Safety Check · gpt56 1.7M · ◷

[abonie]

🤖🕵️ AI review — verify independently.

[Copilot]

Copilot review overview

🟡 Changes recommended

The refactoring can generate invalid SRTP code, discard comments, and appear in explicitly excluded contexts.

Review effort: Balanced
Findings: 2 · 1

Open (3)

• Handle nested parentheses around directly applied lambdas · New
• Guard shorthand conversion with type-check information · New
• Preserve comments in lambda-to-shorthand conversions · New

What changed in this PR

Adds bidirectional Visual Studio refactoring between explicit lambdas and F# _. shorthand, backed by shared compiler syntax-tree logic.

Changes:

• Adds the refactoring, telemetry, resources, and localization.
• Extracts reusable syntax-chain conversion operations.
• Adds compiler-service and editor tests plus release notes.

File	Description
vsintegration/​tests/​FSharp.Editor.Tests/​Refactors/​ConvertDotLambdaTests.fs	Tests refactoring behavior.
vsintegration/​tests/​FSharp.Editor.Tests/​FSharp.Editor.Tests.fsproj	Includes new editor tests.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.zh-Hant.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.zh-Hans.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.tr.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.ru.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.pt-BR.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.pl.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.ko.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.ja.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.it.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.fr.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.es.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.de.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​xlf/​FSharp.Editor.cs.xlf	Adds localized resources.
vsintegration/​src/​FSharp.Editor/​Telemetry/​TelemetryReporter.fs	Defines refactoring telemetry event.
vsintegration/​src/​FSharp.Editor/​Refactor/​ConvertDotLambda.fs	Implements bidirectional conversion.
vsintegration/​src/​FSharp.Editor/​FSharp.Editor.resx	Adds action titles.
vsintegration/​src/​FSharp.Editor/​FSharp.Editor.fsproj	Includes the provider.
tests/​FSharp.Compiler.Service.Tests/​SyntaxTreeOpsTests.fs	Tests syntax-chain operations.
tests/​FSharp.Compiler.Service.Tests/​FSharp.Compiler.Service.Tests.fsproj	Includes syntax tests.
src/​Compiler/​SyntaxTree/​SyntaxTreeOps.fsi	Declares chain conversion helper.
src/​Compiler/​SyntaxTree/​SyntaxTreeOps.fs	Implements shared chain traversal.
docs/​release-notes/​.VisualStudio/​18.vNext.md	Documents the refactoring.
docs/​release-notes/​.FSharp.Compiler.Service/​11.0.200.md	Documents syntax-operation changes.

[abonie]

🤖🕵️ AI review — verify independently.

[abonie]

🤖🕵️ AI review — verify independently.

[T-Gro]

🤖 🕵️ Re-reviewed all recent changes as of 7b928dc; 1 findings

[T-Gro]

The diff is still way too big - apply the NoBloat instructions and CodeCompaction skill.

Especially the massive code comments would benefit not just with drastic reduction, but ideally with well named functions/arguments/bindings/types making the intention clear.

If a well named piece of code just tells what the comment says - delete the comment.

• Delete the 9 non-license implementation comment lines. Rename the internal helpers so dot-lambda, generated-argument, offside-line, and sole-parenthesis intent is in code rather than XML prose.
• Delete the FCS release note: the shared internal syntax walker and its inverse are not user-visible FCS behavior. Keep a compact Visual Studio note for the refactoring action.
• Fold the standalone success Facts and multiline cases into the existing parameterized conversion tables.

[xperiandri]

Pushed e47c07c (names instead of the nine implementation comments), 12d14fd (the tests folded into the tables) and b5496e8 (release notes): the PR now adds one comment line, the license header, down from 10 lines and 1037 characters, and the added lines in total go from 604 to 548. The helpers carry the intent in their names (splitDotLambdaChain, canBeCapturedByGeneratedArgument, lastOffsideSensitiveLine, SoleParenthesizedLambdaArgument), the FCS release note is deleted, and the Visual Studio note is a single short line. The four success Facts and the multi-line cases are rows of the two conversion tables (the former Fact names are kept as display names), so ConvertDotLambdaTests goes from 8 test methods to 3 with the same 61 cases and from 171 to 125 lines; the round trip Fact is now the pair of rows List.map (fun x -> x.P) xs and List.map _.P xs. I left behaviour, the 31-row No action table and the generated xlf files alone; ConvertDotLambdaTests (61), SyntaxTreeOpsTests (14), Language.DotLambdaTests (39) and the AddReturnTypeTests control (22) pass before and after, and FSharp.Editor also builds with Optimize=true.