AD.R

library(JuliaCall)
julia_setup()
julia_library("ForwardDiff")
f <- function(x) sum(x^2L)
julia_assign("ff", f)
julia_command("g = x -> ForwardDiff.gradient(ff, x);")
julia_command("g([2.0])")
## 1-element Array{Float64,1}:
## 4.0
  
julia_command("g([3.0])")
## 1-element Array{Float64,1}:
## 6.0
  
## This is problematic, don't know why, but it seems like a problem related to integer?
julia_command("g([4.0])")
## 1-element Array{Float64,1}:
## 32.0
  
## This works as expected.
julia_command("g([4.01])")
## 1-element Array{Float64,1}:
## 8.02
  
## This doesn't work also...
julia_command("g([1.01,4.01])")
## Error: Error happens when you try to call function sum in Julia.
## DimensionMismatch("dimensions must match")

## But this works...
julia_command("g([1.01,2.01])")
## 2-element Array{Float64,1}:
## 2.02
## 4.02
  
## And this doesn't work!!!
julia_command("g([1.01,2.0])")
## Error: Error happens when you try to call function sum in Julia.
## DimensionMismatch("dimensions must match")
  
julia_command("g([3.01,2.01])")
## 2-element Array{Float64,1}:
## 6.02
## 4.02