Hi all,

I am trying to define a custom STE gradient in Flux. The activation function is the sign function, and its gradient should just be the incoming gradient as is, iff its absolute value is <= 1, or cancelled otherwise. The code I have is below

```
binarize(x) = x>=0 ? true : false
binarize(x::Flux.Tracker.TrackedReal) = Flux.Tracker.track(binarize, x)
@grad function binarize(x)
return binarize(Flux.Tracker.data(x)), Δ -> (abs(x) <= 1 ? Δ : 0, )
end
```

For a random 5x1 array `a`

, i do the following

```
>> a= param(randn(5))
>> Tracked 5-element Array{Float64,1}:
-0.3605564089879154
-0.7853512499733902
0.8102988051980005
-0.9715952052917924
-1.276343849200165
>> c=binarize.(a)
>> 5-element BitArray{1}:
false
false
true
false
false
>> Tracker.back!(c, [1,1,1,1,1])
>> a.grad
>> 5-element Array{Float64,1}:
0.0
0.0
0.0
0.0
0.0
```

There are a couple problems here. First of all I’d expect the gradient to be a TrackedArray.

But more importantly, the returned gradient should not be all zeroes, but ones except for the last element right?

What am I doing wrong here?

Thanks,