Merge pull request #3303 from nojaf/revisit-lambdas

Revisit lambdas

Author: nojaf

AGENTS.md

@@ -20,6 +20,10 @@ All scripts accept a file path or stdin, with optional `--signature` and `--edit
 
 Scripts require a debug build first (`dotnet build src/Fantomas/Fantomas.fsproj`).
 
+## Changelog
+
+When updating `CHANGELOG.md`, add new entries to the **end** of the relevant section (e.g. `### Fixed`), not the top. One entry per issue.
+
 ## Post-task Steps
 
 Run these after completing a task, not during iterative development — analyzers can be slow.

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## [Unreleased]
+## [8.0.0-alpha-008] - 2026-03-25
 
 ### Added
 
@@ -14,6 +14,9 @@
 - `%%` (double-percent) infix operator moved to new line, producing invalid F#. [#2107](https://github.com/fsprojects/fantomas/issues/2107)
 - Indentation warning when formatting `match` with long anonymous record discriminant. [#1903](https://github.com/fsprojects/fantomas/issues/1903)
 - Index-without-dot with variable key followed by unit arguments added spurious spaces, e.g. `dict[key] () ()` became `dict [ key ] () ()`. [#2519](https://github.com/fsprojects/fantomas/issues/2519)
+- Open-ended expressions (lambda, if-then-else, match, ...) in non-last positions of infix, tuple, list/array, and record expressions now stay multiline to preserve semantics. [#3279](https://github.com/fsprojects/fantomas/issues/3279)
+- Lambda in tuple in list on single line changes code meaning. [#3278](https://github.com/fsprojects/fantomas/issues/3278)
+- Custom operator applied to lambda collapses to single line changing semantics. [#3274](https://github.com/fsprojects/fantomas/issues/3274)
 
 ## [8.0.0-alpha-007] - 2026-03-10
 

src/Fantomas.Core.Tests/ControlStructureTests.fs

@@ -417,7 +417,7 @@ elif true then ()
 """
 
 [<Test>]
-let ``multiline if in tuple`` () =
+let ``multiline if in tuple uses comma-leading layout`` () =
     formatSourceString
         """
 (if true then 1 else 2
@@ -428,7 +428,8 @@ let ``multiline if in tuple`` () =
     |> should
         equal
         """
-((if true then 1 else 2), 3)
+(if true then 1 else 2
+ , 3)
 """
 
 // https://docs.microsoft.com/en-us/dotnet/fsharp/style-guide/formatting#formatting-if-expressions

src/Fantomas.Core.Tests/Fantomas.Core.Tests.fsproj

@@ -136,6 +136,7 @@
     <Compile Include="NullnessTests.fs" />
     <Compile Include="AutoPropertiesTests.fs" />
     <Compile Include="PrefixTests.fs" />
+    <Compile Include="RequiresMultilineToPreserveSemanticsTests.fs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Fantomas.Core\Fantomas.Core.fsproj" />

src/Fantomas.Core.Tests/LambdaTests.fs

@@ -1173,7 +1173,7 @@ let dayOfWeekToNum (d: DayOfWeek) =
 """
 
 [<Test>]
-let ``piped lambda with if-then-else, short, 2196`` () =
+let ``piped lambda with if-then-else stays multiline to preserve semantics, 2196`` () =
     formatSourceString
         """
 let foo () =
@@ -1187,7 +1187,9 @@ let foo () =
         equal
         """
 let foo () =
-    f () |> (fun x -> if x then 1 else 2) |> g
+    f ()
+    |> fun x -> if x then 1 else 2
+    |> g
 """
 
 [<Test>]
@@ -1457,7 +1459,7 @@ f
 """
 
 [<Test>]
-let ``lambda in non-last record field should be parenthesized on single line, 3246`` () =
+let ``lambda in non-last record field stays multiline to preserve semantics, 3246`` () =
     formatSourceString
         """
 type Rec = {
@@ -1480,5 +1482,10 @@ let test () : Rec =
         """
 type Rec = { A: int; B: int -> int; C: int }
 
-let test () : Rec = { A = 1; B = (fun x -> x + 1); C = 3 }
+let test () : Rec =
+    {
+        A = 1
+        B = fun x -> x + 1
+        C = 3
+    }
 """

src/Fantomas.Core.Tests/OperatorTests.fs

@@ -1255,7 +1255,7 @@ let ( */ ) = (+)
 """
 
 [<Test>]
-let ``piped lambda on a single line`` () =
+let ``piped lambda stays multiline to preserve semantics`` () =
     formatSourceString
         """
 let a : (unit -> int) list =
@@ -1268,11 +1268,14 @@ let a : (unit -> int) list =
     |> should
         equal
         """
-let a: (unit -> int) list = (fun () -> failwith "": int) |> List.singleton |> id
+let a: (unit -> int) list =
+    fun () -> failwith "": int
+    |> List.singleton
+    |> id
 """
 
 [<Test>]
-let ``piped tuple on a single line`` () =
+let ``piped tuple stays multiline to preserve semantics`` () =
     formatSourceString
         """
 fun i -> sprintf "%i" i, fun () -> i
@@ -1284,11 +1287,13 @@ fun i -> sprintf "%i" i, fun () -> i
     |> should
         equal
         """
-(fun i -> sprintf "%i" i, fun () -> i) |> List.init foo |> Map.ofList
+fun i -> sprintf "%i" i, fun () -> i
+|> List.init foo
+|> Map.ofList
 """
 
 [<Test>]
-let ``lambda piped into non newlineInfixApp`` () =
+let ``lambda piped into non newlineInfixApp stays multiline to preserve semantics`` () =
     formatSourceString
         """
 fun sum count -> sum / float count
@@ -1300,7 +1305,9 @@ fun sum count -> sum / float count
     |> should
         equal
         """
-(fun sum count -> sum / float count) <*| sum xs <*| count
+fun sum count -> sum / float count
+<*| sum xs
+<*| count
 """
 
 [<Test>]