No newline before lambda unless really needed. Fix 503 (#529)

* Tests

* autoNlnByFutureLazy for App(Lambda). Fix #503

src/Fantomas.Tests/AppTests.fs

@@ -0,0 +1,22 @@
+module Fantomas.Tests.AppTests
+
+open NUnit.Framework
+open FsUnit
+open Fantomas.Tests.TestHelper
+
+// the current behavior results in a compile error since the |> is merged to the last line 
+[<Test>]
+let ``no nln before lambda #503``() =
+    formatSourceString false """
+let a =
+    b
+    |> List.exists (fun p ->
+        p.a && p.b |> List.exists (fun o -> o.a = "lorem ipsum dolor sit amet"))
+    """ { config with PageWidth = 80 }
+    |> prepend newline
+    |> should equal """
+let a =
+    b
+    |> List.exists (fun p ->
+        p.a && p.b |> List.exists (fun o -> o.a = "lorem ipsum dolor sit amet"))
+"""

src/Fantomas.Tests/Fantomas.Tests.fsproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\netfx.props" />
   <PropertyGroup>
     <Version>3.0.0</Version>
@@ -20,6 +20,7 @@
     <Compile Include="ComparisonTests.fs" />
     <Compile Include="ControlStructureTests.fs" />
     <Compile Include="PipingTests.fs" />
+    <Compile Include="AppTests.fs" />
     <Compile Include="ModuleTests.fs" />
     <Compile Include="UnionTests.fs" />
     <Compile Include="RecordTests.fs" />

src/Fantomas.Tests/PatternMatchingTests.fs

@@ -201,8 +201,7 @@ with
     |> should equal """
 try
     fst
-        (find
-            (fun (s, (s', ty): int * int) ->
+        (find (fun (s, (s', ty): int * int) ->
             s' = s0 && can (type_match ty ty0) []) (!the_interface))
 with Failure _ -> s0
 """

src/Fantomas/CodePrinter.fs

@@ -730,10 +730,10 @@ and genExpr astContext synExpr =
     let appNlnFun e =
         match e with
         | CompExpr _
-        | Lambda _
         | MatchLambda _
-        | Paren (Lambda _)
         | Paren (MatchLambda _) -> autoNln
+        | Lambda _
+        | Paren (Lambda _) -> autoNlnByFutureLazy
         | _ -> autoNlnByFuture
 
     let kw tokenName f = tokN synExpr.Range tokenName f

src/Fantomas/Context.fs

@@ -399,7 +399,7 @@ let internal autoNlnCheck f sep (ctx : Context) =
     col > ctx.Config.PageWidth
 
 let internal futureNlnCheckMem = Cache.memoizeBy (fun (f, ctx : Context) -> Cache.LambdaEqByRef f, ctx.MemoizeProjection) <| fun (f, ctx) ->
-    if ctx.Writer.IsDummy || not ctx.BreakLines then false else
+    if ctx.Writer.IsDummy || not ctx.BreakLines then (false, false) else
     // Create a dummy context to evaluate length of current operation
     use colWriter = new ColumnIndentedTextWriter(new StringWriter(), isDummy = true)
     let dummyCtx = ctx.With(colWriter, true)
@@ -410,13 +410,22 @@ let internal futureNlnCheckMem = Cache.memoizeBy (fun (f, ctx : Context) -> Cach
         |> Seq.indexed |> Seq.filter (fun (i, _) -> i % 2 = 0) |> Seq.map snd |> String.concat System.String.Empty
     let lines = withoutStringConst.Split([|Environment.NewLine|], StringSplitOptions.None) 
 
-    (lines |> Seq.length) >= 2 || writer.Column > ctx.Config.PageWidth
+    (lines |> Seq.length) >= 2, writer.Column > ctx.Config.PageWidth
 
-let internal futureNlnCheck f (ctx : Context) = futureNlnCheckMem (f, ctx)
+let internal futureNlnCheck f (ctx : Context) =
+    let (isMultiLine, isLong) = futureNlnCheckMem (f, ctx)
+    isMultiLine || isLong
+
+let internal futureNlnCheckLazy f (ctx : Context) =
+    let (isMultiLine, isLong) = futureNlnCheckMem (f, ctx)
+    if isMultiLine then false else isLong
 
 let internal autoNlnByFuture f = ifElseCtx (futureNlnCheck f) (sepNln +> f) f
 let internal autoIndentNlnByFuture f = ifElseCtx (futureNlnCheck f) (indent +> sepNln +> f +> unindent) f
 
+/// like autoNlnByFuture but don't do nln if there is another nln inside f
+let internal autoNlnByFutureLazy f = ifElseCtx (futureNlnCheckLazy f) (sepNln +> f) f
+
 /// Set a checkpoint to break at an appropriate column
 let internal autoNlnOrAddSep f sep (ctx : Context) =
     let isNln = autoNlnCheck f sep ctx