missing `logabsgamma` rules

[Moelf]

JuliaStats/LogExpFunctions.jl#49

I can try to make a PR? is there any example of similar function?

[devmotion]

I added a rule to #79, seems I forgot to add it in that PR.

[Moelf]

Oh sweet, I will wait for it to merge then

[devmotion]

Oh wait, I don't think it's currently possible to define a rule with DiffRules for this function. AFAIK DiffRules only supports real-valued functions with n real-valued arguments but logabsgamma returns a tuple of real values.

[Moelf]

ehhh, can we make a new function that returns logabsgamma(x)[1] then?

[devmotion]

Maybe we could add first \circ logabsgamma on more recent Julia versions since it's possible to dispatch on it there.

However, I am not sure if DiffRules and inparticular downstream packages can deal with rules where there's not a symbol as function name but an to-be-dispatched expression.

Probably the simpler solution is adding a definition of logabsgamma(::Dual) to ForwardDiff (it already depends on SpecialFunctions). Alternatively, you could use a ChainRulesCore compatible AD system (a rule is defined for CR).

[devmotion]

I opened a PR: JuliaDiff/ForwardDiff.jl#585

[devmotion]

The PR is merged and part of ForwardDiff 0.10.30 which is available soon: JuliaRegistries/General#60502