From 9ce1279b62be4bb71f65646960f8282a355625e3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rafa=C5=82=20Gwo=C5=BAdzi=C5=84ski?=
 <rafal.gwozdzinski@ihsmarkit.com>
Date: Tue, 23 Feb 2021 06:39:41 +0100
Subject: [PATCH] Evaluate `traverse` left to right (#418)

---
 src/FSharpPlus/Control/Traversable.fs         | 25 +++++++++++++------
 tests/FSharpPlus.Tests/General.fs             |  8 ++++++
 .../FSharpTests/General/Traversable.fs        |  8 ++++++
 3 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/src/FSharpPlus/Control/Traversable.fs b/src/FSharpPlus/Control/Traversable.fs
index 0f728439c..a48b6c509 100644
--- a/src/FSharpPlus/Control/Traversable.fs
+++ b/src/FSharpPlus/Control/Traversable.fs
@@ -81,8 +81,8 @@ type Traverse =
     static member inline Traverse (t: option<_>, f, [<Optional>]_output: 'R, [<Optional>]_impl: Traverse) : 'R = match t with Some x -> Map.Invoke Some (f x) | _ -> result None
 
     static member inline Traverse (t:Map<_,_>  , f, [<Optional>]_output: 'R, [<Optional>]_impl: Traverse) : 'R =
-       let insert_f k x ys = Map.Invoke (Map.add k) (f x) <*> ys
-       Map.foldBack insert_f t (result Map.empty)
+        let insert_f m k v = Map.Invoke (Map.add k) v <*> m        
+        Map.fold insert_f (result Map.empty) (Map.mapValues f t)
 
     static member inline Traverse (t: Result<'T,'Error>, f: 'T->'``Functor<'U>``, [<Optional>]_output: '``Functor<Result<'U,'Error>>``, [<Optional>]_impl: Traverse) : '``Functor<Result<'U,'Error>>`` =
         match t with
@@ -95,13 +95,24 @@ type Traverse =
         | Choice2Of2 e -> Return.Invoke (Choice<'U,'Error>.Choice2Of2 e)
 
     static member inline Traverse (t:list<_>   ,f , [<Optional>]_output: 'R, [<Optional>]_impl: Traverse) : 'R =
-       let cons_f x ys = Map.Invoke List.cons (f x) <*> ys
-       List.foldBack cons_f t (result [])
+        let rec loop acc = function
+            | [] -> acc
+            | x::xs -> 
+                let v = f x
+                loop (v::acc) xs
+        let cons_f x xs = Map.Invoke List.cons xs <*> x
+        List.fold cons_f (result []) (loop [] t)
 
     static member inline Traverse (t:_ []      ,f , [<Optional>]_output: 'R, [<Optional>]_impl: Traverse) : 'R =
-       let cons x y = Array.append [|x|] y
-       let cons_f x ys = Map.Invoke cons (f x) <*> ys
-       Array.foldBack cons_f t (result [||])
+        let cons x y = Array.append [|x|] y
+        let rec loop acc = function
+            | [||] -> acc
+            | xxs ->
+                let x, xs = Array.head xxs, Array.tail xxs
+                let v = f x
+                loop (cons v acc) xs
+        let cons_f x xs = Map.Invoke cons xs <*> x
+        Array.fold cons_f (result [||]) (loop [||] t)
 
     static member inline Invoke (f: 'T->'``Functor<'U>``) (t: '``Traversable<'T>``) : '``Functor<'Traversable<'U>>`` =
         let inline call_3 (a: ^a, b: ^b, c: ^c, f) = ((^a or ^b or ^c) : (static member Traverse : _*_*_*_ -> _) b, f, c, a)
diff --git a/tests/FSharpPlus.Tests/General.fs b/tests/FSharpPlus.Tests/General.fs
index 9cbf68de3..81f2df4a9 100644
--- a/tests/FSharpPlus.Tests/General.fs
+++ b/tests/FSharpPlus.Tests/General.fs
@@ -1261,6 +1261,14 @@ module Traversable =
         let _ = Seq.sequence    [ZipList [1]; ZipList []; ZipList (seq {failwith "sholdn't get here"})] |> toList
         ()
 
+    [<Test>]
+    let traverse_Order () =
+        SideEffects.reset()
+        let mapper v = SideEffects.add <| sprintf "mapping %d" v
+        let _ = traverse (Option.map mapper) [Some 1; Some 2]
+        SideEffects.are ["mapping 1"; "mapping 2"]
+
+
     [<Test>]
     let traversableForNonPrimitive () =
         let nel = nelist { Some 1 }
diff --git a/tests/FSharpPlusFable.Tests/FSharpTests/General/Traversable.fs b/tests/FSharpPlusFable.Tests/FSharpTests/General/Traversable.fs
index 6749ffebd..52da97e16 100644
--- a/tests/FSharpPlusFable.Tests/FSharpTests/General/Traversable.fs
+++ b/tests/FSharpPlusFable.Tests/FSharpTests/General/Traversable.fs
@@ -92,6 +92,14 @@ let traversable = testList "Traversable" [
     #endif
 
     #if !FABLE_COMPILER || FABLE_COMPILER_3
+    testList "traverse_Order" [
+        testCase "nelist" (fun () ->
+            SideEffects.reset()
+            let mapper v = SideEffects.add <| sprintf "mapping %d" v
+            let _ = traverse (Option.map mapper) [Some 1; Some 2]
+            SideEffects.are ["mapping 1"; "mapping 2"]
+        )]
+
     testList "traversableForNonPrimitive" [
         testCase "nelist" (fun () ->
             let nel = nelist { Some 1 }