From 22599878a45c3676bfc47ac5fc7c9a640e4c7280 Mon Sep 17 00:00:00 2001
From: Joris Dral <joris@well-typed.com>
Date: Mon, 30 Jun 2025 10:05:12 +0200
Subject: [PATCH] A `SerialiseKey` instance for `Void`

---
 src/Database/LSMTree/Internal/Serialise/Class.hs | 10 ++++++----
 src/Database/LSMTree/Internal/Types.hs           |  3 +++
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/Database/LSMTree/Internal/Serialise/Class.hs b/src/Database/LSMTree/Internal/Serialise/Class.hs
index 75d73f6c0..c7859ed56 100644
--- a/src/Database/LSMTree/Internal/Serialise/Class.hs
+++ b/src/Database/LSMTree/Internal/Serialise/Class.hs
@@ -510,11 +510,13 @@ instance SerialiseValue P.ByteArray where
   Void
 -------------------------------------------------------------------------------}
 
-{- |
-This instance is intended for tables without blobs.
+-- | The implementation of 'deserialiseKey' throws an exception.
+instance SerialiseKey Void where
+  serialiseKey = absurd
+  deserialiseKey = error "deserialiseKey: cannot deserialise into Void"
 
-The implementation of @'deserialiseValue'@ throws an excepValuen.
--}
+
+-- | The implementation of 'deserialiseValue' throws an exception.
 instance SerialiseValue Void where
   serialiseValue = absurd
   deserialiseValue = error "deserialiseValue: cannot deserialise into Void"
diff --git a/src/Database/LSMTree/Internal/Types.hs b/src/Database/LSMTree/Internal/Types.hs
index ca8031cef..0d7db7131 100644
--- a/src/Database/LSMTree/Internal/Types.hs
+++ b/src/Database/LSMTree/Internal/Types.hs
@@ -19,6 +19,7 @@ import           Control.DeepSeq (NFData (..), deepseq)
 import           Data.Kind (Type)
 import           Data.Semigroup (Sum)
 import           Data.Typeable
+import           Data.Void (Void)
 import           Data.Word (Word64)
 import qualified Database.LSMTree.Internal.BlobRef as Unsafe
 import           Database.LSMTree.Internal.RawBytes (RawBytes (..))
@@ -225,4 +226,6 @@ instance ResolveValue (ResolveAsFirst v) where
   resolveSerialised :: Proxy (ResolveAsFirst v) -> RawBytes -> RawBytes -> RawBytes
   resolveSerialised _p = const
 
+deriving via ResolveViaSemigroup Void instance ResolveValue Void
+
 deriving via (ResolveViaSemigroup (Sum v)) instance (Num v, SerialiseValue v) => ResolveValue (Sum v)