Add trace

.gitignore

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+dist-newstyle
+.envrc

src/Trace.hs

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+{-# 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