From 723ce8ef9fb9d18ccc81c5a59885001cae4b3c05 Mon Sep 17 00:00:00 2001
From: Mats Rauhala <mats.rauhala@iki.fi>
Date: Wed, 3 Nov 2021 13:17:09 +0200
Subject: [PATCH] Add trace

---
 .gitignore   | 2 ++
 src/Trace.hs | 9 +++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 src/Trace.hs

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..cd7dd01
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+dist-newstyle
+.envrc
diff --git a/src/Trace.hs b/src/Trace.hs
new file mode 100644
index 0000000..823f9ff
--- /dev/null
+++ b/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