From 179a1e8407bf2390c7ba761396c1e56303e89e4e Mon Sep 17 00:00:00 2001
From: Harry <harryscholes@users.noreply.github.com>
Date: Fri, 21 Sep 2018 16:57:54 +0100
Subject: [PATCH 1/2] Correct Custom Gradients docs
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* Fixed a type signature that was incorrect.
* Also, replaced `data(a)` with `a.data`. Don't know if the syntax has changed (recently). This may also need to be corrected in line 121.

MWE:

```julia
using Flux
using Flux.Tracker
using Flux.Tracker: forward, TrackedReal, track, @grad

minus(a, b) = a - b
minus(a::TrackedReal, b::TrackedReal) = Tracker.track(minus, a, b)
@grad function minus(a, b)
    return minus(a.data, b.data), Δ -> (Δ, -Δ)
end

a, b = param(2), param(4)
c = minus(a, b)  # -2.0 (tracked)
Tracker.back!(c)

Tracker.grad(a)  # 1.00
Tracker.grad(b)  # -1.00
```
---
 docs/src/internals/tracker.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/docs/src/internals/tracker.md b/docs/src/internals/tracker.md
index 3d39451d..895e4b52 100644
--- a/docs/src/internals/tracker.md
+++ b/docs/src/internals/tracker.md
@@ -102,14 +102,14 @@ Firstly, we must tell the tracker system to stop when it sees a call to `minus`,
 ```julia
 using Flux.Tracker: TrackedReal, track, @grad
 
-minus(a::TrackedArray, b::TrackedArray) = Tracker.track(minus, a, b)
+minus(a::TrackedReal, b::TrackedReal) = Tracker.track(minus, a, b)
 ```
 
 `track` takes care of building a new `Tracked` object and recording the operation on the tape. We just need to provide a gradient definition.
 
 ```julia
 @grad function minus(a, b)
-  return minus(data(a),data(b)), Δ -> (Δ, -Δ)
+  return minus(a.data, b.data), Δ -> (Δ, -Δ)
 end
 ```
 

From 61c14afee42b513387503eb900e2ebb81fb15d77 Mon Sep 17 00:00:00 2001
From: harryscholes <harryscholes@gmail.com>
Date: Tue, 9 Oct 2018 13:05:38 +0100
Subject: [PATCH 2/2] Add usage example of custom gradients

---
 docs/src/internals/tracker.md | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/docs/src/internals/tracker.md b/docs/src/internals/tracker.md
index 895e4b52..456a9129 100644
--- a/docs/src/internals/tracker.md
+++ b/docs/src/internals/tracker.md
@@ -100,16 +100,16 @@ minus(a, b) = a - b
 Firstly, we must tell the tracker system to stop when it sees a call to `minus`, and record it. We can do this using dispatch:
 
 ```julia
-using Flux.Tracker: TrackedReal, track, @grad
+using Flux.Tracker: TrackedArray, track, @grad
 
-minus(a::TrackedReal, b::TrackedReal) = Tracker.track(minus, a, b)
+minus(a::TrackedArray, b::TrackedArray) = track(minus, a, b)
 ```
 
 `track` takes care of building a new `Tracked` object and recording the operation on the tape. We just need to provide a gradient definition.
 
 ```julia
 @grad function minus(a, b)
-  return minus(a.data, b.data), Δ -> (Δ, -Δ)
+  return minus(data(a), data(b)), Δ -> (Δ, -Δ)
 end
 ```
 
@@ -121,6 +121,19 @@ Note that in the backpropagator we don't call `data(a)`; we *do* in fact want to
 @grad a * b = data(a)*data(b), Δ -> (Δ*b, a*Δ)
 ```
 
+We can then calculate the first derivative of `minus` as follows:
+
+```julia
+a = param([1,2,3])
+b = param([3,2,1])
+
+c = minus(a, b)  # [-2.0 (tracked), 0.0 (tracked), 2.0 (tracked)]
+
+Tracker.back!(c, 1)
+Tracker.grad(a)  # [1.00, 1.00, 1.00]
+Tracker.grad(b)  # [-1.00, -1.00, -1.00]
+```
+
 For multi-argument functions with custom gradients, you likely want to catch not just `minus(::TrackedArray, ::TrackedArray)` but also `minus(::Array, TrackedArray)` and so on. To do so, just define those extra signatures as needed:
 
 ```julia