Option to reduce multiple blank lines from source code

docs/Documentation.md

@@ -147,6 +147,7 @@ fsharp_disable_elmish_syntax=false
 fsharp_keep_indent_in_branch=false
 fsharp_blank_lines_around_nested_multiline_expressions=true
 fsharp_bar_before_discriminated_union_declaration=false
+fsharp_keep_max_number_of_blank_lines=100
 fsharp_strict_mode=false
 ```
 
@@ -1167,7 +1168,7 @@ let main argv =
 ### fsharp_blank_lines_around_nested_multiline_expressions
 
 Surround **nested** multi-line expressions with blank lines.
-Existing blank lines are always preserved (via trivia).  
+Existing blank lines are always preserved (via trivia), with exception when [fsharp_keep_max_number_of_blank_lines](#fsharp_keep_max_number_of_blank_lines) is used.  
 Top level expressions will always follow the [2020 blank lines revision](https://github.com/fsprojects/fantomas/blob/master/docs/FormattingConventions.md#2020-revision) principle.
 Default = true.
 
@@ -1221,6 +1222,29 @@ type MyDU = Short of int
 type MyDU = | Short of int
 ```
 
+### fsharp_keep_max_number_of_blank_lines
+
+Set maximal number of consecutive blank lines to keep from original source. It doesn't change number of new blank lines generated by Fantomas.
+Default=100
+
+`defaultConfig`
+
+```fsharp
+open Foo
+
+
+let x = 42
+```
+
+`{ defaultConfig with KeepMaxNumberOfBlankLines = 1 }`
+
+```fsharp
+open Foo
+
+let x = 42
+```
+
+
 ### fsharp_strict_mode
 
 If being set, pretty printing is only done via ASTs. Compiler directives, inline comments and block comments will be ignored.

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

@@ -91,6 +91,7 @@
     <Compile Include="IndexSyntaxTests.fs" />
     <Compile Include="InsertFinalNewlineTests.fs" />
     <Compile Include="SingleExprTests.fs" />
+    <Compile Include="KeepMaxEmptyLinesTests.fs" />
     <Compile Include="IgnoreFileTests.fs" />
     <Compile Include="Ragnarok\SynBindingValueExpressionTests.fs" />
     <Compile Include="Ragnarok\SynBindingFunctionExpressionTests.fs" />

src/Fantomas.Core.Tests/KeepMaxEmptyLinesTests.fs

@@ -0,0 +1,109 @@
+module Fantomas.Core.Tests.KeepMaxEmptyLinesTests
+
+open NUnit.Framework
+open FsUnit
+open Fantomas.Core.Tests.TestHelper
+open Fantomas.Core.FormatConfig
+
+let checkFormat config source expected =
+    formatSourceString false source config
+    |> prepend newline
+    |> should equal expected
+
+let config x =
+    { config with KeepMaxNumberOfBlankLines = x }
+
+[<Test>]
+let ``reduce 2 empty lines to 1`` () =
+    checkFormat
+        (config 1)
+        """
+open Foo
+
+
+let x = 42
+"""
+        """
+open Foo
+
+let x = 42
+"""
+
+[<Test>]
+let ``reduce 3 empty lines to 2`` () =
+    checkFormat
+        (config 2)
+        """
+open Foo
+
+
+
+let x = 42
+"""
+        """
+open Foo
+
+
+let x = 42
+"""
+
+[<Test>]
+let ``reduce 3 empty lines to 1`` () =
+    checkFormat
+        (config 1)
+        """
+open Foo
+
+
+
+let x = 42
+"""
+        """
+open Foo
+
+let x = 42
+"""
+
+
+[<Test>]
+let ``only generated empty lines`` () =
+    checkFormat
+        (config 0)
+        """
+open Foo
+
+open Goo
+
+module M1 =
+
+    let x = 42
+module M2 = let y = 42
+"""
+        """
+open Foo
+open Goo
+
+module M1 =
+    let x = 42
+
+module M2 =
+    let y = 42
+"""
+
+[<Test>]
+let ``dont reduce empty lines in string`` () =
+    checkFormat
+        (config 1)
+        "
+let x = \"\"\"
+
+
+\"\"\"
+"
+        "
+let x =
+    \"\"\"
+
+
+\"\"\"
+"

src/Fantomas.Core.Tests/TriviaTests.fs

@@ -5,11 +5,12 @@ open Fantomas.Core
 open Fantomas.Core.SourceParser
 open Fantomas.Core.Tests.TestHelper
 open Fantomas.Core.TriviaTypes
+open Fantomas.Core.FormatConfig
 
 let private toTrivia source =
     let sourceText = CodeFormatterImpl.getSourceText source
     let ast, _ = Fantomas.FCS.Parse.parseFile false sourceText []
-    Trivia.collectTrivia sourceText ast
+    Trivia.collectTrivia FormatConfig.Default sourceText ast
 
 let private toTriviaWithDefines source =
     let sourceText = CodeFormatterImpl.getSourceText source
@@ -23,7 +24,7 @@ let private toTriviaWithDefines source =
     let defineCombinations = Defines.getDefineCombination hashDirectives
 
     defineCombinations
-    |> List.map (fun dc -> dc, Trivia.collectTrivia sourceText ast)
+    |> List.map (fun dc -> dc, Trivia.collectTrivia FormatConfig.Default sourceText ast)
     |> Map.ofList
 
 [<Test>]

src/Fantomas.Core/Context.fs

@@ -200,7 +200,7 @@ type internal Context =
     static member Create config (source: ISourceText option) (defineCombination: DefineCombination) (ast: ParsedInput) =
         let trivia, sourceText =
             match source with
-            | Some source when not config.StrictMode -> Trivia.collectTrivia source ast, Some source
+            | Some source when not config.StrictMode -> Trivia.collectTrivia config source ast, Some source
             | _ -> [], None
 
         let triviaByNodes =

src/Fantomas.Core/FormatConfig.fs

@@ -222,6 +222,10 @@ type FormatConfig =
       [<DisplayName("Ragnarok")>]
       Ragnarok: bool
 
+      [<Category("Convention")>]
+      [<DisplayName("Maximum number of consecutive blank lines to keep")>]
+      KeepMaxNumberOfBlankLines: Num
+
       [<Category("Convention")>]
       [<DisplayName("Strict mode")>]
       [<Description("Pretty printing based on ASTs only.\nPlease do not use this setting for formatting hand written code!")>]
@@ -268,4 +272,5 @@ type FormatConfig =
           BlankLinesAroundNestedMultilineExpressions = true
           BarBeforeDiscriminatedUnionDeclaration = false
           Ragnarok = false
+          KeepMaxNumberOfBlankLines = 100
           StrictMode = false }

src/Fantomas.Core/Trivia.fs

@@ -8,6 +8,7 @@ open Fantomas.Core.ISourceTextExtensions
 open Fantomas.Core.SourceParser
 open Fantomas.Core.AstTransformer
 open Fantomas.Core.TriviaTypes
+open Fantomas.Core.FormatConfig
 
 let inline isMainNodeFor nodeType (node: TriviaNodeAssigner) = nodeType = node.Type
 
@@ -289,6 +290,7 @@ let internal collectTriviaFromCodeComments (source: ISourceText) (codeComments:
             { Item = item; Range = r })
 
 let internal collectTriviaFromBlankLines
+    (config: FormatConfig)
     (source: ISourceText)
     (triviaNodes: TriviaNodeAssigner list)
     (codeComments: CommentTrivia list)
@@ -326,29 +328,32 @@ let internal collectTriviaFromBlankLines
 
     let max = source.GetLineCount() - 1
 
-    [ 0..max ]
-    |> List.choose (fun idx ->
+    (0, [ 0..max ])
+    ||> List.chooseState (fun count idx ->
         if ignoreLines.Contains(idx + 1) then
-            None
+            0, None
         else
             let line = source.GetLineString(idx)
 
             if String.isNotNullOrWhitespace line then
-                None
+                0, None
             else
                 let range =
                     let p = Position.mkPos (idx + 1) 0
                     Range.mkFileIndexRange fileIndex p p
 
-                Some { Item = Newline; Range = range })
+                if count < config.KeepMaxNumberOfBlankLines then
+                    (count + 1), Some { Item = Newline; Range = range }
+                else
+                    count, None)
 
 (*
     1. Collect TriviaNode from tokens and AST
     2. Collect TriviaContent from tokens
     3. Merge trivias with triviaNodes
     4. genTrivia should use ranges to identify what extra content should be added from what triviaNode
 *)
-let collectTrivia (source: ISourceText) (ast: ParsedInput) : TriviaNode list =
+let collectTrivia (config: FormatConfig) (source: ISourceText) (ast: ParsedInput) : TriviaNode list =
     let triviaNodesFromAST, directives, codeComments =
         match ast with
         | ParsedInput.ImplFile (ParsedImplFileInput (hds, mns, directives, codeComments)) ->
@@ -363,7 +368,7 @@ let collectTrivia (source: ISourceText) (ast: ParsedInput) : TriviaNode list =
     let trivia =
         [ yield! collectTriviaFromDirectives source directives
           yield! collectTriviaFromCodeComments source codeComments
-          yield! collectTriviaFromBlankLines source triviaNodes codeComments ]
+          yield! collectTriviaFromBlankLines config source triviaNodes codeComments ]
         |> List.sortBy (fun n -> n.Range.Start.Line, n.Range.Start.Column)
 
     let startOfSourceCode = 1

src/Fantomas.Core/Utils.fs

@@ -135,6 +135,15 @@ module List =
             s <- s'
             r)
 
+    let chooseState f state l =
+        let mutable s = state
+
+        l
+        |> List.choose (fun x ->
+            let s', r = f s x
+            s <- s'
+            r)
+
     let isNotEmpty l = (List.isEmpty >> not) l
 
     let moreThanOne =