Merge pull request #11 from insurello/early-return-on-error

Optimize traverse

Author: AntonStrand

README.md

@@ -299,7 +299,7 @@ Solves the same problem as [`bind2`](#bind2) but for five arguments.
 sequence : List<AsyncResult<'a, 'error>> -> AsyncResult<'a list, 'error>
 ```
 
-From time to time you will find yourself having a list of `AsyncResult` (`List<AsyncResult<'a, 'err>`) but you would rather have an `AsyncResult` with a list of values (`AsyncResult<'a list, 'error>`). In those cases `sequence` can help you.
+From time to time you will find yourself having a list of `AsyncResult` (`List<AsyncResult<'a, 'err>`) but you would rather have an `AsyncResult` with a list of values (`AsyncResult<'a list, 'error>`). In those cases `sequence` can help you. `sequence` will make an early return if it reaches an `Error`.
 
 ```fsharp
 fetchUser : int -> AsyncResult<User, string>
@@ -315,7 +315,7 @@ userIds // [1; 2; 3]
 traverse : ('a -> 'b) -> List<AsyncResult<'a, 'err>> -> AsyncResult<'b list, 'err>
 ```
 
-Similar to [`sequence`](#sequence), `traverse` will also change the type from a list of `AsyncResult` to an `AsyncResult` with a list. The difference is that `traverse` allows you to use a transformation function to transform each value in the list of `AsyncResult`.
+Similar to [`sequence`](#sequence), `traverse` will also change the type from a list of `AsyncResult` to an `AsyncResult` with a list. The difference is that `traverse` allows you to use a transformation function to transform each value in the list of `AsyncResult`. `traverse` will make an early return if it reaches an `Error`.
 
 By sending in `id` as the transform function you have implemented `sequence`. Let's have a look how we can solve the example in the `sequence` description using `traverse` instead.
 

src/Insurello.AsyncExtra.Tests/AsyncExtraTests.fs

@@ -94,6 +94,24 @@ let traverseTests =
               let! actual = AsyncResult.traverse transformer input
               Expect.equal actual expected "should equal"
           }
+          testAsync "should make an early return if there is an Error" {
+              let mutable counter = 0
+
+              let transformer x =
+                  counter <- counter + x
+                  x
+
+              let expected = 1
+
+              let input =
+                  [ (AsyncResult.singleton 1)
+                    (AsyncResult.fromResult (Error "Skip next"))
+                    (AsyncResult.singleton 3) ]
+
+              let! _ = AsyncResult.traverse transformer input
+
+              Expect.equal counter expected "should equal"
+          }
           testAsync "should execute async task in sequence" {
               let mutable orderRun = []
 

src/Insurello.AsyncExtra/AsyncExtra.fs

@@ -77,12 +77,14 @@ module AsyncResult =
 
     let traverse : ('a -> 'b) -> List<AsyncResult<'a, 'err>> -> AsyncResult<'b list, 'err> =
         fun transformer list ->
-            let cons head tail = head :: tail
+            let cons tail head = head :: tail
 
-            let mapConcat headR tailR =
-                apply (map (transformer >> cons) headR) tailR
+            let rec fold remaining acc =
+                match remaining with
+                | [] -> acc |> List.rev |> singleton
+                | xA :: xAs -> xA |> bind (transformer >> cons acc >> fold xAs)
 
-            List.foldBack mapConcat list (singleton [])
+            fold list []
 
     let sequence : List<AsyncResult<'a, 'error>> -> AsyncResult<'a list, 'error> = fun list -> traverse id list