You cannot write to the parameters of a formula or class inside a function. Note that writing to parameters is not recommended anyway. Example:
Test {
init:
  @p = (2, 3)      ; Warning, not recommended!
  func tryWrite()
    @p = (2, 3)    ; Error, not allowed!
  endfunc
  
default:
  complex param p
    default = (0, 0)
  endparam
}
See Also
Errors