Add trace

src/Trace.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE DerivingVia #-}
+module Trace where
+
+import Data.Functor.Contravariant (Op(..), Contravariant)
+import Data.Monoid (Ap(..))
+
+newtype Trace m a = Trace { trace :: a -> m () }
+  deriving Contravariant via Op (m ())
+  deriving (Semigroup, Monoid) via Op (Ap m ()) a