Force infix operator on a new line if a lambda is involved (#959)

* Force infix expressions on next line for certain operators.

* Revert "Force infix expressions on next line for certain operators."

This reverts commit 3f9ac69.

* Force infix operator on a new line if a lambda is involved. Fixes #942

src/Fantomas.Tests/OperatorTests.fs

@@ -453,3 +453,29 @@ let ``addition via function`` () =
     |> should equal """
 let a = (+) 7 8
 """
+
+[<Test>]
+let ``lambda piped to lambda should be multiline, 942`` () =
+    formatSourceString false """
+let r (f : 'a -> 'b) (a : 'a) : 'b =
+    fun () ->
+        f a
+    |> fun f -> f ()
+"""  config
+    |> prepend newline
+    |> should equal """
+let r (f: 'a -> 'b) (a: 'a): 'b =
+    fun () -> f a
+    |> fun f -> f ()
+"""
+
+[<Test>]
+let ``combining lines breaks function precedence 488`` () =
+    formatSourceString false """fun () -> ()
+|> Some
+"""  config
+    |> prepend newline
+    |> should equal """
+fun () -> ()
+|> Some
+"""

src/Fantomas/CodePrinter.fs

@@ -1231,15 +1231,21 @@ and genExpr astContext synExpr =
             | (s,_,_)::_ when ((noSpaceInfixOps.Contains s)) -> sepNone
             | _ -> f
 
+        let isLambda e = match e with | Lambda _ -> true | _ -> false
         let expr = genExpr astContext e
+        let shortExpr = expr +> sepAfterExpr sepSpace +> genInfixAppsShort astContext es
+        let longExpr = expr +> sepAfterExpr sepNln +> genInfixApps astContext es
 
         atCurrentColumn
             (fun ctx ->
-                 isShortExpression
-                    ctx.Config.MaxInfixOperatorExpression
-                    (expr +> sepAfterExpr sepSpace +> genInfixAppsShort astContext es)
-                    (expr +> sepAfterExpr sepNln +> genInfixApps astContext es)
-                    ctx)
+                 if isLambda e || List.exists (fun (_,_,e) -> isLambda e) es then
+                    longExpr ctx
+                 else
+                     isShortExpression
+                        ctx.Config.MaxInfixOperatorExpression
+                        shortExpr
+                        longExpr
+                        ctx)
 
     | TernaryApp(e1,e2,e3) ->
         atCurrentColumn (genExpr astContext e1 +> !- "?" +> genExpr astContext e2 +> sepSpace +> !- "<-" +> sepSpace +> genExpr astContext e3)