Implement Array.singleton, List.singleton

Commits:
    implementing "singleton" for array and list

    Add surface area for singleton

    Add null test case for singleton

    Change 'yields' to 'contains' in docs, per code review

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/ArrayModule.fs

@@ -1193,6 +1193,13 @@ type ArrayModule() =
     member this.Partition () =
         this.PartitionTester Array.partition Array.partition    
 
+    [<Test>]
+    member this.Singleton() =
+        Assert.AreEqual([|null|],Array.singleton null)
+        Assert.AreEqual([|"1"|],Array.singleton "1")
+        Assert.AreEqual([|[]|],Array.singleton [])
+        Assert.AreEqual([|[||]|],Array.singleton [||])
+
 #if FX_NO_TPL_PARALLEL
 #else
     [<Test>]

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Collections/ListModule.fs

@@ -688,3 +688,10 @@ type ListModule() =
         let emptyList:int list = [ ]
         let resultEpt = List.contains 4 emptyList
         Assert.IsFalse(resultEpt)
+        
+    [<Test>]
+    member this.Singleton() =
+        Assert.AreEqual([null],List.singleton null)
+        Assert.AreEqual(["1"],List.singleton "1")
+        Assert.AreEqual([[]],List.singleton [])
+        Assert.AreEqual([[||]],List.singleton [||])
\ No newline at end of file

src/fsharp/FSharp.Core.Unittests/SurfaceArea.4.0.fs

@@ -155,6 +155,7 @@ Microsoft.FSharp.Collections.ArrayModule: T[] OfList[T](Microsoft.FSharp.Collect
 Microsoft.FSharp.Collections.ArrayModule: T[] OfSeq[T](System.Collections.Generic.IEnumerable`1[T])
 Microsoft.FSharp.Collections.ArrayModule: T[] Permute[T](Microsoft.FSharp.Core.FSharpFunc`2[System.Int32,System.Int32], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] Reverse[T](T[])
+Microsoft.FSharp.Collections.ArrayModule: T[] Singleton[T](T)
 Microsoft.FSharp.Collections.ArrayModule: T[] SortBy[T,TKey](Microsoft.FSharp.Core.FSharpFunc`2[T,TKey], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] SortWith[T](Microsoft.FSharp.Core.FSharpFunc`2[T,Microsoft.FSharp.Core.FSharpFunc`2[T,System.Int32]], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] Sort[T](T[])
@@ -292,6 +293,7 @@ Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Permute[T](Microsoft.FSharp.Core.FSharpFunc`2[System.Int32,System.Int32], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Replicate[T](Int32, T)
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Reverse[T](Microsoft.FSharp.Collections.FSharpList`1[T])
+Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Singleton[T](T)
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] SortBy[T,TKey](Microsoft.FSharp.Core.FSharpFunc`2[T,TKey], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] SortWith[T](Microsoft.FSharp.Core.FSharpFunc`2[T,Microsoft.FSharp.Core.FSharpFunc`2[T,System.Int32]], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Sort[T](Microsoft.FSharp.Collections.FSharpList`1[T])

src/fsharp/FSharp.Core.Unittests/SurfaceArea.Portable.fs

@@ -149,6 +149,7 @@ Microsoft.FSharp.Collections.ArrayModule: T[] OfList[T](Microsoft.FSharp.Collect
 Microsoft.FSharp.Collections.ArrayModule: T[] OfSeq[T](System.Collections.Generic.IEnumerable`1[T])
 Microsoft.FSharp.Collections.ArrayModule: T[] Permute[T](Microsoft.FSharp.Core.FSharpFunc`2[System.Int32,System.Int32], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] Reverse[T](T[])
+Microsoft.FSharp.Collections.ArrayModule: T[] Singleton[T](T)
 Microsoft.FSharp.Collections.ArrayModule: T[] SortBy[T,TKey](Microsoft.FSharp.Core.FSharpFunc`2[T,TKey], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] SortWith[T](Microsoft.FSharp.Core.FSharpFunc`2[T,Microsoft.FSharp.Core.FSharpFunc`2[T,System.Int32]], T[])
 Microsoft.FSharp.Collections.ArrayModule: T[] Sort[T](T[])
@@ -286,6 +287,7 @@ Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Permute[T](Microsoft.FSharp.Core.FSharpFunc`2[System.Int32,System.Int32], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Replicate[T](Int32, T)
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Reverse[T](Microsoft.FSharp.Collections.FSharpList`1[T])
+Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Singleton[T](T)
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] SortBy[T,TKey](Microsoft.FSharp.Core.FSharpFunc`2[T,TKey], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] SortWith[T](Microsoft.FSharp.Core.FSharpFunc`2[T,Microsoft.FSharp.Core.FSharpFunc`2[T,System.Int32]], Microsoft.FSharp.Collections.FSharpList`1[T])
 Microsoft.FSharp.Collections.ListModule: Microsoft.FSharp.Collections.FSharpList`1[T] Sort[T](Microsoft.FSharp.Collections.FSharpList`1[T])

src/fsharp/FSharp.Core/array.fs

@@ -470,6 +470,9 @@ namespace Microsoft.FSharp.Collections
             let len = array.Length
             scanSubRight f array 0 (len - 1) acc
 
+        [<CompiledName("Singleton")>]
+        let inline singleton value = [|value|]
+
         [<CompiledName("Reduce")>]
         let reduce f (array : _[]) = 
             checkNonNull "array" array

src/fsharp/FSharp.Core/array.fsi

@@ -500,6 +500,14 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("ScanBack")>]
         val scanBack<'T,'State> : folder:('T -> 'State -> 'State) -> array:'T[] -> state:'State -> 'State[]
 
+        /// <summary>Returns an array that contains one item only.</summary>
+        ///
+        /// <param name="value">The input item.</param>
+        ///
+        /// <returns>The result array of one item.</returns>
+        [<CompiledName("Singleton")>]
+        val inline singleton: value:'T -> 'T[]
+
         /// <summary>Sets an element of an array.</summary>
         /// <param name="array">The input array.</param>
         /// <param name="index">The input index.</param>

src/fsharp/FSharp.Core/list.fs

@@ -160,6 +160,9 @@ namespace Microsoft.FSharp.Collections
                 | (h::t) -> let s = f.Invoke(s,h) in loop s t (s :: acc)
             loop s list [s]
 
+        [<CompiledName("Singleton")>]
+        let inline singleton value = [value]
+
         [<CompiledName("Fold2")>]
         let fold2<'T1,'T2,'State> f (acc:'State) (list1:list<'T1>) (list2:list<'T2>) = 
             let f = OptimizedClosures.FSharpFunc<_,_,_,_>.Adapt(f)

src/fsharp/FSharp.Core/list.fsi

@@ -475,6 +475,14 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("ScanBack")>]
         val scanBack<'T,'State> : folder:('T -> 'State -> 'State) -> list:'T list -> state:'State -> 'State list
 
+        /// <summary>Returns a list that contains one item only.</summary>
+        ///
+        /// <param name="value">The input item.</param>
+        ///
+        /// <returns>The result list of one item.</returns>
+        [<CompiledName("Singleton")>]
+        val inline singleton: value:'T -> 'T list
+
         /// <summary>Sorts the given list using the given comparison function.</summary>
         ///
         /// <remarks>This is a stable sort, i.e. the original order of equal elements is preserved.</remarks>