From bba7df44dd0a1efc6329996eb06aa97416832db1 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Mon, 3 Aug 2020 23:47:45 -0700
Subject: [PATCH 1/5] Move List object to List module

---
 src/Fable.Transforms/Fable2Babel.fs           |  16 +-
 src/Fable.Transforms/Replacements.fs          |  24 +-
 src/fable-library/Array.fs                    |  11 +-
 src/fable-library/List.fs                     | 651 +++++++++---------
 src/fable-library/Types.ts                    | 238 +++----
 .../test/bench-compiler/package.json          |   2 +
 tests/Main/ComparisonTests.fs                 |   4 +
 7 files changed, 490 insertions(+), 456 deletions(-)

diff --git a/src/Fable.Transforms/Fable2Babel.fs b/src/Fable.Transforms/Fable2Babel.fs
index 796853a0cd..094ee178cb 100644
--- a/src/Fable.Transforms/Fable2Babel.fs
+++ b/src/Fable.Transforms/Fable2Babel.fs
@@ -373,7 +373,7 @@ module Util =
             makeNativeTypeAnnotation com ctx [genArg] "Array"
 
     and makeListTypeAnnotation com ctx genArg =
-        makeImportTypeAnnotation com ctx [genArg] "Types" "List"
+        makeImportTypeAnnotation com ctx [genArg] "List" "List"
 
     and makeUnionTypeAnnotation com ctx genArgs =
         List.map (typeAnnotation com ctx) genArgs
@@ -809,12 +809,12 @@ module Util =
         // Optimization for bundle size: compile list literals as List.ofArray
         | Replacements.ListLiteral(exprs, t) ->
             [|List.rev exprs |> makeArray com ctx|]
-            |> coreLibCall com ctx r "Types" "newList"
+            |> coreLibCall com ctx r "List" "newList"
         | Fable.NewList (headAndTail, _) ->
             match headAndTail with
-            | None -> coreLibCall com ctx r "Types" "newList" [||]
+            | None -> coreLibCall com ctx r "List" "newList" [||]
             | Some(TransformExpr com ctx head, TransformExpr com ctx tail) ->
-                coreLibCall com ctx r "Types" "cons" [|head; tail|]
+                coreLibCall com ctx r "List" "cons" [|head; tail|]
         | Fable.NewOption (value, t) ->
             match value with
             | Some (TransformExpr com ctx e) ->
@@ -1054,8 +1054,8 @@ module Util =
         let expr = com.TransformAsExpr(ctx, fableExpr)
         match getKind with
         | Fable.ExprGet(TransformExpr com ctx prop) -> getExpr range expr prop
-        | Fable.ListHead -> get range expr "Head"
-        | Fable.ListTail -> get range expr "Tail"
+        | Fable.ListHead -> coreLibCall com ctx range "List" "head" [|expr|]
+        | Fable.ListTail -> coreLibCall com ctx range "List" "tail" [|expr|]
         | Fable.FieldGet(fieldName,_,_) ->
             let expr =
                 match fableExpr with
@@ -1151,7 +1151,7 @@ module Util =
         | Fable.FunctionType _ -> jsTypeof "function" expr
         | Fable.Array _ | Fable.Tuple _ ->
             coreLibCall com ctx None "Util" "isArrayLike" [|com.TransformAsExpr(ctx, expr)|]
-        | Fable.List _ -> jsInstanceof (coreValue com ctx "Types" "List") expr
+        | Fable.List _ -> jsInstanceof (coreValue com ctx "List" "List") expr
         | Replacements.Builtin kind ->
             match kind with
             | Replacements.BclGuid -> jsTypeof "string" expr
@@ -1212,7 +1212,7 @@ module Util =
             let op = if nonEmpty then BinaryUnequal else BinaryEqual
             upcast BinaryExpression(op, com.TransformAsExpr(ctx, expr), NullLiteral(), ?loc=range)
         | Fable.ListTest nonEmpty ->
-            let expr = get range (com.TransformAsExpr(ctx, expr)) "IsEmpty"
+            let expr = coreLibCall com ctx range "List" "isEmpty" [|com.TransformAsExpr(ctx, expr)|]
             if nonEmpty then upcast UnaryExpression(UnaryNot, expr, ?loc=range)
             else expr
         | Fable.UnionCaseTest(uci, ent) ->
diff --git a/src/Fable.Transforms/Replacements.fs b/src/Fable.Transforms/Replacements.fs
index 652eb41bf1..3819f885ac 100644
--- a/src/Fable.Transforms/Replacements.fs
+++ b/src/Fable.Transforms/Replacements.fs
@@ -717,6 +717,8 @@ let identityHash r (arg: Expr) =
         Helper.CoreCall("Util", "structuralHash", Number Int32, [arg], ?loc=r)
     | DeclaredType(ent,_) when ent.IsFSharpUnion || ent.IsFSharpRecord || ent.IsValueType ->
         Helper.CoreCall("Util", "structuralHash", Number Int32, [arg], ?loc=r)
+    | DeclaredType(ent,_) ->
+        Helper.InstanceCall(arg, "GetHashCode", Number Int32, [], ?loc=r)
     | _ ->
         Helper.CoreCall("Util", "identityHash", Number Int32, [arg], ?loc=r)
 
@@ -1838,30 +1840,20 @@ let arrayModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Ex
         Helper.CoreCall("Array", meth, t, args, i.SignatureArgTypes, ?loc=r) |> Some
 
 let lists (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (thisArg: Expr option) (args: Expr list) =
+    let meth = Naming.removeGetSetPrefix i.CompiledName |> Naming.lowerFirst
     match i.CompiledName, thisArg, args with
-    // Use methods for Head and Tail (instead of Get(ListHead) for example) to check for empty lists
-    | ("get_Head" | "get_Tail" | "get_Length" | "get_IsEmpty"), Some callee, _ ->
-        let meth = Naming.removeGetSetPrefix i.CompiledName
-        get r t callee meth |> Some
-    | "get_Item", Some callee, _ ->
-        let meth = Naming.removeGetSetPrefix i.CompiledName
-        Helper.InstanceCall(callee, meth, t, args, i.SignatureArgTypes, ?loc=r) |> Some
-    | "GetSlice", Some x, _ ->
-        let meth = Naming.lowerFirst i.CompiledName
-        let args = match args with [ExprType Unit] -> [x] | args -> args @ [x]
+    | ("get_Head" | "get_Tail" | "get_Length" | "get_IsEmpty"), Some x, _ ->
+        Helper.CoreCall("List", meth, t, [x], i.SignatureArgTypes, ?loc=r) |> Some
+    | ("get_Item" | "GetSlice"), Some x, _ ->
+        Helper.CoreCall("List", meth, t, args @ [x], i.SignatureArgTypes, ?loc=r) |> Some
+    | ("get_Empty" | "Cons"), None, _ ->
         Helper.CoreCall("List", meth, t, args, i.SignatureArgTypes, ?loc=r) |> Some
-    | "get_Empty", None, _ -> NewList(None, (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
-    | "Cons", None, [h;t] -> NewList(Some(h,t), (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
     | ("GetHashCode" | "Equals" | "CompareTo"), Some callee, _ ->
         Helper.InstanceCall(callee, i.CompiledName, t, args, i.SignatureArgTypes, ?loc=r) |> Some
     | _ -> None
 
 let listModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Expr option) (args: Expr list) =
     match i.CompiledName, args with
-    | "IsEmpty", [x] -> Test(x, ListTest false, r) |> Some
-    | "Empty", _ -> NewList(None, (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
-    | "Singleton", [x] ->
-        NewList(Some(x, Value(NewList(None, t), None)), (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
     // Use a cast to give it better chances of optimization (e.g. converting list
     // literals to arrays) after the beta reduction pass
     | "ToSeq", [x] -> toSeq t x |> Some
diff --git a/src/fable-library/Array.fs b/src/fable-library/Array.fs
index 5710ffd995..8909faf584 100644
--- a/src/fable-library/Array.fs
+++ b/src/fable-library/Array.fs
@@ -123,7 +123,6 @@ module Helpers =
 open Helpers
 
 let private indexNotFoundMsg = "An index satisfying the predicate was not found in the collection."
-let inline indexNotFound() = failwith indexNotFoundMsg
 
 // Pay attention when benchmarking to append and filter functions below
 // if implementing via native JS array .concat() and .filter() do not fall behind due to js-native transitions.
@@ -478,7 +477,7 @@ let partition (f: 'T -> bool) (source: 'T[]) ([<Inject>] cons: IArrayCons<'T>) =
 let find (predicate: 'T -> bool) (array: 'T[]): 'T =
     match findImpl predicate array with
     | Some res -> res
-    | None -> indexNotFound()
+    | None -> failwith indexNotFoundMsg
 
 let tryFind (predicate: 'T -> bool) (array: 'T[]): 'T option =
     findImpl predicate array
@@ -486,7 +485,7 @@ let tryFind (predicate: 'T -> bool) (array: 'T[]): 'T option =
 let findIndex (predicate: 'T -> bool) (array: 'T[]): int =
     match findIndexImpl predicate array with
     | index when index > -1 -> index
-    | _ -> indexNotFound()
+    | _ -> failwith indexNotFoundMsg
 
 let tryFindIndex (predicate: 'T -> bool) (array: 'T[]): int option =
     match findIndexImpl predicate array with
@@ -496,7 +495,7 @@ let tryFindIndex (predicate: 'T -> bool) (array: 'T[]): int option =
 let pick chooser (array: _[]) =
     let rec loop i =
         if i >= array.Length then
-            indexNotFound()
+            failwith indexNotFoundMsg
         else
             match chooser array.[i] with
             | None -> loop(i+1)
@@ -513,7 +512,7 @@ let tryPick chooser (array: _[]) =
 
 let findBack predicate (array: _[]) =
     let rec loop i =
-        if i < 0 then indexNotFound()
+        if i < 0 then failwith indexNotFoundMsg
         elif predicate array.[i] then array.[i]
         else loop (i - 1)
     loop (array.Length - 1)
@@ -534,7 +533,7 @@ let findLastIndex predicate (array: _[]) =
 
 let findIndexBack predicate (array: _[]) =
     let rec loop i =
-        if i < 0 then indexNotFound()
+        if i < 0 then failwith indexNotFoundMsg
         elif predicate array.[i] then i
         else loop (i - 1)
     loop (array.Length - 1)
diff --git a/src/fable-library/List.fs b/src/fable-library/List.fs
index c266ca9be0..c84aad202b 100644
--- a/src/fable-library/List.fs
+++ b/src/fable-library/List.fs
@@ -3,156 +3,237 @@ module List
 // Disables warn:1204 raised by use of LanguagePrimitives.ErrorStrings.*
 #nowarn "1204"
 
-open System.Collections.Generic
 open Fable.Core
 
-let head = function
-    | x::_ -> x
-    | _ -> failwith "List was empty"
+let msgListWasEmpty = "List was empty"
+let msgListNoMatch = "List did not contain any matching elements"
+
+type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
+    let mutable hashCode = None
+
+    static member Empty = new List<'T>(0, ResizeArray<'T>())
+    static member internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
+
+    member inline internal _.Add(x: 'T) =
+        let values =
+            if Count = Values.Count
+            then Values
+            else Values.GetRange(0, Count)
+        values.Add(x)
+        new List<'T>(values.Count, values)
+
+    member inline _.IsEmpty = Count <= 0
+    member inline _.Length = Count
+
+    member _.Head =
+        if Count > 0
+        then Values.[Count - 1]
+        else failwith msgListWasEmpty
+
+    member _.Tail =
+        if Count > 0
+        then new List<'T>(Count - 1, Values)
+        else failwith msgListWasEmpty
+
+    member inline _.Item with get(index) =
+        Values.[Count - 1 - index]
+
+    override xs.ToString() =
+        "[" + System.String.Join("; ", xs) + "]"
+
+    override xs.Equals(other: obj) =
+        let ys = other :?> 'T list
+        if xs.Length <> ys.Length then false
+        elif xs.GetHashCode() <> ys.GetHashCode() then false
+        else Seq.forall2 (Unchecked.equals) xs ys
+        // else (xs :> System.IComparable).CompareTo(other) = 0
+
+    override xs.GetHashCode() =
+        match hashCode with
+        | Some h -> h
+        | None ->
+            let inline combineHash i x y = (x <<< 1) + y + 631 * i
+            let len = min (xs.Length - 1) 18 // limit the hash count
+            let mutable h = 0
+            for i = 0 to len do
+                h <- combineHash i h (hash xs.[i])
+            hashCode <- Some h
+            h
+
+    interface System.IComparable with
+        member xs.CompareTo(other: obj) =
+            Seq.compareWith compare xs (other :?> 'T list)
+            // List.CompareWith compare xs (other :?> 'T list)
+
+    interface System.Collections.Generic.IEnumerable<'T> with
+        member xs.GetEnumerator(): System.Collections.Generic.IEnumerator<'T> =
+            let elems = seq { for i=xs.Length - 1 downto 0 do yield Values.[i] }
+            elems.GetEnumerator()
+
+            // new ListEnumerator<'T>(xs) :> System.Collections.Generic.IEnumerator<'T>
+
+            // let mutable i = Count
+            // {
+            //     new System.Collections.Generic.IEnumerator<'T> with
+            //         member _.Current = Values.[i]
+            //     interface System.Collections.IEnumerator with
+            //         member _.Current: obj = box (Values.[i])
+            //         member _.MoveNext() = i <- i - 1; i >= 0
+            //         member _.Reset() = i <- Count
+            //     interface System.IDisposable with
+            //         member _.Dispose(): unit = ()
+            // }
+
+    interface System.Collections.IEnumerable with
+        member xs.GetEnumerator(): System.Collections.IEnumerator =
+            ((xs :> System.Collections.Generic.IEnumerable<'T>).GetEnumerator() :> System.Collections.IEnumerator)
+
+    // static member internal CompareWith (comparer: 'T -> 'T -> int) (xs: 'T list) (ys: 'T list): int =
+    //     if obj.ReferenceEquals(xs, ys)
+    //     then 0
+    //     else
+    //         if xs.IsEmpty then
+    //             if ys.IsEmpty then 0 else -1
+    //         elif ys.IsEmpty then 1
+    //         else
+    //             let mutable i = 0
+    //             let mutable result = 0
+    //             if xs.Length > ys.Length then 1
+    //             elif xs.Length < ys.Length then -1
+    //             else
+    //                 while i < xs.Length && result = 0 do
+    //                     result <- comparer xs.[i] ys.[i]
+    //                     i <- i + 1
+    //                 result
+
+// and ListEnumerator<'T when 'T: comparison>(xs: List<'T>) =
+//     let mutable i = -1
+//     interface System.Collections.Generic.IEnumerator<'T> with
+//         member __.Current = xs.[i]
+//     interface System.Collections.IEnumerator with
+//         member __.Current = box (xs.[i])
+//         member __.MoveNext() = i <- i + 1; i < xs.Length
+//         member __.Reset() = i <- -1
+//     interface System.IDisposable with
+//         member __.Dispose() = ()
+
+and 'T list when 'T: comparison = List<'T>
+
+let newList (values: ResizeArray<'T>) = new List<'T>(values.Count, values)
+
+let empty () = List.Empty
+
+let cons (x: 'T) (xs: 'T list) = List.Cons (x, xs)
+
+let singleton x = cons x List.Empty
+
+let isEmpty (xs: 'T list) = xs.IsEmpty
+
+let length (xs: 'T list) = xs.Length
+
+let head (xs: 'T list) = xs.Head
+
+let tryHead (xs: 'T list) =
+    if xs.Length > 0
+    then Some xs.[0]
+    else None
+
+let tail (xs: 'T list) = xs.Tail
+
+let last (xs: 'T list) =
+    if xs.Length > 0
+    then xs.[xs.Length - 1]
+    else failwith msgListWasEmpty
+
+let tryLast (xs: 'T list) =
+    if xs.Length > 0
+    then Some xs.[xs.Length - 1]
+    else None
 
-let tryHead = function
-    | x::_ -> Some x
-    | _ -> None
-
-let tail = function
-    | _::xs -> xs
-    | _ -> failwith "List was empty"
+let compareWith (comparer: 'T -> 'T -> int) (xs: 'T list) (ys: 'T list): int =
+    Seq.compareWith comparer xs ys
+    //List.CompareWith comparer xs ys
 
-let rec last = function
-    | [] -> failwith "List was empty"
-    | [x] -> x
-    | _::xs -> last xs
+let fold (folder: 'acc -> 'T -> 'acc) (state: 'acc) (xs: 'T list) =
+    let mutable acc = state
+    for i = 0 to xs.Length - 1 do
+        acc <- folder acc xs.[i]
+    acc
 
-let rec tryLast = function
-    | [] -> None
-    | [x] -> Some x
-    | _::xs -> tryLast xs
+let foldBack (folder: 'T -> 'acc -> 'acc) (xs: 'T list) (state: 'acc) =
+    let mutable acc = state
+    for i = xs.Length - 1 downto 0 do
+        acc <- folder xs.[i] acc
+    acc
 
-let compareWith (comparer: 'T -> 'T -> int) (xs: 'T list) (ys: 'T list): int =
-    if obj.ReferenceEquals(xs, ys)
-    then 0
-    else
-        let rec loop xs ys =
-            match xs, ys with
-            | [], [] -> 0
-            | [], _ -> -1
-            | _, [] -> 1
-            | x::xs, y::ys ->
-                match comparer x y with
-                | 0 -> loop xs ys
-                | res -> res
-        loop xs ys
-
-let rec foldIndexedAux f i acc = function
-    | [] -> acc
-    | x::xs -> foldIndexedAux f (i+1) (f i acc x) xs
-
-let foldIndexed<'a,'acc> f (state: 'acc) (xs: 'a list) =
-    foldIndexedAux f 0 state xs
-
-let rec fold<'a,'acc> f (state: 'acc) (xs: 'a list) =
-    match xs with
-    | [] -> state
-    | h::t -> fold f (f state h) t
-
-let reverse xs =
-    fold (fun acc x -> x::acc) [] xs
-
-let foldBack<'a,'acc> f (xs: 'a list) (state: 'acc) =
-    fold (fun acc x -> f x acc) state (reverse xs)
+let reverse (xs: 'a list) =
+    fold (fun acc x -> cons x acc) List.Empty xs
 
 let toSeq (xs: 'a list): 'a seq =
     Seq.map id xs
 
 let ofSeq (xs: 'a seq): 'a list =
-    Seq.fold (fun acc x -> x::acc) [] xs
+    Seq.fold (fun acc x -> cons x acc) List.Empty xs
     |> reverse
 
 let concat (lists: seq<'a list>) =
-    Seq.fold (fold (fun acc x -> x::acc)) [] lists
+    Seq.fold (fold (fun acc x -> cons x acc)) List.Empty lists
     |> reverse
 
-let rec foldIndexed2Aux f i acc bs cs =
-    match bs, cs with
-    | [], [] -> acc
-    | x::xs, y::ys -> foldIndexed2Aux f (i+1) (f i acc x y) xs ys
-    | _ -> invalidOp "Lists had different lengths"
-
-let foldIndexed2<'a, 'b, 'acc> f (state: 'acc) (xs: 'a list) (ys: 'b list) =
-    foldIndexed2Aux f 0 state xs ys
-
-let fold2<'a, 'b, 'acc> f (state: 'acc) (xs: 'a list) (ys: 'b list) =
+let fold2 f (state: 'acc) (xs: 'a list) (ys: 'b list) =
     Seq.fold2 f state xs ys
 
-let foldBack2<'a, 'b, 'acc> f (xs: 'a list) (ys: 'b list) (state: 'acc) =
+let foldBack2 f (xs: 'a list) (ys: 'b list) (state: 'acc) =
     Seq.foldBack2 f xs ys state
 
-let unfold f state =
-    let rec unfoldInner acc state =
-        match f state with
-        | None -> reverse acc
-        | Some (x,state) -> unfoldInner (x::acc) state
-    unfoldInner [] state
-
-let rec foldIndexed3Aux f i acc bs cs ds =
-    match bs, cs, ds with
-    | [], [], [] -> acc
-    | x::xs, y::ys, z::zs -> foldIndexed3Aux f (i+1) (f i acc x y z) xs ys zs
-    | _ -> invalidOp "Lists had different lengths"
-
-let foldIndexed3<'a, 'b, 'c, 'acc> f (seed: 'acc) (xs: 'a list) (ys: 'b list) (zs: 'c list) =
-    foldIndexed3Aux f 0 seed xs ys zs
-
-let fold3<'a, 'b, 'c, 'acc> f (state: 'acc) (xs: 'a list) (ys: 'b list) (zs: 'c list) =
-    foldIndexed3 (fun _ acc x y z -> f acc x y z) state xs ys zs
+let unfold (gen: 'acc -> ('T * 'acc) option) (state: 'acc) =
+    let rec loop st acc =
+        match gen st with
+        | None -> acc
+        | Some (x, st) -> loop st (cons x acc)
+    loop state List.Empty
+    |> reverse
 
-let scan<'a, 'acc> f (state: 'acc) (xs: 'a list) =
+let scan f (state: 'acc) (xs: 'a list) =
     Seq.scan f state xs |> ofSeq
 
-let scanBack<'a, 'acc> f (xs: 'a list) (state: 'acc) =
+let scanBack f (xs: 'a list) (state: 'acc) =
     Seq.scanBack f xs state |> ofSeq
 
-let length xs =
-    fold (fun acc _ -> acc + 1) 0 xs
-
-let append xs ys =
-    fold (fun acc x -> x::acc) ys (reverse xs)
+let append (xs: 'a list) (ys: 'a list) =
+    fold (fun acc x -> cons x acc) ys (reverse xs)
 
 let collect (f: 'a -> 'b list) (xs: 'a list) =
     Seq.collect f xs |> ofSeq
 
-let map f xs =
-    fold (fun acc x -> f x::acc) [] xs
+let mapIndexed (f: int -> 'a -> 'b) (xs: 'a list) =
+    let rec loop i acc =
+        if i < xs.Length
+        then loop (i + 1) (cons (f i xs.[i]) acc)
+        else acc
+    loop 0 List.Empty
     |> reverse
 
-let mapIndexed f xs =
-    foldIndexed (fun i acc x -> f i x::acc) [] xs
-    |> reverse
+let map (f: 'a -> 'b) (xs: 'a list) =
+    mapIndexed (fun i x -> f x) xs
 
-let indexed xs =
-    mapIndexed (fun i x -> (i,x)) xs
+let indexed (xs: 'a list) =
+    mapIndexed (fun i x -> (i, x)) xs
 
 let map2 f xs ys =
-    fold2 (fun acc x y -> f x y::acc) [] xs ys
-    |> reverse
+    Seq.map2 f xs ys |> ofSeq
 
 let mapIndexed2 f xs ys =
-    foldIndexed2 (fun i acc x y  -> f i x y:: acc) [] xs ys
-    |> reverse
+    Seq.mapi2 f xs ys |> ofSeq
 
 let map3 f xs ys zs =
-    fold3 (fun acc x y z -> f x y z::acc) [] xs ys zs
-    |> reverse
-
-let mapIndexed3 f xs ys zs =
-    foldIndexed3 (fun i acc x y z -> f i x y z:: acc) [] xs ys zs
-    |> reverse
+    Seq.map3 f xs ys zs |> ofSeq
 
 let mapFold (f: 'S -> 'T -> 'R * 'S) s xs =
-    let foldFn (nxs, fs) x =
+    let folder (nxs, fs) x =
         let nx, fs = f fs x
-        nx::nxs, fs
-    let nxs, s = fold foldFn ([], s) xs
+        cons nx nxs, fs
+    let nxs, s = fold folder (List.Empty, s) xs
     reverse nxs, s
 
 let mapFoldBack (f: 'T -> 'S -> 'R * 'S) xs s =
@@ -165,132 +246,138 @@ let iterate2 f xs ys =
     fold2 (fun () x y -> f x y) () xs ys
 
 let iterateIndexed f xs =
-    foldIndexed (fun i () x -> f i x) () xs
+    fold (fun i x -> f i x; i + 1) 0 xs |> ignore
 
 let iterateIndexed2 f xs ys =
-    foldIndexed2 (fun i () x y -> f i x y) () xs ys
+    fold2 (fun i x y -> f i x y; i + 1) 0 xs ys |> ignore
 
-let ofArray (xs: IList<'T>) =
-    // Array.foldBack (fun x acc -> x::acc) xs []
-    let mutable res = []
+let ofArray (xs: System.Collections.Generic.IList<'T>) =
+    let mutable res = List.Empty
     for i = xs.Count - 1 downto 0 do
-        res <- xs.[i]::res
+        res <- cons xs.[i] res
     res
 
-let empty<'a> : 'a list = []
-
-let isEmpty = function
-    | [] -> true
-    | _ -> false
-
-let rec tryPickIndexedAux f i = function
-    | [] -> None
-    | x::xs ->
-        let result = f i x
-        match result with
-        | Some _ -> result
-        | None -> tryPickIndexedAux f (i+1) xs
-
-let tryPickIndexed f xs =
-    tryPickIndexedAux f 0 xs
+let tryPickIndexed (f: int -> 'a -> 'b option) (xs: 'a list) =
+    let rec loop i =
+        let res = f i xs.[i]
+        match res with
+        | Some _ -> res
+        | None -> if i < xs.Length - 1 then loop (i + 1) else None
+    if xs.Length > 0 then loop 0 else None
+
+let tryPickIndexedBack (f: int -> 'a -> 'b option) (xs: 'a list) =
+    let rec loop i =
+        let res = f i xs.[i]
+        match res with
+        | Some _ -> res
+        | None -> if i > 0 then loop (i - 1) else None
+    if xs.Length > 0 then loop (xs.Length - 1) else None
 
 let tryPick f xs =
     tryPickIndexed (fun _ x -> f x) xs
 
 let pick f xs =
     match tryPick f xs with
-    | None -> invalidOp "List did not contain any matching elements"
+    | None -> invalidOp msgListNoMatch
     | Some x -> x
 
 let tryFindIndexed f xs =
     tryPickIndexed (fun i x -> if f i x then Some x else None) xs
 
-let tryFind f xs =
-    tryPickIndexed (fun _ x -> if f x then Some x else None) xs
+let tryFindIndexedBack f xs =
+    tryPickIndexedBack (fun i x -> if f i x then Some x else None) xs
 
 let findIndexed f xs =
     match tryFindIndexed f xs with
-    | None -> invalidOp "List did not contain any matching elements"
+    | None -> invalidOp msgListNoMatch
+    | Some x -> x
+
+let findIndexedBack f xs =
+    match tryFindIndexedBack f xs with
+    | None -> invalidOp msgListNoMatch
     | Some x -> x
 
 let find f xs =
     findIndexed (fun _ x -> f x) xs
 
 let findBack f xs =
-    xs |> reverse |> find f
+    findIndexedBack (fun _ x -> f x) xs
+
+let tryFind f xs =
+    tryPickIndexed (fun _ x -> if f x then Some x else None) xs
 
 let tryFindBack f xs =
-    xs |> reverse |> tryFind f
+    tryPickIndexedBack (fun _ x -> if f x then Some x else None) xs
 
 let tryFindIndex f xs: int option =
     tryPickIndexed (fun i x -> if f x then Some i else None) xs
 
 let tryFindIndexBack f xs: int option =
-    List.toArray xs
-    |> Array.tryFindIndexBack f
+    tryPickIndexedBack (fun i x -> if f x then Some i else None) xs
 
 let findIndex f xs: int =
     match tryFindIndex f xs with
-    | None -> invalidOp "List did not contain any matching elements"
+    | None -> invalidOp msgListNoMatch
     | Some x -> x
 
 let findIndexBack f xs: int =
-    List.toArray xs
-    |> Array.findIndexBack f
+    match tryFindIndexBack f xs with
+    | None -> invalidOp msgListNoMatch
+    | Some x -> x
 
-let item n xs =
-    findIndexed (fun i _ -> n = i) xs
+let item n (xs: 'a list) =
+    if n >= 0 && n < xs.Length
+    then xs.[n]
+    else failwith "Index out of range"
 
-let tryItem n xs =
-    tryFindIndexed (fun i _ -> n = i) xs
+let tryItem n (xs: 'a list) =
+    if n >= 0 && n < xs.Length
+    then Some xs.[n]
+    else None
 
 let filter f xs =
     fold (fun acc x ->
-        if f x then x::acc
-        else acc) [] xs |> reverse
+        if f x
+        then cons x acc
+        else acc) List.Empty xs
+    |> reverse
 
 let partition f xs =
     fold (fun (lacc, racc) x ->
-        if f x then x::lacc, racc
-        else lacc,x::racc) ([],[]) (reverse xs)
+        if f x then cons x lacc, racc
+        else lacc, cons x racc) (List.Empty, List.Empty) (reverse xs)
 
 let choose f xs =
     fold (fun acc x ->
         match f x with
-        | Some y -> y:: acc
-        | None -> acc) [] xs |> reverse
-
-let contains<'T> (value: 'T) (list: 'T list) ([<Inject>] eq: IEqualityComparer<'T>) =
-    let rec loop xs =
-        match xs with
-        | [] -> false
-        | v::rest ->
-            if eq.Equals (value, v)
-            then true
-            else loop rest
-    loop list
-
-let except (itemsToExclude: seq<'t>) (array: 't list) ([<Inject>] eq: IEqualityComparer<'t>): 't list =
-    if isEmpty array then array
+        | Some y -> cons y acc
+        | None -> acc) List.Empty xs |> reverse
+
+let contains (value: 'T) (xs: 'T list) ([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'T>) =
+    tryFindIndex (fun v -> eq.Equals (value, v)) xs |> Option.isSome
+
+let except (itemsToExclude: seq<'t>) (xs: 't list) ([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'t>): 't list =
+    if isEmpty xs then xs
     else
-        let cached = HashSet(itemsToExclude, eq)
-        array |> filter cached.Add
+        let cached = System.Collections.Generic.HashSet(itemsToExclude, eq)
+        xs |> filter cached.Add
 
 let initialize n f =
-    let mutable xs = []
-    for i = 0 to n-1 do xs <- (f i)::xs
-    reverse xs
+    let mutable res = List.Empty
+    for i = 0 to n - 1 do
+        res <- cons (f i) res
+    res |> reverse
 
 let replicate n x =
     initialize n (fun _ -> x)
 
-let reduce f = function
-    | [] -> invalidOp "List was empty"
-    | h::t -> fold f h t
+let reduce f (xs: 't list) =
+    if isEmpty xs then invalidOp msgListWasEmpty
+    else fold f (head xs) (tail xs)
 
-let reduceBack f = function
-    | [] -> invalidOp "List was empty"
-    | h::t -> foldBack f t h
+let reduceBack f (xs: 't list) =
+    if isEmpty xs then invalidOp msgListWasEmpty
+    else foldBack f (tail xs) (head xs)
 
 let forAll f xs =
     fold (fun acc x -> acc && f x) true xs
@@ -298,21 +385,20 @@ let forAll f xs =
 let forAll2 f xs ys =
     fold2 (fun acc x y -> acc && f x y) true xs ys
 
-let rec exists f = function
-    | [] -> false
-    | x::xs -> f x || exists f xs
+let exists f xs =
+    tryFindIndex f xs |> Option.isSome
 
-let rec exists2 f bs cs =
-    match bs, cs with
-    | [], [] -> false
-    | x::xs, y::ys -> f x y || exists2 f xs ys
+let rec exists2 f xs ys =
+    match length xs, length ys with
+    | 0, 0 -> false
+    | x, y when x = y -> f (head xs) (head ys) || exists2 f (tail xs) (tail ys)
     | _ -> invalidOp "Lists had different lengths"
 
 let unzip xs =
-    foldBack (fun (x, y) (lacc, racc) -> x::lacc, y::racc) xs ([],[])
+    foldBack (fun (x, y) (lacc, racc) -> cons x lacc, cons y racc) xs (List.Empty, List.Empty)
 
 let unzip3 xs =
-    foldBack (fun (x, y, z) (lacc, macc, racc) -> x::lacc, y::macc, z::racc) xs ([],[],[])
+    foldBack (fun (x, y, z) (lacc, macc, racc) -> cons x lacc, cons y macc, cons z racc) xs (List.Empty, List.Empty, List.Empty)
 
 let zip xs ys =
     map2 (fun x y -> x, y) xs ys
@@ -320,20 +406,22 @@ let zip xs ys =
 let zip3 xs ys zs =
     map3 (fun x y z -> x, y, z) xs ys zs
 
-let sort (xs: 'T list) ([<Inject>] comparer: IComparer<'T>): 'T list =
-    Array.sortInPlaceWith (fun x y -> comparer.Compare(x, y)) (List.toArray xs) |> ofArray
+let sortWith (comparison: 'T -> 'T -> int) (xs: 'T list): 'T list =
+    let values = ResizeArray(xs)
+    values.Sort(System.Comparison<_>(comparison))
+    values |> ofSeq
 
-let sortBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: IComparer<'b>): 'a list =
-    Array.sortInPlaceWith (fun x y -> comparer.Compare(projection x, projection y)) (List.toArray xs) |> ofArray
+let sort (xs: 'T list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'T>): 'T list =
+    sortWith (fun x y -> comparer.Compare(x, y)) xs
 
-let sortDescending (xs: 'T list) ([<Inject>] comparer: IComparer<'T>): 'T list =
-    Array.sortInPlaceWith (fun x y -> comparer.Compare(x, y) * -1) (List.toArray xs) |> ofArray
+let sortBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'b>): 'a list =
+    sortWith (fun x y -> comparer.Compare(projection x, projection y)) xs
 
-let sortByDescending (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: IComparer<'b>): 'a list =
-    Array.sortInPlaceWith (fun x y -> comparer.Compare(projection x, projection y) * -1) (List.toArray xs) |> ofArray
+let sortDescending (xs: 'T list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'T>): 'T list =
+    sortWith (fun x y -> comparer.Compare(x, y) * -1) xs
 
-let sortWith (comparer: 'T -> 'T -> int) (xs: 'T list): 'T list =
-    Array.sortInPlaceWith comparer (List.toArray xs) |> ofArray
+let sortByDescending (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'b>): 'a list =
+    sortWith (fun x y -> comparer.Compare(projection x, projection y) * -1) xs
 
 let sum (xs: 'T list) ([<Inject>] adder: IGenericAdder<'T>): 'T =
     fold (fun acc x -> adder.Add(acc, x)) (adder.GetZero()) xs
@@ -341,16 +429,16 @@ let sum (xs: 'T list) ([<Inject>] adder: IGenericAdder<'T>): 'T =
 let sumBy (f: 'T -> 'T2) (xs: 'T list) ([<Inject>] adder: IGenericAdder<'T2>): 'T2 =
     fold (fun acc x -> adder.Add(acc, f x)) (adder.GetZero()) xs
 
-let maxBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: IComparer<'b>): 'a =
+let maxBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'b>): 'a =
     reduce (fun x y -> if comparer.Compare(projection y, projection x) > 0 then y else x) xs
 
-let max (li:'a list) ([<Inject>] comparer: IComparer<'a>): 'a =
+let max (li:'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'a>): 'a =
     reduce (fun x y -> if comparer.Compare(y, x) > 0 then y else x) li
 
-let minBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: IComparer<'b>): 'a =
+let minBy (projection: 'a -> 'b) (xs: 'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'b>): 'a =
     reduce (fun x y -> if comparer.Compare(projection y, projection x) > 0 then x else y) xs
 
-let min (xs: 'a list) ([<Inject>] comparer: IComparer<'a>): 'a =
+let min (xs: 'a list) ([<Inject>] comparer: System.Collections.Generic.IComparer<'a>): 'a =
     reduce (fun x y -> if comparer.Compare(y, x) > 0 then x else y) xs
 
 let average (xs: 'T list) ([<Inject>] averager: IGenericAverager<'T>): 'T =
@@ -361,124 +449,78 @@ let averageBy (f: 'T -> 'T2) (xs: 'T list) ([<Inject>] averager: IGenericAverage
     let total = fold (fun acc x -> averager.Add(acc, f x)) (averager.GetZero()) xs
     averager.DivideByInt(total, length xs)
 
-let permute f xs =
-    xs
-    |> List.toArray
+let permute f (xs: 'T list) =
+    Array.ofSeq xs Array.DynamicArrayCons
     |> Array.permute f
     |> ofArray
 
 let chunkBySize (chunkSize: int) (xs: 'T list): 'T list list =
-    xs
-    |> List.toArray
+    Array.ofSeq xs Array.DynamicArrayCons
     |> Array.chunkBySize chunkSize
     |> ofArray
     |> map ofArray
 
-let skip i xs =
-    let rec skipInner i xs =
-        match i, xs with
-        | 0, _ -> xs
-        | _, [] -> failwith "The input sequence has an insufficient number of elements."
-        | _, _::xs -> skipInner (i - 1) xs
-    match i, xs with
-    | i, _ when i < 0 -> failwith "The input must be non-negative."
-    | 0, _ -> xs
-    | 1, _::xs -> xs
-    | i, xs -> skipInner i xs
-
-let rec skipWhile predicate xs =
-    match xs with
-    | h::t when predicate h -> skipWhile predicate t
-    | _ -> xs
-
-// TODO: Is there a more efficient algorithm?
-let rec takeSplitAux error i acc xs =
-    match i, xs with
-    | 0, _ -> reverse acc, xs
-    | _, [] ->
-        if error then
-            failwith "The input sequence has an insufficient number of elements."
-        else
-            reverse acc, xs
-    | _, x::xs -> takeSplitAux error (i - 1) (x::acc) xs
+let skip i (xs: 'T list) =
+    Seq.skip i xs |> ofSeq
 
-let take i xs =
-    match i, xs with
-    | i, _ when i < 0 -> failwith "The input must be non-negative."
-    | 0, _ -> []
-    | 1, x::_ -> [x]
-    | i, xs -> takeSplitAux true i [] xs |> fst
-
-let rec takeWhile predicate (xs: 'T list) =
-    match xs with
-    | [] -> xs
-    | x::([] as nil) -> if predicate x then xs else nil
-    | x::xs ->
-        if not (predicate x) then []
-        else x::(takeWhile predicate xs)
+let skipWhile predicate (xs: 'T list) =
+    Seq.skipWhile predicate xs |> ofSeq
 
-let truncate i xs =
-    match i, xs with
-    | i, _ when i < 0 -> failwith "The input must be non-negative."
-    | 0, _ -> []
-    | 1, x::_ -> [x]
-    | i, xs -> takeSplitAux false i [] xs |> fst
+let take i xs =
+    Seq.take i xs |> ofSeq
 
-let splitAt i xs =
-    match i, xs with
-    | i, _ when i < 0 -> failwith "The input must be non-negative."
-    | 0, _ -> [],xs
-    | 1, x::xs -> [x],xs
-    | i, xs -> takeSplitAux true i [] xs
+let takeWhile predicate (xs: 'T list) =
+   Seq.takeWhile predicate xs |> ofSeq
 
-let outOfRange() = failwith "Index out of range"
+let truncate i xs =
+    Seq.truncate i xs |> ofSeq
 
 let getSlice (lower: int option) (upper: int option) (xs: 'T list) =
     let lower = defaultArg lower 0
-    let hasUpper = Option.isSome upper
-    if lower < 0 then outOfRange()
-    elif hasUpper && upper.Value < lower then []
+    let upper = defaultArg upper (xs.Length - 1)
+    if lower < 0 || upper >= xs.Length then failwith "Index out of range"
+    elif upper < lower then List.Empty
     else
-        let mutable lastIndex = -1
-        let res =
-            ([], xs) ||> foldIndexed (fun i acc x ->
-                lastIndex <- i
-                if lower <= i && (not hasUpper || i <= upper.Value) then x::acc
-                else acc)
-        if lower > (lastIndex + 1) || (hasUpper && upper.Value > lastIndex) then outOfRange()
-        reverse res
-
-let distinctBy (projection: 'T -> 'Key) (xs: 'T list) ([<Inject>] eq: IEqualityComparer<'Key>) =
-    let hashSet = HashSet<'Key>(eq)
+        let values = ResizeArray(upper - lower + 1)
+        for i = lower to upper do values.Add(xs.[i])
+        values |> ofSeq
+
+let splitAt i (xs: 'T list) =
+    if i < 0 then invalidArg "index" LanguagePrimitives.ErrorStrings.InputMustBeNonNegativeString
+    if i > xs.Length then invalidArg "index" "The input sequence has an insufficient number of elements."
+    take i xs, skip i xs
+
+let distinctBy (projection: 'T -> 'Key) (xs: 'T list) ([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'Key>) =
+    let hashSet = System.Collections.Generic.HashSet<'Key>(eq)
     xs |> filter (projection >> hashSet.Add)
 
-let distinct (xs: 'T list) ([<Inject>] eq: IEqualityComparer<'T>) =
+let distinct (xs: 'T list) ([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'T>) =
     distinctBy id xs eq
 
 let exactlyOne (xs: 'T list) =
-    match xs with
-    | [] -> invalidArg "list" LanguagePrimitives.ErrorStrings.InputSequenceEmptyString
-    | [x] -> x
-    | x1::x2::xs -> invalidArg "list" "Input list too long"
-
-let groupBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: IEqualityComparer<'Key>): ('Key * 'T list) list =
-    let dict = Dictionary<'Key, 'T list>(eq)
-    let mutable keys = []
+    match xs.Length with
+    | 1 -> head xs
+    | 0 -> invalidArg "list" LanguagePrimitives.ErrorStrings.InputSequenceEmptyString
+    | _ -> invalidArg "list" "Input list too long"
+
+let groupBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'Key>): ('Key * 'T list) list =
+    let dict = System.Collections.Generic.Dictionary<'Key, 'T list>(eq)
+    let mutable keys = List.Empty
     xs |> iterate (fun v ->
         let key = projection v
         match dict.TryGetValue(key) with
         | true, prev ->
-            dict.[key] <- v::prev
+            dict.[key] <- cons v prev
         | false, _ ->
-            dict.Add(key, [v])
-            keys <- key::keys )
-    let mutable result = []
-    keys |> iterate (fun key -> result <- (key, reverse dict.[key]) :: result)
+            dict.Add(key, cons v List.Empty)
+            keys <- cons key keys )
+    let mutable result = List.Empty
+    keys |> iterate (fun key -> result <- cons (key, reverse dict.[key]) result)
     result
 
-let countBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: IEqualityComparer<'Key>) =
-    let dict = Dictionary<'Key, int>(eq)
-    let mutable keys = []
+let countBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'Key>) =
+    let dict = System.Collections.Generic.Dictionary<'Key, int>(eq)
+    let mutable keys = List.Empty
     xs |> iterate (fun v ->
         let key = projection v
         match dict.TryGetValue(key) with
@@ -486,29 +528,24 @@ let countBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: IEqualityCompa
             dict.[key] <- prev + 1
         | false, _ ->
             dict.[key] <- 1
-            keys <- key::keys )
-    let mutable result = []
-    keys |> iterate (fun key -> result <- (key, dict.[key]) :: result)
+            keys <- cons key keys )
+    let mutable result = List.Empty
+    keys |> iterate (fun key -> result <- cons (key, dict.[key]) result)
     result
 
-let where predicate source =
-    filter predicate source
+let where predicate (xs: 'T list) =
+    filter predicate xs
 
-let pairwise source =
-    Seq.pairwise source
-    |> ofSeq
+let pairwise (xs: 'T list) =
+    Seq.pairwise xs |> ofSeq
 
-let windowed (windowSize: int) (source: 'T list): 'T list list =
-    if windowSize <= 0 then
-        failwith "windowSize must be positive"
-    let mutable res = []
-    for i = length source downto windowSize do
-        res <- (getSlice (Some(i-windowSize)) (Some(i-1)) source) :: res
-    res
+let windowed (windowSize: int) (xs: 'T list): 'T list list =
+    Seq.windowed windowSize xs
+    |> ofSeq
+    |> map ofArray
 
-let splitInto (chunks: int) (source: 'T list): 'T list list =
-    source
-    |> List.toArray
+let splitInto (chunks: int) (xs: 'T list): 'T list list =
+    Array.ofSeq xs Array.DynamicArrayCons
     |> Array.splitInto chunks
     |> ofArray
     |> map ofArray
diff --git a/src/fable-library/Types.ts b/src/fable-library/Types.ts
index cfb4d4bcea..ae7035b2d6 100644
--- a/src/fable-library/Types.ts
+++ b/src/fable-library/Types.ts
@@ -47,139 +47,139 @@ export class SystemObject implements IEquatable<any> {
   }
 }
 
-function compareList<T>(self: List<T>, other: List<T>) {
-  if (self === other) {
-    return 0;
-  } else {
-    if (other == null) {
-      return -1;
-    }
-    const selfLen = self.Length;
-    const otherLen = other.Length;
-    const minLen = Math.min(selfLen, otherLen);
-    for (let i = 0; i < minLen; i++) {
-      const res = compare(self.Item(i), other.Item(i));
-      if (res !== 0) { return res; }
-    }
-    return selfLen > otherLen ? 1 : (selfLen < otherLen ? -1 : 0);
-  }
-}
+// function compareList<T>(self: List<T>, other: List<T>) {
+//   if (self === other) {
+//     return 0;
+//   } else {
+//     if (other == null) {
+//       return -1;
+//     }
+//     const selfLen = self.Length;
+//     const otherLen = other.Length;
+//     const minLen = Math.min(selfLen, otherLen);
+//     for (let i = 0; i < minLen; i++) {
+//       const res = compare(self.Item(i), other.Item(i));
+//       if (res !== 0) { return res; }
+//     }
+//     return selfLen > otherLen ? 1 : (selfLen < otherLen ? -1 : 0);
+//   }
+// }
 
-export function newList<T>(vals: T[]): List<T> {
-  return new List(vals);
-}
+// export function newList<T>(vals: T[]): List<T> {
+//   return new List(vals);
+// }
 
-export function cons<T>(head: T, tail: List<T>): List<T> {
-  // If this points to the last index of the stack, push the new value into it.
-  // Otherwise, this becomes an "actual" tail.
-  if (tail.vals.length === tail.idx + 1) {
-    tail.vals.push(head);
-    return new List(tail.vals, tail.tail);
-  } else {
-    return new List([head], tail);
-  }
-}
+// export function cons<T>(head: T, tail: List<T>): List<T> {
+//   // If this points to the last index of the stack, push the new value into it.
+//   // Otherwise, this becomes an "actual" tail.
+//   if (tail.vals.length === tail.idx + 1) {
+//     tail.vals.push(head);
+//     return new List(tail.vals, tail.tail);
+//   } else {
+//     return new List([head], tail);
+//   }
+// }
 
-/**
- * F# list is represented in runtime by an optimized type that uses a stack (a reverted JS array)
- * to store the values, so we can a have a big list represented by a single object (plus the stack).
- * It also allows for optimizations in the List module.
- */
-export class List<T> implements IEquatable<List<T>>, IComparable<List<T>>, Iterable<T> {
-  public vals: T[];
-  public idx: number;
-  public tail?: List<T>;
-
-  constructor(vals?: T[], tail?: List<T>, idx?: number) {
-    this.vals = vals ?? [];
-    this.idx = idx ?? this.vals.length - 1;
-    this.tail = tail;
-  }
+// /**
+//  * F# list is represented in runtime by an optimized type that uses a stack (a reverted JS array)
+//  * to store the values, so we can a have a big list represented by a single object (plus the stack).
+//  * It also allows for optimizations in the List module.
+//  */
+// export class List<T> implements IEquatable<List<T>>, IComparable<List<T>>, Iterable<T> {
+//   public vals: T[];
+//   public idx: number;
+//   public tail?: List<T>;
+
+//   constructor(vals?: T[], tail?: List<T>, idx?: number) {
+//     this.vals = vals ?? [];
+//     this.idx = idx ?? this.vals.length - 1;
+//     this.tail = tail;
+//   }
 
-  public Item(i: number): T {
-    if (i < 0) {
-      throw new Error("Index out of range");
-    } else if (i <= this.idx) {
-      return this.vals[this.idx - i];
-    } else if (this.tail) {
-      return this.tail.Item(i - this.idx - 1);
-    } else {
-      throw new Error("Index out of range");
-    }
-  }
+//   public Item(i: number): T {
+//     if (i < 0) {
+//       throw new Error("Index out of range");
+//     } else if (i <= this.idx) {
+//       return this.vals[this.idx - i];
+//     } else if (this.tail) {
+//       return this.tail.Item(i - this.idx - 1);
+//     } else {
+//       throw new Error("Index out of range");
+//     }
+//   }
 
-  public get Head(): T {
-    if (this.idx >= 0) {
-      return this.vals[this.idx];
-    } else if (this.idx < 0 && this.tail) {
-      return this.tail.Head;
-    } else  {
-      throw new Error("List was empty");
-    }
-  }
+//   public get Head(): T {
+//     if (this.idx >= 0) {
+//       return this.vals[this.idx];
+//     } else if (this.idx < 0 && this.tail) {
+//       return this.tail.Head;
+//     } else  {
+//       throw new Error("List was empty");
+//     }
+//   }
 
-  public get Tail(): List<T> | undefined {
-    if (this.idx === 0 && this.tail) {
-      return this.tail;
-    } else if (this.idx >= 0) {
-      return new List(this.vals, this.tail, this.idx - 1);
-    } else {
-      return this.tail?.Tail;
-    }
-  }
+//   public get Tail(): List<T> | undefined {
+//     if (this.idx === 0 && this.tail) {
+//       return this.tail;
+//     } else if (this.idx >= 0) {
+//       return new List(this.vals, this.tail, this.idx - 1);
+//     } else {
+//       return this.tail?.Tail;
+//     }
+//   }
 
-  public get IsEmpty(): boolean {
-    return this.idx < 0 && (this.tail?.IsEmpty ?? true);
-  }
+//   public get IsEmpty(): boolean {
+//     return this.idx < 0 && (this.tail?.IsEmpty ?? true);
+//   }
 
-  public get Length(): number {
-    return this.idx + 1 + (this.tail?.Length ?? 0);
-  }
+//   public get Length(): number {
+//     return this.idx + 1 + (this.tail?.Length ?? 0);
+//   }
 
-  public toString() {
-    return "[" + Array.from(this).join("; ") + "]";
-  }
+//   public toString() {
+//     return "[" + Array.from(this).join("; ") + "]";
+//   }
 
-  public toJSON() {
-    return Array.from(this);
-  }
+//   public toJSON() {
+//     return Array.from(this);
+//   }
 
-  public [Symbol.iterator](): Iterator<T> {
-    let curIdx = this.idx;
-    let li: List<T> = this;
-    return {
-      next: (): IteratorResult<T> => {
-        while (curIdx < 0 && li.tail) {
-          li = li.tail;
-          curIdx = li.idx;
-        }
-        return (curIdx < 0)
-          ? { done: true, value: undefined }
-          : { done: false, value: li.vals[curIdx--] };
-      }
-    };
-  }
+//   public [Symbol.iterator](): Iterator<T> {
+//     let curIdx = this.idx;
+//     let li: List<T> = this;
+//     return {
+//       next: (): IteratorResult<T> => {
+//         while (curIdx < 0 && li.tail) {
+//           li = li.tail;
+//           curIdx = li.idx;
+//         }
+//         return (curIdx < 0)
+//           ? { done: true, value: undefined }
+//           : { done: false, value: li.vals[curIdx--] };
+//       }
+//     };
+//   }
 
-  public GetHashCode() {
-    if (this.idx < 0) {
-      return 0;
-    } else {
-      const hashes: number[] = new Array(this.idx + 1);
-      for (let i = this.idx; i >= 0; i--) {
-        hashes[i] = structuralHash(this.vals[i]);
-      }
-      return combineHashCodes(hashes);
-    }
-  }
+//   public GetHashCode() {
+//     if (this.idx < 0) {
+//       return 0;
+//     } else {
+//       const hashes: number[] = new Array(this.idx + 1);
+//       for (let i = this.idx; i >= 0; i--) {
+//         hashes[i] = structuralHash(this.vals[i]);
+//       }
+//       return combineHashCodes(hashes);
+//     }
+//   }
 
-  public Equals(other: List<T>): boolean {
-    return compareList(this, other) === 0;
-  }
+//   public Equals(other: List<T>): boolean {
+//     return compareList(this, other) === 0;
+//   }
 
-  public CompareTo(other: List<T>): number {
-    return compareList(this, other);
-  }
-}
+//   public CompareTo(other: List<T>): number {
+//     return compareList(this, other);
+//   }
+// }
 
 export class Union extends SystemObject implements IComparable<any> {
   public tag: number;
diff --git a/src/fable-standalone/test/bench-compiler/package.json b/src/fable-standalone/test/bench-compiler/package.json
index b13e897170..02f4a7d9f6 100644
--- a/src/fable-standalone/test/bench-compiler/package.json
+++ b/src/fable-standalone/test/bench-compiler/package.json
@@ -34,7 +34,9 @@
     "tsc-lib-init": "npm run tsc -- --init --target es2020 --module es2020 --allowJs",
     "tsc-lib": "npm run tsc -- -p ./out-lib --outDir ./out-lib-js",
 
+    "build-test-node": "node out-node/app.js ../../../../../fable-test/fable-test.fsproj out-test && npm run build-test-transform",
     "build-test-node-ts": "node out-node/app.js ../../../../../fable-test/fable-test.fsproj out-test --classTypes --typescript && npm run build-test-transform -- --typescript",
+    "build-test-dotnet": "dotnet run -c Release ../../../../../fable-test/fable-test.fsproj out-test && npm run build-test-transform",
     "build-test-dotnet-ts": "dotnet run -c Release ../../../../../fable-test/fable-test.fsproj out-test --classTypes --typescript && npm run build-test-transform -- --typescript",
     "build-test-dotnet-opt": "dotnet run -c Release ../../../../../fable-test/fable-test.fsproj out-test --optimize-fcs && npm run build-test-transform",
     "build-test-transform": "node transform ../../../../../fable-test/fable-test.fsproj out-test ../../../../build/fable-library",
diff --git a/tests/Main/ComparisonTests.fs b/tests/Main/ComparisonTests.fs
index a9d1b23fd2..190858f800 100644
--- a/tests/Main/ComparisonTests.fs
+++ b/tests/Main/ComparisonTests.fs
@@ -434,6 +434,10 @@ let tests =
         (OTest(1).GetHashCode(), OTest(1).GetHashCode()) ||> notEqual
         (OTest(2).GetHashCode(), OTest(1).GetHashCode()) ||> notEqual
 
+    testCase "GetHashCode with objects that overwrite it works" <| fun () ->
+        (Test(1).GetHashCode(), Test(1).GetHashCode()) ||> equal
+        (Test(2).GetHashCode(), Test(1).GetHashCode()) ||> notEqual
+
     testCase "GetHashCode with same object works" <| fun () ->
         let o = OTest(1)
         let h1 = o.GetHashCode()

From 806cd6306062b3c93c64ca440a976fd96d52dc0a Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Tue, 4 Aug 2020 19:42:34 -0700
Subject: [PATCH 2/5] List module update

---
 src/fable-library/List.fs       | 38 ++++++++++++++++-----------------
 src/fable-library/MutableMap.fs |  9 ++++----
 src/fable-library/MutableSet.fs |  9 ++++----
 3 files changed, 29 insertions(+), 27 deletions(-)

diff --git a/src/fable-library/List.fs b/src/fable-library/List.fs
index c84aad202b..e88531e995 100644
--- a/src/fable-library/List.fs
+++ b/src/fable-library/List.fs
@@ -11,8 +11,8 @@ let msgListNoMatch = "List did not contain any matching elements"
 type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
     let mutable hashCode = None
 
-    static member Empty = new List<'T>(0, ResizeArray<'T>())
-    static member internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
+    static member inline Empty = new List<'T>(0, ResizeArray<'T>())
+    static member inline internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
 
     member inline internal _.Add(x: 'T) =
         let values =
@@ -25,12 +25,12 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
     member inline _.IsEmpty = Count <= 0
     member inline _.Length = Count
 
-    member _.Head =
+    member inline _.Head =
         if Count > 0
         then Values.[Count - 1]
         else failwith msgListWasEmpty
 
-    member _.Tail =
+    member inline _.Tail =
         if Count > 0
         then new List<'T>(Count - 1, Values)
         else failwith msgListWasEmpty
@@ -67,22 +67,22 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
 
     interface System.Collections.Generic.IEnumerable<'T> with
         member xs.GetEnumerator(): System.Collections.Generic.IEnumerator<'T> =
-            let elems = seq { for i=xs.Length - 1 downto 0 do yield Values.[i] }
-            elems.GetEnumerator()
+            // let elems = seq { for i=xs.Length - 1 downto 0 do yield Values.[i] }
+            // elems.GetEnumerator()
 
             // new ListEnumerator<'T>(xs) :> System.Collections.Generic.IEnumerator<'T>
 
-            // let mutable i = Count
-            // {
-            //     new System.Collections.Generic.IEnumerator<'T> with
-            //         member _.Current = Values.[i]
-            //     interface System.Collections.IEnumerator with
-            //         member _.Current: obj = box (Values.[i])
-            //         member _.MoveNext() = i <- i - 1; i >= 0
-            //         member _.Reset() = i <- Count
-            //     interface System.IDisposable with
-            //         member _.Dispose(): unit = ()
-            // }
+            let mutable i = Count
+            {
+                new System.Collections.Generic.IEnumerator<'T> with
+                    member _.Current = Values.[i]
+                interface System.Collections.IEnumerator with
+                    member _.Current: obj = box (Values.[i])
+                    member _.MoveNext() = i <- i - 1; i >= 0
+                    member _.Reset() = i <- Count
+                interface System.IDisposable with
+                    member _.Dispose(): unit = ()
+            }
 
     interface System.Collections.IEnumerable with
         member xs.GetEnumerator(): System.Collections.IEnumerator =
@@ -170,7 +170,7 @@ let reverse (xs: 'a list) =
     fold (fun acc x -> cons x acc) List.Empty xs
 
 let toSeq (xs: 'a list): 'a seq =
-    Seq.map id xs
+    xs :> System.Collections.Generic.IEnumerable<'a>
 
 let ofSeq (xs: 'a seq): 'a list =
     Seq.fold (fun acc x -> cons x acc) List.Empty xs
@@ -201,7 +201,7 @@ let scanBack f (xs: 'a list) (state: 'acc) =
     Seq.scanBack f xs state |> ofSeq
 
 let append (xs: 'a list) (ys: 'a list) =
-    fold (fun acc x -> cons x acc) ys (reverse xs)
+    foldBack (fun x acc -> cons x acc) xs ys
 
 let collect (f: 'a -> 'b list) (xs: 'a list) =
     Seq.collect f xs |> ofSeq
diff --git a/src/fable-library/MutableMap.fs b/src/fable-library/MutableMap.fs
index 5f6e66eb3c..24588063fb 100644
--- a/src/fable-library/MutableMap.fs
+++ b/src/fable-library/MutableMap.fs
@@ -91,10 +91,11 @@ type MutableMap<'Key, 'Value when 'Key: equality>(pairs: KeyValuePair<'Key, 'Val
 
     interface IEnumerable<KeyValuePair<'Key, 'Value>> with
         member this.GetEnumerator(): IEnumerator<KeyValuePair<'Key, 'Value>> =
-            let elems = seq {
-                for pairs in hashMap.Values do
-                    for pair in pairs do
-                        yield pair }
+            // let elems = seq {
+            //     for pairs in hashMap.Values do
+            //         for pair in pairs do
+            //             yield pair }
+            let elems = Seq.concat hashMap.Values
             elems.GetEnumerator()
 
     interface ICollection<KeyValuePair<'Key, 'Value>> with
diff --git a/src/fable-library/MutableSet.fs b/src/fable-library/MutableSet.fs
index d744a49eaf..6e6cbeb970 100644
--- a/src/fable-library/MutableSet.fs
+++ b/src/fable-library/MutableSet.fs
@@ -79,10 +79,11 @@ type MutableSet<'T when 'T: equality>(items: 'T seq, comparer: IEqualityComparer
 
     interface IEnumerable<'T> with
         member this.GetEnumerator(): IEnumerator<'T> =
-            let elems = seq {
-                for values in hashMap.Values do
-                    for value in values do
-                        yield value }
+            // let elems = seq {
+            //     for values in hashMap.Values do
+            //         for value in values do
+            //             yield value }
+            let elems = Seq.concat hashMap.Values
             elems.GetEnumerator()
 
     interface ICollection<'T> with

From 3c1d85016eb7f53c53b2edacec580a16a799d030 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Tue, 11 Aug 2020 14:06:08 -0700
Subject: [PATCH 3/5] Optimized IEnumerator.get_Current

---
 src/Fable.Transforms/FSharp2Fable.Util.fs |  5 +-
 src/Fable.Transforms/FSharp2Fable.fs      | 19 ++++--
 src/fable-library/List.fs                 | 74 ++++++-----------------
 src/fable-library/Seq.ts                  |  6 +-
 4 files changed, 40 insertions(+), 64 deletions(-)

diff --git a/src/Fable.Transforms/FSharp2Fable.Util.fs b/src/Fable.Transforms/FSharp2Fable.Util.fs
index 1d7df4b9eb..156b05d89b 100644
--- a/src/Fable.Transforms/FSharp2Fable.Util.fs
+++ b/src/Fable.Transforms/FSharp2Fable.Util.fs
@@ -1039,7 +1039,10 @@ module Util =
         | [arg] when memb.IsPropertySetterMethod ->
             let t = memb.CurriedParameterGroups.[0].[0].Type |> makeType com Map.empty
             Fable.Set(callee, Fable.FieldSet(name, t), arg, r)
-        | _ when memb.IsPropertyGetterMethod && countNonCurriedParams memb = 0 ->
+        | _ when memb.IsPropertyGetterMethod && countNonCurriedParams memb = 0
+            // performance optimization, compile get_Current as instance call instead of a getter
+            && memb.FullName <> "System.Collections.IEnumerator.get_Current"
+            && memb.FullName <> "System.Collections.Generic.IEnumerator.get_Current" ->
             let t = memb.ReturnParameter.Type |> makeType com Map.empty
             let kind = Fable.FieldGet(name, true, t)
             Fable.Get(callee, kind, typ, r)
diff --git a/src/Fable.Transforms/FSharp2Fable.fs b/src/Fable.Transforms/FSharp2Fable.fs
index dbaf0aefbd..163ed010f0 100644
--- a/src/Fable.Transforms/FSharp2Fable.fs
+++ b/src/Fable.Transforms/FSharp2Fable.fs
@@ -171,7 +171,7 @@ let private transformTraitCall com (ctx: Context) r typ (sourceTypes: FSharpType
 let private transformObjExpr (com: IFableCompiler) (ctx: Context) (objType: FSharpType)
                     baseCallExpr (overrides: FSharpObjectExprOverride list) otherOverrides =
 
-    let mapOverride (over: FSharpObjectExprOverride) =
+    let mapOverride (typ: FSharpType) (over: FSharpObjectExprOverride) =
       trampoline {
         let ctx, args = bindMemberArgs com ctx over.CurriedParameterGroups
         let! body = transformExpr com ctx over.Body
@@ -179,7 +179,12 @@ let private transformObjExpr (com: IFableCompiler) (ctx: Context) (objType: FSha
         let name, kind =
             match over.Signature.Name with
             | Naming.StartsWith "get_" name when countNonCurriedParamsForOverride over = 0 ->
-                name, Fable.ObjectGetter
+                // performance optimization, compile get_Current as instance call instead of a getter
+                if over.Signature.Name = "get_Current" &&
+                    (typ.TypeDefinition.FullName = "System.Collections.Generic.IEnumerator`1" ||
+                     typ.TypeDefinition.FullName = "System.Collections.IEnumerator")
+                then name, Fable.ObjectMethod false
+                else name, Fable.ObjectGetter
             | Naming.StartsWith "set_" name when countNonCurriedParamsForOverride over = 1 ->
                 name, Fable.ObjectSetter
             | name ->
@@ -222,8 +227,8 @@ let private transformObjExpr (com: IFableCompiler) (ctx: Context) (objType: FSha
 
       let! members =
         (objType, overrides)::otherOverrides
-        |> trampolineListMap (fun (_typ, overrides) ->
-            overrides |> trampolineListMap mapOverride)
+        |> trampolineListMap (fun (typ, overrides) ->
+            overrides |> trampolineListMap (mapOverride typ))
 
       return Fable.ObjectExpr(members |> List.concat, makeType com ctx.GenericArgs objType, baseCall)
     }
@@ -985,7 +990,11 @@ let private transformAttachedMember (com: FableCompiler) (ctx: Context)
         let kind =
             match args with
             | [_thisArg; unitArg] when memb.IsPropertyGetterMethod && unitArg.Type = Fable.Unit ->
-                Fable.ObjectGetter
+                // performance optimization, compile get_Current as instance call instead of a getter
+                if (memb.CompiledName = "System-Collections-Generic-IEnumerator`1-get_Current" ||
+                    memb.CompiledName = "System-Collections-IEnumerator-get_Current")
+                then Fable.ObjectMethod false
+                else Fable.ObjectGetter
             | [_thisArg; _valueArg] when memb.IsPropertySetterMethod ->
                 Fable.ObjectSetter
             | _ when memb.CompiledName = "System-Collections-Generic-IEnumerable`1-GetEnumerator" ->
diff --git a/src/fable-library/List.fs b/src/fable-library/List.fs
index e88531e995..e3e4d7406e 100644
--- a/src/fable-library/List.fs
+++ b/src/fable-library/List.fs
@@ -11,10 +11,10 @@ let msgListNoMatch = "List did not contain any matching elements"
 type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
     let mutable hashCode = None
 
-    static member inline Empty = new List<'T>(0, ResizeArray<'T>())
-    static member inline internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
+    static member Empty = new List<'T>(0, ResizeArray<'T>())
+    static member internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
 
-    member inline internal _.Add(x: 'T) =
+    member internal _.Add(x: 'T) =
         let values =
             if Count = Values.Count
             then Values
@@ -22,20 +22,20 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
         values.Add(x)
         new List<'T>(values.Count, values)
 
-    member inline _.IsEmpty = Count <= 0
-    member inline _.Length = Count
+    member _.IsEmpty = Count <= 0
+    member _.Length = Count
 
-    member inline _.Head =
+    member _.Head =
         if Count > 0
         then Values.[Count - 1]
         else failwith msgListWasEmpty
 
-    member inline _.Tail =
+    member _.Tail =
         if Count > 0
         then new List<'T>(Count - 1, Values)
         else failwith msgListWasEmpty
 
-    member inline _.Item with get(index) =
+    member _.Item with get(index) =
         Values.[Count - 1 - index]
 
     override xs.ToString() =
@@ -46,7 +46,6 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
         if xs.Length <> ys.Length then false
         elif xs.GetHashCode() <> ys.GetHashCode() then false
         else Seq.forall2 (Unchecked.equals) xs ys
-        // else (xs :> System.IComparable).CompareTo(other) = 0
 
     override xs.GetHashCode() =
         match hashCode with
@@ -63,59 +62,25 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
     interface System.IComparable with
         member xs.CompareTo(other: obj) =
             Seq.compareWith compare xs (other :?> 'T list)
-            // List.CompareWith compare xs (other :?> 'T list)
 
     interface System.Collections.Generic.IEnumerable<'T> with
         member xs.GetEnumerator(): System.Collections.Generic.IEnumerator<'T> =
-            // let elems = seq { for i=xs.Length - 1 downto 0 do yield Values.[i] }
-            // elems.GetEnumerator()
-
-            // new ListEnumerator<'T>(xs) :> System.Collections.Generic.IEnumerator<'T>
-
-            let mutable i = Count
-            {
-                new System.Collections.Generic.IEnumerator<'T> with
-                    member _.Current = Values.[i]
-                interface System.Collections.IEnumerator with
-                    member _.Current: obj = box (Values.[i])
-                    member _.MoveNext() = i <- i - 1; i >= 0
-                    member _.Reset() = i <- Count
-                interface System.IDisposable with
-                    member _.Dispose(): unit = ()
-            }
+            new ListEnumerator<'T>(xs) :> System.Collections.Generic.IEnumerator<'T>
 
     interface System.Collections.IEnumerable with
         member xs.GetEnumerator(): System.Collections.IEnumerator =
             ((xs :> System.Collections.Generic.IEnumerable<'T>).GetEnumerator() :> System.Collections.IEnumerator)
 
-    // static member internal CompareWith (comparer: 'T -> 'T -> int) (xs: 'T list) (ys: 'T list): int =
-    //     if obj.ReferenceEquals(xs, ys)
-    //     then 0
-    //     else
-    //         if xs.IsEmpty then
-    //             if ys.IsEmpty then 0 else -1
-    //         elif ys.IsEmpty then 1
-    //         else
-    //             let mutable i = 0
-    //             let mutable result = 0
-    //             if xs.Length > ys.Length then 1
-    //             elif xs.Length < ys.Length then -1
-    //             else
-    //                 while i < xs.Length && result = 0 do
-    //                     result <- comparer xs.[i] ys.[i]
-    //                     i <- i + 1
-    //                 result
-
-// and ListEnumerator<'T when 'T: comparison>(xs: List<'T>) =
-//     let mutable i = -1
-//     interface System.Collections.Generic.IEnumerator<'T> with
-//         member __.Current = xs.[i]
-//     interface System.Collections.IEnumerator with
-//         member __.Current = box (xs.[i])
-//         member __.MoveNext() = i <- i + 1; i < xs.Length
-//         member __.Reset() = i <- -1
-//     interface System.IDisposable with
-//         member __.Dispose() = ()
+and ListEnumerator<'T when 'T: comparison>(xs: List<'T>) =
+    let mutable i = -1
+    interface System.Collections.Generic.IEnumerator<'T> with
+        member __.Current = xs.[i]
+    interface System.Collections.IEnumerator with
+        member __.Current = box (xs.[i])
+        member __.MoveNext() = i <- i + 1; i < xs.Length
+        member __.Reset() = i <- -1
+    interface System.IDisposable with
+        member __.Dispose() = ()
 
 and 'T list when 'T: comparison = List<'T>
 
@@ -152,7 +117,6 @@ let tryLast (xs: 'T list) =
 
 let compareWith (comparer: 'T -> 'T -> int) (xs: 'T list) (ys: 'T list): int =
     Seq.compareWith comparer xs ys
-    //List.CompareWith comparer xs ys
 
 let fold (folder: 'acc -> 'T -> 'acc) (state: 'acc) (xs: 'T list) =
     let mutable acc = state
diff --git a/src/fable-library/Seq.ts b/src/fable-library/Seq.ts
index 5a8f9dae20..c2ab165dcc 100644
--- a/src/fable-library/Seq.ts
+++ b/src/fable-library/Seq.ts
@@ -4,7 +4,7 @@ import { Option, some, value } from "./Option";
 import { compare, equals, IComparer, IDisposable } from "./Util";
 
 export interface IEnumerator<T> {
-  Current: T | undefined;
+  Current(): T | undefined; // intentionally not a getter (for performance reasons)
   MoveNext(): boolean;
   Reset(): void;
 }
@@ -32,7 +32,7 @@ export class Enumerator<T> implements IEnumerator<T>, IDisposable {
     this.current = cur.value;
     return !cur.done;
   }
-  get Current() {
+  Current() {
     return this.current;
   }
   public Reset() {
@@ -51,7 +51,7 @@ export function toIterator<T>(en: IEnumerator<T>): Iterator<T> {
   return {
     next() {
       return en.MoveNext()
-        ? { done: false, value: en.Current }
+        ? { done: false, value: en.Current() }
         : { done: true, value: undefined };
     },
   } as Iterator<T>;

From cdf6b30228f53d51f065787079cbee1051286d72 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Tue, 11 Aug 2020 15:55:55 -0700
Subject: [PATCH 4/5] Optimized List.reverse

---
 src/fable-library/List.fs | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/src/fable-library/List.fs b/src/fable-library/List.fs
index e3e4d7406e..85ea78b2d2 100644
--- a/src/fable-library/List.fs
+++ b/src/fable-library/List.fs
@@ -22,6 +22,17 @@ type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
         values.Add(x)
         new List<'T>(values.Count, values)
 
+    member internal _.Reverse() =
+        let values = Values.GetRange(0, Count) // copy values
+        values.Reverse()
+        new List<'T>(values.Count, values)
+
+    // This is a destructive internal optimization that
+    // can only be performed on newly constructed lists.
+    member internal xs.ReverseInPlace() =
+        Values.Reverse()
+        xs
+
     member _.IsEmpty = Count <= 0
     member _.Length = Count
 
@@ -131,18 +142,21 @@ let foldBack (folder: 'T -> 'acc -> 'acc) (xs: 'T list) (state: 'acc) =
     acc
 
 let reverse (xs: 'a list) =
-    fold (fun acc x -> cons x acc) List.Empty xs
+    xs.Reverse()
+
+let reverseInPlace (xs: 'a list) =
+    xs.ReverseInPlace()
 
 let toSeq (xs: 'a list): 'a seq =
     xs :> System.Collections.Generic.IEnumerable<'a>
 
 let ofSeq (xs: 'a seq): 'a list =
     Seq.fold (fun acc x -> cons x acc) List.Empty xs
-    |> reverse
+    |> reverseInPlace
 
 let concat (lists: seq<'a list>) =
     Seq.fold (fold (fun acc x -> cons x acc)) List.Empty lists
-    |> reverse
+    |> reverseInPlace
 
 let fold2 f (state: 'acc) (xs: 'a list) (ys: 'b list) =
     Seq.fold2 f state xs ys
@@ -153,10 +167,9 @@ let foldBack2 f (xs: 'a list) (ys: 'b list) (state: 'acc) =
 let unfold (gen: 'acc -> ('T * 'acc) option) (state: 'acc) =
     let rec loop st acc =
         match gen st with
-        | None -> acc
+        | None -> reverse acc
         | Some (x, st) -> loop st (cons x acc)
     loop state List.Empty
-    |> reverse
 
 let scan f (state: 'acc) (xs: 'a list) =
     Seq.scan f state xs |> ofSeq
@@ -174,9 +187,8 @@ let mapIndexed (f: int -> 'a -> 'b) (xs: 'a list) =
     let rec loop i acc =
         if i < xs.Length
         then loop (i + 1) (cons (f i xs.[i]) acc)
-        else acc
+        else reverseInPlace acc
     loop 0 List.Empty
-    |> reverse
 
 let map (f: 'a -> 'b) (xs: 'a list) =
     mapIndexed (fun i x -> f x) xs
@@ -304,7 +316,7 @@ let filter f xs =
         if f x
         then cons x acc
         else acc) List.Empty xs
-    |> reverse
+    |> reverseInPlace
 
 let partition f xs =
     fold (fun (lacc, racc) x ->
@@ -315,7 +327,8 @@ let choose f xs =
     fold (fun acc x ->
         match f x with
         | Some y -> cons y acc
-        | None -> acc) List.Empty xs |> reverse
+        | None -> acc) List.Empty xs
+    |> reverseInPlace
 
 let contains (value: 'T) (xs: 'T list) ([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'T>) =
     tryFindIndex (fun v -> eq.Equals (value, v)) xs |> Option.isSome
@@ -330,7 +343,7 @@ let initialize n f =
     let mutable res = List.Empty
     for i = 0 to n - 1 do
         res <- cons (f i) res
-    res |> reverse
+    res |> reverseInPlace
 
 let replicate n x =
     initialize n (fun _ -> x)

From 231304774500412a224e0bb42295b116200d5439 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Sun, 16 Aug 2020 20:06:21 -0700
Subject: [PATCH 5/5] Fixed Seq.compareWith

---
 src/Fable.Transforms/Fable2Babel.fs  |  9 ++-
 src/Fable.Transforms/Replacements.fs |  8 ++-
 src/fable-library/List.fs            | 94 +++++++++++++++-------------
 src/fable-library/Seq.ts             | 14 ++++-
 4 files changed, 76 insertions(+), 49 deletions(-)

diff --git a/src/Fable.Transforms/Fable2Babel.fs b/src/Fable.Transforms/Fable2Babel.fs
index 094ee178cb..4e3fd22710 100644
--- a/src/Fable.Transforms/Fable2Babel.fs
+++ b/src/Fable.Transforms/Fable2Babel.fs
@@ -810,9 +810,13 @@ module Util =
         | Replacements.ListLiteral(exprs, t) ->
             [|List.rev exprs |> makeArray com ctx|]
             |> coreLibCall com ctx r "List" "newList"
+            // match exprs with
+            // | [] -> coreLibCall com ctx r "List" "empty" [||]
+            // | [TransformExpr com ctx expr] -> coreLibCall com ctx r "List" "singleton" [|expr|]
+            // | exprs -> [|makeArray com ctx exprs|] |> coreLibCall com ctx r "List" "ofArray"
         | Fable.NewList (headAndTail, _) ->
             match headAndTail with
-            | None -> coreLibCall com ctx r "List" "newList" [||]
+            | None -> coreLibCall com ctx r "List" "empty" [||]
             | Some(TransformExpr com ctx head, TransformExpr com ctx tail) ->
                 coreLibCall com ctx r "List" "cons" [|head; tail|]
         | Fable.NewOption (value, t) ->
@@ -1054,6 +1058,8 @@ module Util =
         let expr = com.TransformAsExpr(ctx, fableExpr)
         match getKind with
         | Fable.ExprGet(TransformExpr com ctx prop) -> getExpr range expr prop
+        // | Fable.ListHead -> get range expr "Head"
+        // | Fable.ListTail -> get range expr "Tail"
         | Fable.ListHead -> coreLibCall com ctx range "List" "head" [|expr|]
         | Fable.ListTail -> coreLibCall com ctx range "List" "tail" [|expr|]
         | Fable.FieldGet(fieldName,_,_) ->
@@ -1212,6 +1218,7 @@ module Util =
             let op = if nonEmpty then BinaryUnequal else BinaryEqual
             upcast BinaryExpression(op, com.TransformAsExpr(ctx, expr), NullLiteral(), ?loc=range)
         | Fable.ListTest nonEmpty ->
+            // let expr = get range (com.TransformAsExpr(ctx, expr)) "IsEmpty"
             let expr = coreLibCall com ctx range "List" "isEmpty" [|com.TransformAsExpr(ctx, expr)|]
             if nonEmpty then upcast UnaryExpression(UnaryNot, expr, ?loc=range)
             else expr
diff --git a/src/Fable.Transforms/Replacements.fs b/src/Fable.Transforms/Replacements.fs
index 3819f885ac..50d8ccba88 100644
--- a/src/Fable.Transforms/Replacements.fs
+++ b/src/Fable.Transforms/Replacements.fs
@@ -1842,8 +1842,9 @@ let arrayModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Ex
 let lists (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (thisArg: Expr option) (args: Expr list) =
     let meth = Naming.removeGetSetPrefix i.CompiledName |> Naming.lowerFirst
     match i.CompiledName, thisArg, args with
-    | ("get_Head" | "get_Tail" | "get_Length" | "get_IsEmpty"), Some x, _ ->
+    | ("get_Head" | "get_Tail" | "get_IsEmpty" | "get_Length"), Some x, _ ->
         Helper.CoreCall("List", meth, t, [x], i.SignatureArgTypes, ?loc=r) |> Some
+        // get r t x meth |> Some
     | ("get_Item" | "GetSlice"), Some x, _ ->
         Helper.CoreCall("List", meth, t, args @ [x], i.SignatureArgTypes, ?loc=r) |> Some
     | ("get_Empty" | "Cons"), None, _ ->
@@ -1854,6 +1855,11 @@ let lists (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (thisArg: Ex
 
 let listModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Expr option) (args: Expr list) =
     match i.CompiledName, args with
+    // | ("Head" | "Tail" | "IsEmpty") as meth, [x] -> get r t x (Naming.lowerFirst meth) |> Some
+    // | "IsEmpty", [x] -> Test(x, ListTest false, r) |> Some
+    // | "Empty", _ -> NewList(None, (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
+    // | "Singleton", [x] ->
+    //     NewList(Some(x, Value(NewList(None, t), None)), (genArg com ctx r 0 i.GenericArgs)) |> makeValue r |> Some
     // Use a cast to give it better chances of optimization (e.g. converting list
     // literals to arrays) after the beta reduction pass
     | "ToSeq", [x] -> toSeq t x |> Some
diff --git a/src/fable-library/List.fs b/src/fable-library/List.fs
index 85ea78b2d2..9308a53759 100644
--- a/src/fable-library/List.fs
+++ b/src/fable-library/List.fs
@@ -8,67 +8,71 @@ open Fable.Core
 let msgListWasEmpty = "List was empty"
 let msgListNoMatch = "List did not contain any matching elements"
 
-type List<'T when 'T: comparison>(Count: int, Values: ResizeArray<'T>) =
-    let mutable hashCode = None
+[<CustomEquality; CustomComparison>]
+type List<'T when 'T: comparison> =
+    { Count: int; Values: ResizeArray<'T> }
 
-    static member Empty = new List<'T>(0, ResizeArray<'T>())
-    static member internal Cons (x: 'T, xs: 'T list) = xs.Add(x)
-
-    member internal _.Add(x: 'T) =
+    member inline internal xs.Add(x: 'T) =
         let values =
-            if Count = Values.Count
-            then Values
-            else Values.GetRange(0, Count)
+            if xs.Count = xs.Values.Count
+            then xs.Values
+            else xs.Values.GetRange(0, xs.Count)
         values.Add(x)
-        new List<'T>(values.Count, values)
+        { Count = values.Count; Values = values }
 
-    member internal _.Reverse() =
-        let values = Values.GetRange(0, Count) // copy values
+    member inline internal xs.Reverse() =
+        let values = xs.Values.GetRange(0, xs.Count) // copy values
         values.Reverse()
-        new List<'T>(values.Count, values)
+        { Count = values.Count; Values = values }
 
     // This is a destructive internal optimization that
     // can only be performed on newly constructed lists.
-    member internal xs.ReverseInPlace() =
-        Values.Reverse()
+    member inline internal xs.ReverseInPlace() =
+        xs.Values.Reverse()
         xs
 
-    member _.IsEmpty = Count <= 0
-    member _.Length = Count
+    static member inline Singleton(x: 'T) =
+        let values = ResizeArray<'T>()
+        values.Add(x)
+        { Count = 1; Values = values }
+
+    static member inline Empty = { Count = 0; Values = ResizeArray<'T>() }
+    static member inline Cons (x: 'T, xs: 'T list) = xs.Add(x)
+
+    member inline xs.IsEmpty = xs.Count <= 0
+    member inline xs.Length = xs.Count
 
-    member _.Head =
-        if Count > 0
-        then Values.[Count - 1]
+    member inline xs.Head =
+        if xs.Count > 0
+        then xs.Values.[xs.Count - 1]
         else failwith msgListWasEmpty
 
-    member _.Tail =
-        if Count > 0
-        then new List<'T>(Count - 1, Values)
+    member inline xs.Tail =
+        if xs.Count > 0
+        then { Count = xs.Count - 1; Values = xs.Values }
         else failwith msgListWasEmpty
 
-    member _.Item with get(index) =
-        Values.[Count - 1 - index]
+    member inline xs.Item with get (index: int) =
+        xs.Values.[xs.Count - 1 - index]
 
     override xs.ToString() =
         "[" + System.String.Join("; ", xs) + "]"
 
     override xs.Equals(other: obj) =
-        let ys = other :?> 'T list
-        if xs.Length <> ys.Length then false
-        elif xs.GetHashCode() <> ys.GetHashCode() then false
-        else Seq.forall2 (Unchecked.equals) xs ys
+        if obj.ReferenceEquals(xs, other)
+        then true
+        else
+            let ys = other :?> 'T list
+            if xs.Length <> ys.Length then false
+            else Seq.forall2 (Unchecked.equals) xs ys
 
     override xs.GetHashCode() =
-        match hashCode with
-        | Some h -> h
-        | None ->
-            let inline combineHash i x y = (x <<< 1) + y + 631 * i
-            let len = min (xs.Length - 1) 18 // limit the hash count
-            let mutable h = 0
-            for i = 0 to len do
-                h <- combineHash i h (hash xs.[i])
-            hashCode <- Some h
-            h
+        let inline combineHash i x y = (x <<< 1) + y + 631 * i
+        let len = min (xs.Length - 1) 18 // limit the hash count
+        let mutable h = 0
+        for i = 0 to len do
+            h <- combineHash i h (hash xs.[i])
+        h
 
     interface System.IComparable with
         member xs.CompareTo(other: obj) =
@@ -95,13 +99,13 @@ and ListEnumerator<'T when 'T: comparison>(xs: List<'T>) =
 
 and 'T list when 'T: comparison = List<'T>
 
-let newList (values: ResizeArray<'T>) = new List<'T>(values.Count, values)
+let newList (values: ResizeArray<'T>) = { Count = values.Count; Values = values }
 
 let empty () = List.Empty
 
 let cons (x: 'T) (xs: 'T list) = List.Cons (x, xs)
 
-let singleton x = cons x List.Empty
+let singleton (x: 'T) = List.Singleton (x)
 
 let isEmpty (xs: 'T list) = xs.IsEmpty
 
@@ -144,7 +148,7 @@ let foldBack (folder: 'T -> 'acc -> 'acc) (xs: 'T list) (state: 'acc) =
 let reverse (xs: 'a list) =
     xs.Reverse()
 
-let reverseInPlace (xs: 'a list) =
+let inline reverseInPlace (xs: 'a list) =
     xs.ReverseInPlace()
 
 let toSeq (xs: 'a list): 'a seq =
@@ -167,7 +171,7 @@ let foldBack2 f (xs: 'a list) (ys: 'b list) (state: 'acc) =
 let unfold (gen: 'acc -> ('T * 'acc) option) (state: 'acc) =
     let rec loop st acc =
         match gen st with
-        | None -> reverse acc
+        | None -> reverseInPlace acc
         | Some (x, st) -> loop st (cons x acc)
     loop state List.Empty
 
@@ -210,7 +214,7 @@ let mapFold (f: 'S -> 'T -> 'R * 'S) s xs =
         let nx, fs = f fs x
         cons nx nxs, fs
     let nxs, s = fold folder (List.Empty, s) xs
-    reverse nxs, s
+    reverseInPlace nxs, s
 
 let mapFoldBack (f: 'T -> 'S -> 'R * 'S) xs s =
     mapFold (fun s v -> f v s) s (reverse xs)
@@ -492,7 +496,7 @@ let groupBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: System.Collect
             dict.Add(key, cons v List.Empty)
             keys <- cons key keys )
     let mutable result = List.Empty
-    keys |> iterate (fun key -> result <- cons (key, reverse dict.[key]) result)
+    keys |> iterate (fun key -> result <- cons (key, reverseInPlace dict.[key]) result)
     result
 
 let countBy (projection: 'T -> 'Key) (xs: 'T list)([<Inject>] eq: System.Collections.Generic.IEqualityComparer<'Key>) =
diff --git a/src/fable-library/Seq.ts b/src/fable-library/Seq.ts
index c2ab165dcc..40c2aa03d1 100644
--- a/src/fable-library/Seq.ts
+++ b/src/fable-library/Seq.ts
@@ -191,8 +191,18 @@ export function choose<T, U>(f: (x: T) => U, xs: Iterable<T>) {
 }
 
 export function compareWith<T>(f: (x: T, y: T) => number, xs: Iterable<T>, ys: Iterable<T>) {
-  const nonZero = tryFind((i: number) => i !== 0, map2(f, xs, ys));
-  return nonZero != null ? value(nonZero) : length(xs) - length(ys);
+  if (xs === ys) { return 0; }
+  let cur1: IteratorResult<T>;
+  let cur2: IteratorResult<T>;
+  let c = 0;
+  for (const iter1 = xs[Symbol.iterator](), iter2 = ys[Symbol.iterator](); ;) {
+    cur1 = iter1.next();
+    cur2 = iter2.next();
+    if (cur1.done || cur2.done) { break; }
+    c = f(cur1.value, cur2.value);
+    if (c !== 0) { break; }
+  }
+  return (c !== 0) ? c : (cur1.done && !cur2.done) ? -1 : (!cur1.done && cur2.done) ? 1 : 0;
 }
 
 export function delay<T>(f: () => Iterable<T>): Iterable<T> {