R optim, can I pass a list to parameter :par?

Question:

I am using optim to optimise my function. However, my function have 2 paramters, first is a scalar, second is a dynamic length vector.

I initialized parameters with a list and then pass into optim function. Raise the following error, it seems par in optim can only get a vactor input? Since my second parameter has dynamic length, it's difficult to use a vector of parameters and then slice with y <- para_vector[slice] to pass them to y.

For instance:

# I dont know the length of second elements
para <- c(1,3,2, ... ,283)
x <- para[1]
# I can't slice the vector and pass value to y
y <- para[2:?]

Code:

obj <- function(para){
  ### input:
  ### para is a list with 2 elements.first element is scalar, second element is vector 

  # x is a scalar, stored in the first position of para list
  x <- para[[1]]
  # y is a vector with unfixed length, so I store it in the second element in a list
  y <- para[[2]] 

  value <- x^2 + sum(y^2)
  return(value)
}

para_initial = list(1,c(0,1))

optim(par = list(1,c(0,1)),fn = obj)

Output:

Error in optim(par = list(1, c(0, 1)), fn = obj) : (list) object cannot be coerced to type 'double'