add finallyDo to helper functions (#34)

Author: ni-ko-o-kin

src/ConcurrentTask.elm

@@ -5,7 +5,7 @@ module ConcurrentTask exposing
     , onResponseDecoderFailure, onJsException
     , mapError, onError
     , succeed, fail, andThen
-    , fromResult, toResult, andThenDo, return, debug
+    , fromResult, toResult, andThenDo, finallyDo, return, debug
     , race
     , batch, sequence
     , map, andMap, map2, map3, map4, map5
@@ -69,7 +69,7 @@ Lift `UnexpectedError`s into regular task flow.
 
 These are some general helpers that can make chaining, combining and debugging tasks more convenient.
 
-@docs fromResult, toResult, andThenDo, return, debug
+@docs fromResult, toResult, andThenDo, finallyDo, return, debug
 
 
 # Racing Helpers
@@ -696,6 +696,40 @@ andThenDo t2 t1 =
     t1 |> andThen (\_ -> t2)
 
 
+{-| Always executes the Task, regardless of whether the previous Task has succeeded or failed.
+
+The behavior is similar to JavaScript's `finally` block in a `try-catch-finally` statement.
+
+This can, for example, be used to ensure that a resource is always released after being locked:
+
+    import ConcurrentTask exposing (ConcurrentTask)
+
+    lockResource : ConcurrentTask String ()
+    lockResource =
+        succeed ()
+
+    unlockResource : ConcurrentTask String ()
+    unlockResource =
+        succeed ()
+
+    performOperation : ConcurrentTask String ()
+    performOperation =
+        fail "operation failed"
+
+    secureOperation : ConcurrentTask String ()
+    secureOperation =
+        lockResource
+            |> andThenDo performOperation
+            |> finallyDo unlockResource
+
+-}
+finallyDo : ConcurrentTask x a -> ConcurrentTask x b -> ConcurrentTask x a
+finallyDo t2 t1 =
+    t1
+        |> onError (\e -> t2 |> andThenDo (fail e))
+        |> andThenDo t2
+
+
 {-| Succeed with a hardcoded value after the previous Task.
 
 Maybe you want to do some Tasks on a User but allow it to be chained onwards:

tests/TaskTest.elm

@@ -23,6 +23,7 @@ suite =
         [ hardcoded
         , successResponses
         , errors
+        , helpers
         ]
 
 
@@ -448,6 +449,35 @@ hardcoded =
         ]
 
 
+helpers : Test
+helpers =
+    describe "Task Helpers"
+        [ describe "finallyDo"
+            [ test "Runs the followup task when the first task succeeds" <|
+                \_ ->
+                    createTask
+                        |> ConcurrentTask.finallyDo createTask
+                        |> runTask
+                            [ ( 0, Encode.string "first" )
+                            , ( 1, Encode.string "followup" )
+                            ]
+                        |> Expect.equal (Internal.Success "followup")
+            , test "Runs the followup task when the first task fails" <|
+                \_ ->
+                    createTask
+                        |> ConcurrentTask.finallyDo (ConcurrentTask.succeed "followup")
+                        |> evalTask
+                            [ ( 0, jsException "error" Encode.null )
+                            , ( 1, Encode.string "followup" )
+                            ]
+                        |> Expect.all
+                            [ Tuple.second >> Expect.equal (Internal.Error "error")
+                            , Tuple.first >> Ids.get >> Expect.equal "1"
+                            ]
+            ]
+        ]
+
+
 
 -- Task Runner