could not find function about httr HOT 7 CLOSED

> library(httr)
> oauth_app
function (appname, key, secret = NULL) 
{
    if (is.null(secret)) {
        env_name <- str_c(toupper(appname), "_CONSUMER_SECRET")
        secret <- Sys.getenv(env_name)
        if (secret == "") {
            stop("Couldn't find secret in environment variable ", 
                env_name, call. = FALSE)
        }
        message("Using secret stored in environment variable ", 
            env_name)
    }
    list(secret = secret, key = key)
}
<environment: namespace:httr>
> oauth2.0_token
Error: object 'oauth2.0_token' not found
> oauth2.0
Error: object 'oauth2.0' not found
> oauth_app()
Error in toupper(appname) : 
  argument "appname" is missing, with no default
> oauth2.0_token()
Error: could not find function "oauth2.0_token"