POC double indentation.

src/Fantomas.Core.Tests/ExperimentalDoubleIdentParametersTests.fs

@@ -0,0 +1,30 @@
+module Fantomas.Core.Tests.ExperimentalDoubleIdentParametersTests
+
+open NUnit.Framework
+open FsUnit
+open Fantomas.Core.Tests.TestHelpers
+
+let config =
+    { config with
+        ExperimentalDoubleIndentParameters = true
+        MaxLineLength = 10 }
+
+[<Test>]
+let ``initial sample`` () =
+    formatSourceString
+        """
+let sillyfuncWithParams parameterName1 ignoredParameterName2 ignoredParameterName3 =
+    longBody
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+let sillyfuncWithParams
+        parameterName1
+        ignoredParameterName2
+        ignoredParameterName3
+        =
+    longBody
+"""

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

@@ -132,6 +132,7 @@
     <Compile Include="DotLambdaTests.fs" />
     <Compile Include="ConstraintIntersectionTests.fs" />
     <Compile Include="BeginEndTests.fs" />
+    <Compile Include="ExperimentalDoubleIdentParametersTests.fs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Fantomas.Core\Fantomas.Core.fsproj" />

src/Fantomas.Core/CodePrinter.fs

@@ -2879,7 +2879,7 @@ let genBinding (b: BindingNode) (ctx: Context) : Context =
                      +> afterLetKeyword
                      +> sepSpace
                      +> genFunctionName
-                     +> indent
+                     +> ifElseCtx (fun ctx -> ctx.Config.ExperimentalDoubleIndentParameters) (indent +> indent) indent
                      +> sepNln
                      +> genParameters
                      +> onlyIf (not endsWithTupleParameter || alternativeSyntax) sepNln
@@ -2890,7 +2890,10 @@ let genBinding (b: BindingNode) (ctx: Context) : Context =
                                  sepNln +> genSingleTextNode b.Equals
                              else
                                  sepSpace +> genSingleTextNode b.Equals)
-                     +> unindent
+                     +> ifElseCtx
+                         (fun ctx -> ctx.Config.ExperimentalDoubleIndentParameters)
+                         (unindent +> unindent)
+                         unindent
                      +> onlyIf hasTriviaAfterLeadingKeyword unindent)
                         ctx
 

src/Fantomas.Core/FormatConfig.fs

@@ -227,7 +227,12 @@ type FormatConfig =
 
       [<Category("Convention")>]
       [<DisplayName("Applies the Stroustrup style to the final (two) array or list argument(s) in a function application")>]
-      ExperimentalElmish: bool }
+      ExperimentalElmish: bool
+
+      [<Category("Indentation")>]
+      [<DisplayName("Double indent parameters")>]
+      [<Description("Use double indentation for parameters.")>]
+      ExperimentalDoubleIndentParameters: bool }
 
     member x.IsStroustrupStyle = x.MultilineBracketStyle = Stroustrup
 
@@ -268,4 +273,5 @@ type FormatConfig =
           MultilineBracketStyle = Cramped
           KeepMaxNumberOfBlankLines = 100
           NewlineBeforeMultilineComputationExpression = true
-          ExperimentalElmish = false }
+          ExperimentalElmish = false
+          ExperimentalDoubleIndentParameters = false }

src/Fantomas.Tests/EditorConfigurationTests.fs

@@ -507,7 +507,7 @@ fsharp_multiline_bracket_style = cramped
     Assert.That(config.MultilineBracketStyle, Is.EqualTo Cramped)
 
 [<Test>]
-let fsharp_prefer_computation_expression_name_on_same_line () =
+let fsharp_newline_before_multiline_computation_expression () =
     let rootDir = tempName ()
 
     let editorConfig =
@@ -526,7 +526,7 @@ fsharp_newline_before_multiline_computation_expression = false
     Assert.That(config.NewlineBeforeMultilineComputationExpression, Is.False)
 
 [<Test>]
-let fsharp_stroustrup_final_list_arguments () =
+let fsharp_experimental_elmish () =
     let rootDir = tempName ()
 
     let editorConfig =
@@ -543,3 +543,22 @@ fsharp_experimental_elmish = true
     let config = EditorConfig.readConfiguration fsharpFile.FSharpFile
 
     Assert.That(config.ExperimentalElmish, Is.True)
+
+[<Test>]
+let fsharp_experimental_double_indent_parameters () =
+    let rootDir = tempName ()
+
+    let editorConfig =
+        """
+[*.fs]
+fsharp_experimental_double_indent_parameters = true
+"""
+
+    use configFixture =
+        new ConfigurationFile(defaultConfig, rootDir, content = editorConfig)
+
+    use fsharpFile = new FSharpFile(rootDir)
+
+    let config = EditorConfig.readConfiguration fsharpFile.FSharpFile
+
+    Assert.That(config.ExperimentalDoubleIndentParameters, Is.True)