Merge pull request #412 from andrewMacmurray/stack-safe-helpers

Makes BackendTask.combine stack safe

src/BackendTask.elm

@@ -88,6 +88,7 @@ Any place in your `elm-pages` app where the framework lets you pass in a value o
 
 import FatalError exposing (FatalError)
 import Json.Encode
+import List.Chunks
 import Pages.StaticHttpRequest exposing (RawRequest(..))
 
 
@@ -175,6 +176,26 @@ resolve =
 -}
 combine : List (BackendTask error value) -> BackendTask error (List value)
 combine items =
+    items
+        |> List.Chunks.chunk 100
+        |> List.map combineHelp
+        |> List.Chunks.chunk 100
+        |> List.map combineHelp
+        |> List.Chunks.chunk 100
+        |> List.map combineHelp
+        |> combineHelp
+        |> map (List.concat >> List.concat >> List.concat)
+
+
+{-| `combineHelp` on its own will overflow the stack with larger lists of tasks
+
+dividing the tasks into smaller nested chunks and recombining makes `combine` stack safe
+
+There's probably a way of doing this without the Lists but it's a neat trick to safely combine lots of tasks!
+
+-}
+combineHelp : List (BackendTask error value) -> BackendTask error (List value)
+combineHelp items =
     List.foldl (map2 (::)) (succeed []) items |> map List.reverse
 
 

src/List/Chunks.elm

@@ -0,0 +1,51 @@
+module List.Chunks exposing (chunk)
+
+import Array exposing (Array)
+
+
+{-| Adapted from <https://package.elm-lang.org/packages/krisajenkins/elm-exts/latest/Exts-List>
+
+Split a list into chunks of length `n`.
+
+Be aware that the last sub-list may be smaller than `n`-items long.
+
+For example `chunk 3 [1..10] => [[1,2,3], [4,5,6], [7,8,9], [10]]`
+
+-}
+chunk : Int -> List a -> List (List a)
+chunk n xs =
+    if n < 1 then
+        List.singleton xs
+
+    else
+        evaluate (chunkInternal n xs Array.empty)
+
+
+chunkInternal : Int -> List a -> Array (List a) -> Trampoline (List (List a))
+chunkInternal n xs acc =
+    -- elm-review: known-unoptimized-recursion
+    if List.isEmpty xs then
+        Done (Array.toList acc)
+
+    else
+        Jump
+            (\_ ->
+                chunkInternal n
+                    (List.drop n xs)
+                    (Array.push (List.take n xs) acc)
+            )
+
+
+type Trampoline a
+    = Done a
+    | Jump (() -> Trampoline a)
+
+
+evaluate : Trampoline a -> a
+evaluate trampoline =
+    case trampoline of
+        Done value ->
+            value
+
+        Jump f ->
+            evaluate (f ())