From 63ed03865494887f7da339817de30fc193ee6c79 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@github.com>
Date: Thu, 16 Jun 2016 11:58:17 -0400
Subject: [PATCH] :memo: why OVERLAPPABLE.

---
 src/Data/Record.hs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/Data/Record.hs b/src/Data/Record.hs
index 70b2a6df9..2f62148a0 100644
--- a/src/Data/Record.hs
+++ b/src/Data/Record.hs
@@ -33,6 +33,8 @@ class HasField (fields :: [*]) (field :: *) where
 
 -- Instances
 
+-- OVERLAPPABLE is required for the HasField instances so that we can handle the two cases: either the head of the non-empty h-list is the requested field, or it isn’t. The third possible case (the h-list is empty) is rejected at compile-time.
+
 instance {-# OVERLAPPABLE #-} HasField fields field => HasField (notIt ': fields) field where
   getField (RCons _ t) = getField t