Lack rule for ifelse

[wupeifan]

I was wondering whether a rule for ifelse(condition, exp1, exp2) can be created? Thanks a lot!

[JianghuiDu]

Most of the times ifelse can be replaced by combinations of max and mix which are included in DiffRules.jl.
Something like this https://exceljet.net/replace-ugly-ifs-with-max-or-min.

[wupeifan]

I raised this issue exactly because I want to avoid max and min combinations, but thanks for your suggestions.

[mcabbott]

Should be simple to add, and worth having IMO.

[wupeifan]

I think it should be something like D(ifelse(condition, exp1, exp2)) = ifelse(condition, D(exp1), D(exp2)). If it is just this simple then I can make a little PR, but I don't know whether there are other potential issues.

[mcabbott]

I was thinking it looks like this, but I'm not sure that false is actually the right thing... in Zygote etc. it would be nothing:

@define_diffrule Base.:ifelse(p, x, y) = :(false), :($p), :(!$p)

Trying this out with a package that uses this (old Zygote, maybe Tracker) would increase my confidence.