Provided an optimized GetEnumerator for scan

Author: manofstick

src/fsharp/FSharp.Core/iseq.fs

@@ -108,6 +108,20 @@ namespace Microsoft.FSharp.Collections
                     member __.GetHashCode o    = c.GetHashCode o._1
                     member __.Equals (lhs,rhs) = c.Equals (lhs._1, rhs._1) }
 
+        [<AbstractClass>]
+        type PreferGetEnumerator<'T>() =
+            inherit EnumerableBase<'T>()
+
+            abstract GetEnumerator: unit -> IEnumerator<'T>
+            abstract GetSeq : unit -> ISeq<'T>
+
+            interface IEnumerable<'T> with
+                member this.GetEnumerator () : IEnumerator<'T> = this.GetEnumerator ()
+
+            interface ISeq<'T> with
+                member this.PushTransform<'U> (next:ITransformFactory<'T,'U>) : ISeq<'U> = (this.GetSeq()).PushTransform next
+                member this.Fold<'Result> (f:PipeIdx->Folder<'T,'Result>) : 'Result = (this.GetSeq()).Fold f
+
         [<CompiledName "Empty">]
         let empty<'T> = Microsoft.FSharp.Collections.SeqComposition.Core.EmptyEnumerable<'T>.Instance
 
@@ -592,17 +606,79 @@ namespace Microsoft.FSharp.Collections
         let concat (sources:ISeq<#ISeq<'T>>) : ISeq<'T> =
             upcast (ThinConcatEnumerable (sources, id))
 
+        (*
+            Represents the following seq comprehension, but they don't work at this level
+
+            seq {
+                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt folder 
+                let mutable state = initialState
+                yield state
+                for item in enumerable do
+                    state <- f.Invoke (state, item)
+                    yield state }
+        *)
+        type ScanEnumerator<'T,'State>(folder:'State->'T->'State, initialState:'State, enumerable:seq<'T>) =
+            let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt folder
+
+            let mutable state      = 0 (*Pre-start*)
+            let mutable enumerator = Unchecked.defaultof<IEnumerator<'T>>
+            let mutable current    = initialState
+
+            interface IEnumerator<'State> with
+                member this.Current: 'State =
+                    match state with
+                    | 0(*PreStart*)      -> notStarted()
+                    | 1(*GetEnumerator*) -> current
+                    | 2(*MoveNext*)      -> current
+                    | _(*Finished*)      -> alreadyFinished()
+
+            interface IEnumerator with
+                member this.Current : obj =
+                    box (this:>IEnumerator<'State>).Current
+                
+                member this.MoveNext () : bool = 
+                    match state with
+                    | 0(*PreStart*) ->
+                        state <- 1(*GetEnumerator*)
+                        true
+                    | 1(*GetEnumerator*) ->
+                        enumerator <- enumerable.GetEnumerator ()
+                        state <- 2(*MoveNext*)
+                        (this:>IEnumerator).MoveNext ()
+                    | 2(*MoveNext*) ->
+                        if enumerator.MoveNext () then
+                            current <- f.Invoke (current, enumerator.Current)
+                            true
+                        else
+                            current <- Unchecked.defaultof<_>
+                            state <- 3(*Finished*)
+                            false
+                    | _(*Finished*) -> alreadyFinished()
+                    
+                member this.Reset () : unit = noReset ()
+
+            interface IDisposable with
+                member this.Dispose(): unit = 
+                    if isNotNull enumerator then
+                        enumerator.Dispose ()
+
         [<CompiledName "Scan">]
-        let inline scan (folder:'State->'T->'State) (initialState:'State) (source:ISeq<'T>) :ISeq<'State> =
-            let head = singleton initialState
-            let tail = 
-                source.PushTransform { new ITransformFactory<'T,'State> with
-                    override __.Compose _ _ next =
-                        upcast { new Transform<'T,'V,'State>(next, initialState) with
-                            override this.ProcessNext (input:'T) : bool =
-                                this.State <- folder this.State input
-                                TailCall.avoid (next.ProcessNext this.State) } }
-            concat (ofList [ head ; tail ])
+        let scan (folder:'State->'T->'State) (initialState:'State) (source:ISeq<'T>) : ISeq<'State> =
+            upcast { new PreferGetEnumerator<'State>() with
+                member this.GetEnumerator () =
+                    upcast new ScanEnumerator<'T,'State>(folder, initialState, source)
+
+                member this.GetSeq () = 
+                    let head = singleton initialState
+                    let tail = 
+                        source.PushTransform { new ITransformFactory<'T,'State> with
+                            override __.Compose _ _ next =
+                                let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt folder
+                                upcast { new Transform<'T,'V,'State>(next, initialState) with
+                                    override this.ProcessNext (input:'T) : bool =
+                                        this.State <- f.Invoke (this.State, input)
+                                        TailCall.avoid (next.ProcessNext this.State) } }
+                    concat (ofList [ head ; tail ]) }
 
         [<CompiledName "Skip">]
         let skip (skipCount:int) (source:ISeq<'T>) : ISeq<'T> =

src/fsharp/FSharp.Core/iseq.fsi

@@ -217,7 +217,7 @@ namespace Microsoft.FSharp.Collections
     val inline reduce : f:('T->'T->'T) -> source:ISeq<'T> -> 'T
 
     [<CompiledName "Scan">]
-    val inline scan : folder:('State->'T->'State) -> initialState:'State -> source:ISeq<'T> -> ISeq<'State>
+    val scan : folder:('State->'T->'State) -> initialState:'State -> source:ISeq<'T> -> ISeq<'State>
 
     [<CompiledName "Skip">]
     val skip : skipCount:int -> source:ISeq<'T> -> ISeq<'T>

src/fsharp/FSharp.Core/seqcore.fsi

@@ -74,6 +74,9 @@ namespace Microsoft.FSharp.Collections.SeqComposition
         abstract member Append : ISeq<'T> -> ISeq<'T>
         abstract member Length : unit -> int
         abstract member GetRaw : unit -> seq<'T>
+        default Append : ISeq<'T> -> ISeq<'T>
+        default Length : unit -> int
+        default GetRaw : unit -> seq<'T>
         interface ISeq<'T>
 
     [<AbstractClass>]