Change ListGen.traverse from monadic to applicative

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Version ?.?.?
 
 - Add `Tree.apply`. Change `Gen.apply` from monadic to applicative. Revert runtime optimization of `Gen.integral`. ([#398][398], [@TysonMN][TysonMN])
+- Change `ListGen.traverse` from monadic to applicative. ([#399][399], [@TysonMN][TysonMN])
 
 ## Version 0.12.0 (2021-12-12)
 
@@ -189,6 +190,8 @@
 [porges]:
   https://github.com/porges
 
+[399]:
+  https://github.com/hedgehogqa/fsharp-hedgehog/pull/399
 [398]:
   https://github.com/hedgehogqa/fsharp-hedgehog/pull/398
 [386]:

src/Hedgehog/ListGen.fs

@@ -1,15 +1,15 @@
 module Hedgehog.ListGen
 
+
 let traverse (f: 'a -> Gen<'b>) (ma: list<'a>) : Gen<list<'b>> =
-    let rec loop input output =
-        match input with
-        | [] -> output |> List.rev |> Gen.constant
-        | a :: input ->
-            gen {
-                let! b = f a
-                return! loop input (b :: output)
-            }
-    loop ma []
+  List.empty
+  |> Gen.constant
+  |> List.fold (fun glb a ->
+    (fun list b -> List.append list [b])
+    |> Gen.constant
+    |> Gen.apply glb
+    |> Gen.apply (f a))
+  <| ma
 
 let sequence (gens : List<Gen<'a>>) : Gen<List<'a>> =
     gens |> traverse id