Diagnosing rrule issues

[mcabbott]

@e3c6

function mylog_pullback(δ)
        if x > 0
            dx = 1/x
        else
            dx = oftype(1/x, Inf)
        end
        return NoTangent(), dx
    end

This needs to be dx * δ, then I think it should work (at least for positive x).

@oxinabox

we can’t tell if a type like Int actually represents a continous quanity and it is just a special case of what is conceptually a float (etc) and so we should perturb it.
Or if it actually represents an an index like d in size(x, d) , in which case we should not pertube it, and the correct tangent is NoTangent since it is not pertable.
We assume it is the later.

One possibility here would be for the tester to try evaluating the function at a non-integer point. size(x, nextfloat(float(d))) will be an error, from which you infer that d::Int represents a non-perturbable parameter.

(But not sure it’s worth the effort & complication.)