Document ActionFieldsG

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## Description ✍️
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Add some documentation on `ActionFieldG` type.

PR-URL: https://github.com/hasura/graphql-engine-mono/pull/3945
GitOrigin-RevId: d9543ed7a8fe3ccfe9f5267c3a2ac71fb040f4db

server/src-lib/Hasura/GraphQL/Execute/Action.hs

@@ -139,9 +139,9 @@ resolveActionExecution env logger _userInfo AnnActionExecution {..} ActionExecCo
           _aaeResponseTransform
 
 -- | Build action response from the Webhook JSON response when there are no relationships defined
-makeActionResponseNoRelations :: RA.ActionFieldsG Void -> ActionWebhookResponse -> AO.Value
+makeActionResponseNoRelations :: RA.ActionFields -> ActionWebhookResponse -> AO.Value
 makeActionResponseNoRelations annFields webhookResponse =
-  let mkResponseObject :: RA.ActionFieldsG Void -> HashMap Text J.Value -> AO.Value
+  let mkResponseObject :: RA.ActionFields -> HashMap Text J.Value -> AO.Value
       mkResponseObject fields obj =
         AO.object $
           flip mapMaybe fields $ \(fieldName, annField) ->
@@ -603,7 +603,7 @@ processOutputSelectionSet ::
   RS.ArgumentExp v ->
   GraphQLType ->
   [(PGCol, PGScalarType)] ->
-  RA.ActionFieldsG Void ->
+  RA.ActionFields ->
   StringifyNumbers ->
   RS.AnnSimpleSelectG ('Postgres 'Vanilla) Void v
 processOutputSelectionSet tableRowInput actionOutputType definitionList actionFields =

server/src-lib/Hasura/GraphQL/Execute/RemoteJoin/Collect.hs

@@ -400,7 +400,7 @@ transformAnnFields fields = do
 -- 'RemoteJoin'.
 transformActionFields ::
   ActionFieldsG (RemoteRelationshipField UnpreparedValue) ->
-  Collector (ActionFieldsG Void)
+  Collector ActionFields
 transformActionFields fields = do
   -- Produces a list of transformed fields that may or may not have an
   -- associated remote join.

server/src-lib/Hasura/RQL/IR/Action.hs

@@ -16,11 +16,20 @@ import Hasura.Prelude
 import Hasura.RQL.Types.Common (FieldName, Fields)
 import Language.GraphQL.Draft.Syntax qualified as G
 
+-- | Internal representation for a selection of fields on the result of an action.
+-- Type parameter r will be either
+-- r ~ (RemoteRelationshipField UnpreparedValue) when the AST is emitted by the parser.
+-- r ~ Void when an execution tree is constructed so that a backend is
+-- absolved of dealing with remote relationships.
 data ActionFieldG (r :: Type)
-  = ACFScalar G.Name
-  | ACFRemote (ActionRemoteRelationshipSelect r)
-  | ACFExpression Text
-  | ACFNestedObject G.Name !(ActionFieldsG r)
+  = -- | Scalar value. G.Name is the original field name from the object type.
+    ACFScalar G.Name
+  | -- | Remote relationship
+    ACFRemote (ActionRemoteRelationshipSelect r)
+  | -- | Constant text value (used for __typename fields)
+    ACFExpression Text
+  | -- | Nested object. G.Name is the original field name from the object type.
+    ACFNestedObject G.Name !(ActionFieldsG r)
   deriving (Eq, Show, Functor, Foldable, Traversable)
 
 type ActionFieldsG r = Fields (ActionFieldG r)