SUGGESTION: add a 'ResolveAsFirst' newtype wrapper

This is one way to recover some desirable behaviour from the `Normal`/`Monoidal`
API split. In particular, if a user does not intend to use `Mupserts`, then in
the full API they still have to define `ResolveValue` instances. The
`ResolveAsFirst` wrapper provides a sensible default instance when this is the
case.

src/Database/LSMTree.hs

@@ -92,6 +92,8 @@ module Database.LSMTree (
     -- ** Properties
   , resolveValueValidOutput
   , resolveValueAssociativity
+    -- ** DerivingVia wrappers
+  , ResolveAsFirst (..)
 
     -- * Utility types
   , IOLike
@@ -113,6 +115,7 @@ import qualified Database.LSMTree.Common as Common
 import qualified Database.LSMTree.Internal as Internal
 import qualified Database.LSMTree.Internal.BlobRef as Internal
 import qualified Database.LSMTree.Internal.Entry as Entry
+import qualified Database.LSMTree.Internal.RawBytes as RB
 import qualified Database.LSMTree.Internal.Serialise as Internal
 import qualified Database.LSMTree.Internal.Snapshot as Internal
 import qualified Database.LSMTree.Internal.Vector as V
@@ -536,3 +539,20 @@ union = error "union: not yet implemented" $ union @m @k @v @b
 
 resolve ::  forall v. ResolveValue v => Proxy v -> Internal.ResolveSerialisedValue
 resolve = coerce . resolveValue
+
+-- | Newtype wrapper for use with deriving via, which resolves values as a
+-- 'Data.Semigroup.First' semigroup.
+--
+-- The instance makes 'Insert's isomorphic to 'Mupsert's. This can be a sensible
+-- default if there is no intent to use 'Mupsert's.
+newtype ResolveAsFirst v = ResolveAsFirst v
+  deriving stock (Show, Eq, Ord)
+  deriving newtype SerialiseValue
+
+instance ResolveValue (ResolveAsFirst v) where
+  resolveValue ::
+       Proxy (ResolveAsFirst v)
+    -> RB.RawBytes
+    -> RB.RawBytes
+    -> RB.RawBytes
+  resolveValue _ x _ = x