Add fsharp_keep_single_case_union_multiline setting

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Added
+
+- New `fsharp_keep_single_case_union_multiline` setting to keep single-case discriminated union declarations on a separate line instead of collapsing them onto the type name. [#3377](https://github.com/fsprojects/fantomas/pull/3377)
+
 ## [8.0.0-alpha-012] - 2026-04-16
 
 ### Changed

docs/docs/end-users/Configuration.fsx

@@ -1020,7 +1020,7 @@ printfn
 (*** include-output ***)
 
 formatCode
-    """ 
+    """
     type MyDU = Short of int
     """
     """
@@ -1029,6 +1029,29 @@ fsharp_bar_before_discriminated_union_declaration = true
 
 (*** include-output ***)
 
+(**
+<fantomas-setting green></fantomas-setting>
+### fsharp_keep_single_case_union_multiline
+<copy-to-clipboard text="fsharp_keep_single_case_union_multiline = true"></copy-to-clipboard>
+
+Keep the declaration of a single-case discriminated union on a separate line, instead of collapsing it onto the same line as the type name when it is short enough to fit.
+*)
+
+(*** hide ***)
+printfn
+    $"# Default\n{toEditorConfigName (nameof FormatConfig.Default.KeepSingleCaseUnionMultiline)} = {FormatConfig.Default.KeepSingleCaseUnionMultiline.ToString().ToLower()}"
+(*** include-output ***)
+
+formatCode
+    """
+    type MyDU = Short of int
+    """
+    """
+fsharp_keep_single_case_union_multiline = true
+    """
+
+(*** include-output ***)
+
 (**
 ## Other
 

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

@@ -82,6 +82,7 @@
     <Compile Include="BlankLinesAroundNestedMultilineExpressions.fs" />
     <Compile Include="ColMultilineItemTests.fs" />
     <Compile Include="BarBeforeDiscriminatedUnionDeclarationTests.fs" />
+    <Compile Include="KeepSingleCaseUnionMultilineTests.fs" />
     <Compile Include="SpaceBeforeColonTests.fs" />
     <Compile Include="LibraryOnlySynExprTests.fs" />
     <Compile Include="IndexSyntaxTests.fs" />

src/Fantomas.Core.Tests/KeepSingleCaseUnionMultilineTests.fs

@@ -0,0 +1,141 @@
+module Fantomas.Core.Tests.KeepSingleCaseUnionMultilineTests
+
+open NUnit.Framework
+open FsUnit
+open Fantomas.Core.Tests.TestHelpers
+
+let config =
+    { config with
+        KeepSingleCaseUnionMultiline = true }
+
+[<Test>]
+let ``single case union with field is kept multiline`` () =
+    formatSourceString
+        """
+type MyDU = Short of int
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyDU =
+    | Short of int
+"""
+
+[<Test>]
+let ``single case union without field is kept multiline`` () =
+    formatSourceString
+        """
+type A = | A
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type A =
+    | A
+"""
+
+[<Test>]
+let ``single case union with access modifier is kept multiline`` () =
+    formatSourceString
+        """
+type Foo = private Foo of int
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type Foo =
+    private | Foo of int
+"""
+
+[<Test>]
+let ``result is idempotent`` () =
+    let source =
+        """
+type MyDU =
+    | Short of int
+"""
+
+    formatSourceString source config
+    |> fun formatted -> formatSourceString formatted config
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyDU =
+    | Short of int
+"""
+
+[<Test>]
+let ``combines with bar before discriminated union declaration`` () =
+    formatSourceString
+        """
+type MyDU = Short of int
+"""
+        { config with
+            BarBeforeDiscriminatedUnionDeclaration = true }
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyDU =
+    | Short of int
+"""
+
+[<Test>]
+let ``multi case union is unaffected`` () =
+    formatSourceString
+        """
+type MyDU =
+    | Short of int
+    | Long of string
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyDU =
+    | Short of int
+    | Long of string
+"""
+
+[<Test>]
+let ``single case union with member stays multiline`` () =
+    formatSourceString
+        """
+type MyDU =
+    | Short of int
+    member this.Value = 1
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyDU =
+    | Short of int
+
+    member this.Value = 1
+"""
+
+[<Test>]
+let ``single case union in signature file is kept multiline`` () =
+    formatSignatureString
+        """namespace meh
+
+type Foo = Bar of int
+"""
+        config
+    |> should
+        equal
+        """namespace meh
+
+type Foo =
+    | Bar of int
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -3549,10 +3549,13 @@ let genTypeDefn (td: TypeDefn) =
                         genSingleTextNode vis +> onlyIfNot singleCase.XmlDoc.IsNone sepNln)
                     +> genUnionCase hasVerticalBar singleCase
 
-                expressionFitsOnRestOfLine
-                    (sepSpace +> genCase hasVerticalBar)
-                    (indentSepNlnUnindent (genCase true))
-                    ctx
+                if ctx.Config.KeepSingleCaseUnionMultiline then
+                    indentSepNlnUnindent (genCase true) ctx
+                else
+                    expressionFitsOnRestOfLine
+                        (sepSpace +> genCase hasVerticalBar)
+                        (indentSepNlnUnindent (genCase true))
+                        ctx
             | xs ->
                 indentSepNlnUnindent
                     (opt sepNln node.Accessibility genSingleTextNode

src/Fantomas.Core/FormatConfig.fs

@@ -221,6 +221,11 @@ type FormatConfig =
       [<DisplayName("Add a bar before Discriminated Union declarations")>]
       BarBeforeDiscriminatedUnionDeclaration: bool
 
+      [<Category("Convention")>]
+      [<DisplayName("Keep single-case Discriminated Union declarations multiline")>]
+      [<Description("Do not collapse a short single-case union onto the same line as the type name.")>]
+      KeepSingleCaseUnionMultiline: bool
+
       [<Category("Convention")>]
       [<DisplayName("How to format bracket expressions (arrays, objects, etc.) that span multiple lines")>]
       [<Description("Possible options include cramped (default), aligned, and stroustrup")>]
@@ -273,6 +278,7 @@ type FormatConfig =
           ExperimentalKeepIndentInBranch = false
           BlankLinesAroundNestedMultilineExpressions = true
           BarBeforeDiscriminatedUnionDeclaration = false
+          KeepSingleCaseUnionMultiline = false
           MultilineBracketStyle = Aligned
           KeepMaxNumberOfBlankLines = 100
           NewlineBeforeMultilineComputationExpression = true