ReverseDiffOverload

Reverse mode differentiation for pre-defined functions.

Using reverse mode differentiation is very simple, just call,

reversediff(function, args...)

for instance, to differentiate f, defined by,

f(a, b) = begin
    c = a*b
    dot(c, c) + dot(c, b)
end

at, [1. 2; 3 4], [1., 2], call,

reversediff(f, [1. 2; 3 4], [1., 2])

which returns a tuple containing the return value, and the differential of the return value with respect to every argument,

(173.0,(
2x2 Array{Float64,2}:
 11.0  22.0
 24.0  48.0,

[88.0,129.0])

Testing

It is good practice, at this early stage, to test that reversediff produces the correct results for your problem. To do such a test on your function, simply call,

testdiff(g, args...)

for instance,

testdiff(f, [1. 2; 3 4], [1., 2])

An error will be generated if finite difference and our method give different results.

Troubleshooting

Of course, things aren't always quite that simple. There are three common sources of bugs. First, reversediff differentiates with respect to every argument, so every argument should have type Float64, or Array{Float64}. Notice the care taken in the example to make sure that we don't get an array of Int. Second, the type signature of the function you're trying to differentiate may be too constrained - the function needs to let values of type Call propagate through until they reach known functions. Notice, f, defined above had no type constraints. Third, you may be trying to use a function whose differential is not yet defined. You can provide define new definitions using the macro @d1, for one argument functions, or @d2, for two argument functions, for instance, to redefine the differentials for *, we might use,

ReverseDiff.@d2(*, d*y', x'*d)

Where x is the first argument to the function, y is the second argument, and d is the differential of the objective with respect to the result of the function call. Note that you can also annotate the types of x and y, using,

ReverseDiff.@d2(*, d*y', x'*d, AbstractArray, AbstractArray)