add seq and forEach to Task

platform/Task.roc

@@ -13,6 +13,8 @@ module [
     fromResult,
     batch,
     result,
+    seq, 
+    forEach,
 ]
 
 import Effect
@@ -244,3 +246,34 @@ result = \task ->
             \res -> res |> ok |> InternalTask.toEffect
 
     InternalTask.fromEffect effect
+## Apply a task repeatedly to a list of items, and return a list of the resulting values
+##
+## ```
+## authors : List ID
+## getAuthor : ID -> Task Author [DbError]
+##
+## getAuthors : Task (List Author) [DbError]
+## getAuthors = Task.list authors getAuthor
+## ```
+##
+seq : List (Task ok err) -> Task (List ok) err
+seq = \tasks ->
+    List.walk tasks (Task.ok []) \state, task ->
+        value <- task |> Task.await
+
+        state |> Task.map \values -> List.append values value
+
+## Apply a task repeatedly for each item in a list
+##
+## ```
+## authors : List Author
+## saveAuthor : Author -> Task {} [DbError]
+##
+## saveAuthors : Task (List Author) [DbError]
+## saveAuthors = Task.forEach authors saveAuthor
+## ```
+##
+forEach : List a, (a -> Task {} b) -> Task {} b
+forEach = \items, fn ->
+    List.walk items (Task.ok {}) \state, item ->
+        state |> Task.await \_ -> fn item