Allow IOG's contra-tracer

Author: jasagredo

lsm-tree.cabal

@@ -613,7 +613,7 @@ library
     , bytestring              ^>=0.11.4.0 || ^>=0.12.1.0
     , cborg                   ^>=0.2.10.0
     , containers              ^>=0.6      || ^>=0.7
-    , contra-tracer           ^>=0.2
+    , contra-tracer           ^>=0.1      || ^>=0.2
     , crc32c                  ^>=0.2.1
     , deepseq                 ^>=1.4      || ^>=1.5
     , filepath

src/Database/LSMTree/Internal/Unsafe.hs

@@ -315,12 +315,19 @@ data TableTrace =
 #endif
   deriving stock Show
 
-contramapTraceMerge :: Monad m => Tracer m TableTrace -> Tracer m (AtLevel MergeTrace)
+contramapTraceMerge :: forall m. Monad m => Tracer m TableTrace -> Tracer m (AtLevel MergeTrace)
+#if MIN_VERSION_contra_tracer(0,2,0)
 #ifdef DEBUG_TRACES
 contramapTraceMerge t = TraceMerge `contramap` t
 #else
 contramapTraceMerge t = traceMaybe (const Nothing) t
 #endif
+#else
+contramapTraceMerge _t = nullTracer
+  where
+    -- See #766
+    _unused = pure @m ()
+#endif
 
 -- | Trace messages related to cursors.
 data CursorTrace =