From 85432ccfb6f2d1c8049c5bf45c7fa123fcf9928c Mon Sep 17 00:00:00 2001
From: liboz <ringil.sword@gmail.com>
Date: Thu, 13 Oct 2016 06:19:09 -0400
Subject: [PATCH] except

---
 src/fsharp/FSharp.Core/seq.fs | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/src/fsharp/FSharp.Core/seq.fs b/src/fsharp/FSharp.Core/seq.fs
index 429525ff527..31c7afea626 100644
--- a/src/fsharp/FSharp.Core/seq.fs
+++ b/src/fsharp/FSharp.Core/seq.fs
@@ -697,6 +697,10 @@ namespace Microsoft.FSharp.Collections
             and DistinctByFactory<'T,'Key when 'Key: equality> (keyFunction:'T-> 'Key) =
                 inherit SeqComponentFactory<'T,'T> ()
                 override __.Create<'V> (_result:Result<'V>) (next:SeqComponent<'T,'V>) : SeqComponent<'T,'V> = upcast DistinctBy (keyFunction, next) 
+            
+            and ExceptFactory<'T when 'T: equality> (itemsToExclude: seq<'T>) =
+                inherit SeqComponentFactory<'T,'T> ()
+                override __.Create<'V> (_result:Result<'V>) (next:SeqComponent<'T,'V>) : SeqComponent<'T,'V> = upcast Except (itemsToExclude, next) 
 
             and FilterFactory<'T> (filter:'T->bool) =
                 inherit SeqComponentFactory<'T,'T> ()
@@ -791,6 +795,17 @@ namespace Microsoft.FSharp.Collections
                     else
                         false
 
+            and Except<'T,'V when 'T: equality> (itemsToExclude: seq<'T>, next:SeqComponent<'T,'V>) =
+                inherit SeqComponent<'T,'V>(next)
+
+                let cached = lazy(HashSet(itemsToExclude, HashIdentity.Structural))
+
+                override __.ProcessNext (input:'T) : bool = 
+                    if cached.Value.Add input then
+                        Helpers.avoidTailCall (next.ProcessNext input)
+                    else
+                        false
+
             and Filter<'T,'V> (filter:'T->bool, next:SeqComponent<'T,'V>) =
                 inherit SeqComponent<'T,'V>(next)
 
@@ -2311,17 +2326,7 @@ namespace Microsoft.FSharp.Collections
         [<CompiledName("Except")>]
         let except (itemsToExclude: seq<'T>) (source: seq<'T>) =
             checkNonNull "itemsToExclude" itemsToExclude
-            checkNonNull "source" source
-
-            seq {
-                use e = source.GetEnumerator()
-                if e.MoveNext() then
-                    let cached = HashSet(itemsToExclude, HashIdentity.Structural)
-                    let next = e.Current
-                    if (cached.Add next) then yield next
-                    while e.MoveNext() do
-                        let next = e.Current
-                        if (cached.Add next) then yield next }
+            source |> seqFactory (SeqComposer.ExceptFactory itemsToExclude)
 
         [<CompiledName("ChunkBySize")>]
         let chunkBySize chunkSize (source : seq<_>) =