From 837a3a695928dda6aeaaf63ecba2a61a59d17a91 Mon Sep 17 00:00:00 2001
From: Mario Rogic <hello@mario.net.au>
Date: Fri, 25 Oct 2024 23:09:57 +1000
Subject: [PATCH] WIP failing test for nested extensible records and
 preliminary thinking

---
 .../Test/Wire_Record_Extensible3_Tricky.elm   |   4 -
 .../Test/Wire_Record_Extensible6_Nested.elm   | 124 ++++++++++++++++++
 2 files changed, 124 insertions(+), 4 deletions(-)
 create mode 100644 test/scenario-alltypes/src/Test/Wire_Record_Extensible6_Nested.elm

diff --git a/test/scenario-alltypes/src/Test/Wire_Record_Extensible3_Tricky.elm b/test/scenario-alltypes/src/Test/Wire_Record_Extensible3_Tricky.elm
index a0460b2e..5c8bebe8 100644
--- a/test/scenario-alltypes/src/Test/Wire_Record_Extensible3_Tricky.elm
+++ b/test/scenario-alltypes/src/Test/Wire_Record_Extensible3_Tricky.elm
@@ -34,10 +34,6 @@ expected_w3_decode_Config =
 
    Essence from datetimepicker-legacy/src/DateTimePicker/Config.elm
 
-    @TODO remove this now?:
-   Tests neutered as our generation types are more specific than old Source based ones,
-   but the actual test was for a generation failure so this ensures the gen at least type checks
-
 -}
 
 
diff --git a/test/scenario-alltypes/src/Test/Wire_Record_Extensible6_Nested.elm b/test/scenario-alltypes/src/Test/Wire_Record_Extensible6_Nested.elm
new file mode 100644
index 00000000..16820ef0
--- /dev/null
+++ b/test/scenario-alltypes/src/Test/Wire_Record_Extensible6_Nested.elm
@@ -0,0 +1,124 @@
+module Test.Wire_Record_Extensible6_Nested exposing (..)
+
+import Bytes.Decode
+import Bytes.Encode
+import Lamdera.Wire3
+
+
+type alias Derp =
+    A { dog : () }
+
+
+
+-- So working backwards from a concrete example, what would this encoder/decoder look like?
+
+
+expected_w3_encode_Derp : Derp -> Lamdera.Wire3.Encoder
+expected_w3_encode_Derp =
+    \w3_rec_var0 ->
+        Lamdera.Wire3.encodeSequenceWithoutLength
+            [ Lamdera.Wire3.encodeUnit w3_rec_var0.a
+            , Lamdera.Wire3.encodeUnit w3_rec_var0.dog
+            ]
+
+
+expected_w3_decode_Derp =
+    Lamdera.Wire3.succeedDecode
+        (\a0 dog0 -> { a = a0, dog = dog0 })
+        |> Lamdera.Wire3.andMapDecode
+            Lamdera.Wire3.decodeUnit
+        |> Lamdera.Wire3.andMapDecode
+            Lamdera.Wire3.decodeUnit
+
+
+
+{-
+
+   Notice how the attributes `.a` and `.dog` are both statically called – that's because
+   Wire's record encoding is lexicographically ordered by field name.
+
+   We're specifically _not_ calling w3_encode_A, because if we did, we'd have to chose
+   between one of two implementations:
+
+    Lamdera.Wire3.encodeSequenceWithoutLength
+        (w3_encode_A w3_rec_var0
+            ++ [ Lamdera.Wire3.encodeUnit w3_rec_var0.dog
+               ]
+        )
+
+    Lamdera.Wire3.encodeSequenceWithoutLength
+        ([ Lamdera.Wire3.encodeUnit w3_rec_var0.dog
+         ]
+            ++ w3_encode_A w3_rec_var0
+        )
+
+    In this specific case, the first one is correct, because `a` comes before `dog`,
+    but if the attribute was `x`, it would be the second one.
+
+    So to fix _that_, we'd need to return some extra info such that a caller could
+    sort the attributes... aaand now we're really getting into the weeds.
+
+
+    So we're effectively saying "if you're going to use an extensible record, then use a
+    type alias that concretely fills all type vars, and then use that type instead".
+
+    This leaves us with an outstanding problem for anonymous extensible records, i.e.:
+
+    type alias SomeRecord =
+        { herp : String
+        , derp : Int
+        , extensible : A { dog : String }
+        }
+
+    Here we've got the `A { dog : String }` in the middle of the record, and we can't
+    just use `A { dog : String }` as a type, because it's not a concrete type, and as
+    we just discussed, we can't make wire encoding/decoding functions for it on the fly,
+    because we can't sort the attributes. It needs to be concrete.
+
+
+-}
+-- None of these can actually be used, because type variables must be fully applied,
+-- meaning the holey extensible record itself is never directly as a value.
+
+
+type alias A x =
+    { x | a : () }
+
+
+expected_w3_encode_A : ({ x | a : () } -> Lamdera.Wire3.Encoder) -> (A x -> Lamdera.Wire3.Encoder)
+expected_w3_encode_A w3_x_c_x =
+    w3_x_c_x
+
+
+expected_w3_decode_A : Bytes.Decode.Decoder (A { a : () })
+expected_w3_decode_A =
+    -- "Cannot decode an extensible record with unfilled type vars"
+    Lamdera.Wire3.failDecode
+
+
+type alias B x =
+    A { x | b : () }
+
+
+expected_w3_encode_B : (B x -> Lamdera.Wire3.Encoder) -> (B x -> Lamdera.Wire3.Encoder)
+expected_w3_encode_B w3_x_c_x =
+    w3_x_c_x
+
+
+expected_w3_decode_B w3_x_c_x =
+    -- "Cannot decode an extensible record with unfilled type vars"
+    Lamdera.Wire3.failDecode
+
+
+type alias C x =
+    B { x | c : () }
+
+
+expected_w3_encode_C : (C x -> Lamdera.Wire3.Encoder) -> (C x -> Lamdera.Wire3.Encoder)
+expected_w3_encode_C w3_x_c_x =
+    w3_x_c_x
+
+
+expected_w3_decode_C w3_x_c_x =
+    -- "Cannot decode an extensible record with unfilled type vars"
+    Lamdera.Wire3.failDecode