Formatting single case DU on the same line. fsprojects#192

src/Fantomas.Tests/UnionTests.fs

@@ -126,8 +126,7 @@ let main argv =
    | _ -> 0""" config
     |> prepend newline
     |> should equal """
-type TestUnion =
-    | Test of A : int * B : int
+type TestUnion = Test of A : int * B : int
 
 [<EntryPoint>]
 let main argv =
@@ -154,4 +153,42 @@ type uColor =
 
 let col3 =
     Microsoft.FSharp.Core.LanguagePrimitives.EnumOfValue<uint32, uColor>(2u)
+"""
+
+[<Test>]
+let ``Single case DUs on same line`` () =
+    formatSourceString false """
+type CustomerId = 
+    | CustomerId of int
+    """ config
+    |> prepend newline
+    |> should equal """
+type CustomerId = CustomerId of int
+"""
+
+[<Test>]
+let ``Single case DU with private access modifier`` () =
+   formatSourceString false """
+type CustomerId =
+   private 
+   | CustomerId of int
+   """ config
+   |> prepend newline
+   |> should equal """
+type CustomerId = private CustomerId of int
+"""
+
+[<Test>]
+let ``Single case DU with member should be on a newline`` () =
+    formatSourceString false """
+type CustomerId =
+    | CustomerId of int
+    member this.Test() =
+        printfn "%A" this
+    """ config
+    |> prepend newline
+    |> should equal """
+type CustomerId =
+    | CustomerId of int
+    member this.Test() = printfn "%A" this
 """

src/Fantomas/CodePrinter.fs

@@ -8,6 +8,7 @@ open Fantomas
 open Fantomas.FormatConfig
 open Fantomas.SourceParser
 open Fantomas.SourceTransformer
+open Fantomas.SourceTransformer
 
 /// This type consists of contextual information which is important for formatting
 type ASTContext =
@@ -690,9 +691,18 @@ and genTypeDefn astContext (TypeDef(ats, px, ao, tds, tcs, tdr, ms, s)) =
         +> unindent
 
     | Simple(TDSRUnion(ao', xs)) ->
+        let unionCases =  
+            match xs with
+            | [] -> id
+            | [x] when List.isEmpty ms -> 
+                indent +> sepSpace +> opt sepSpace ao' genAccess
+                +> genUnionCase { astContext with HasVerticalBar = false } x
+            | xs ->
+                indent +> sepNln +> opt sepNln ao' genAccess 
+                +> col sepNln xs (genUnionCase { astContext with HasVerticalBar = true })
+
         typeName +> sepEq 
-        +> indent +> sepNln +> opt sepNln ao' genAccess 
-        +> col sepNln xs (genUnionCase { astContext with HasVerticalBar = true })
+        +> unionCases
         +> genMemberDefnList { astContext with IsInterface = false } ms
         +> unindent