title: How to interpret error "elements..... must be named" when sourcing an R6 class?
body: <p>Code snippets taken from <a href="https://github.com/chernan/RESTfulAccessExamples/blob/master/bioDBnet.Rmd" rel="noreferrer">chernan's sample REST queries</a> are utilized to define an R6 class one private method, two public attributes and a constructor:</p>
  
  <pre><code>library(R6)
  library(RCurl)
  library(RJSONIO)
  Symbol &lt;- R6Class("Symbol",
    private = list(
      #
      # define a generic function to send an HTTP GET request
      #
      rcurl_request &lt;- function(service_url, parameters) {
        # Collapse all parameters into one string
        all_parameters &lt;- paste(
          sapply(names(parameters), 
                 FUN=function(param_name, parameters) {
                   paste(param_name, paste(parameters[[param_name]], collapse=','), collapse='', sep='=')
                 }, 
                 parameters),
          collapse="&amp;")
        # Paste base URL and parameters
        requested_url &lt;- paste0(service_url, all_parameters)
        # Encode URL (in case there would be any space character for instance)
        requested_url &lt;- URLencode(requested_url)
        # Start request to service
        response &lt;- getURL(requested_url, .opts = list(ssl.verifypeer = FALSE))
        return(response)
      }
    ),                
    public = list(
      species = NULL,
      bdbnJSON = NULL,
      initialize = function(symbol, bdbSpecies){
        parameters &lt;- list(method="dbfind", 
                           inputValues=symbol,
                           output="geneid",
                           taxonId=bdbSpecies,
                           format="col"
        )
        base_url = "https://biodbnet-abcc.ncifcrf.gov/webServices/rest.php/"
        json_url = paste0(base_url, "biodbnetRestApi.json?")    
        self$bdbnJSON &lt;- rcurl_request(json_url, parameters)
      }
    )                  
  )
  </code></pre>
  
  <p>Now I source the containing file and get a strange error.  </p>
  
  <pre><code>&gt; source("parseSymbol.R")
  Error in R6Class("Symbol", private = list(rcurl_request &lt;- function(service_url,  : 
    All elements of public, private, and active must be named.
  </code></pre>
  
  <p>This looks like it should mean that I tried to do something like "self$[something I didn't declare in public or private] &lt;- new value" but there appears to be no such mistake.  What is going on here?</p>