From 7a75bd34268bbb2686da34185d93fd649a76d231 Mon Sep 17 00:00:00 2001
From: Jarrett Revels <jarrettrevels@gmail.com>
Date: Tue, 19 Feb 2019 13:22:41 -0500
Subject: [PATCH] add hook for abs definition that downstream packages can
 overload

---
 src/rules.jl | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/rules.jl b/src/rules.jl
index 47e7576..5ed0ca8 100644
--- a/src/rules.jl
+++ b/src/rules.jl
@@ -70,7 +70,11 @@ else
         :(  SpecialFunctions.digamma($x)  )
 end
 @define_diffrule Base.transpose(x)            = :(  1                                  )
-@define_diffrule Base.abs(x)                  = :( signbit($x) ? -one($x) : one($x)    )
+@define_diffrule Base.abs(x)                  = :( DiffRules._abs_deriv($x)            )
+
+# We provide this hook for special number types like `Interval`
+# that need their own special definition of `abs`.
+_abs_deriv(x) = signbit(x) ? -one(x) : one(x)
 
 # binary #
 #--------#