David Widmann

> 1. It is quite a burden to maintain (in particular when adding new features). Is there a concrete example of a recent feature for which Tracker support was not...

> due to its lack of support for ChainRules. Both Zygote and ReverseDiff now suppports it There is at least interest in adding something similar to what's done in ReverseDiff...

Maybe https://github.com/JuliaStats/Distributions.jl/blob/master/src/common.jl#L149 messes with Tracker's broadcasting? What happens if you add the same definition for your custom struct?

OK, I figured out why this is happening: It's caused by the heuristic in https://github.com/FluxML/Tracker.jl/blob/043da25fe491a5c705898e4de90cb8567f93373f/src/lib/array.jl#L558 and the definition of `eltype(::Normal{T}) = T` (well, actually Distributions defines it on types as...

I'm not completely sure about the motivation of this check but to me it seems the heuristic is supposed to drop tracking information in cases where the output is known...

> Anything with eltype `Bool` gets rejected (including bools themselves), so as long as the conditional remains the same, The fix for Distributions (and e.g. samplers in general: https://docs.julialang.org/en/v1/stdlib/Random/#A-simple-sampler-without-pre-computed-data) requires...

I don't follow completely - what are you after in the end? Derivatives of the kernel functions with respect to one or both arguments? IIRC many kernels in KernelFunctions already...

IPMCMC is implemented in https://www.github.com/TuringLang/AdvancedPS.jl/tree/master/src%2Fcontrib%2FAdvancedSMCExtensions.jl, but currently broken. It just has to be updated to the new interface which hopefully is quite straightforward.

Sure, one can always roll a custom `mcmcsample`. The only downside is that you lose some of the default features. BTW if you want to have more control over the...

> @devmotion what are your current thoughts on AbstractDifferentiation? It's the package everyone seems to want to use at some point in the future but basically nobody uses right now...