how to do async web getter?

crawl3.nim

# this doesn't work.  I think I need a dispatcher and
# some polling, not sure where to put it.

import
  asyncdispatch, asyncnet, htmlparser, xmltree, httpclient, strutils,
  strtabs, streams, uri, sets, os

var num_running = 0
 
proc crawl(url: string, client: PAsyncHttpClient = newAsyncHttpClient()) {.async.} =
  num_running += 1
  echo("Crawling ", url)
  let resp = await client.get(url)
 
  if resp.status.startswith("200"):
    echo len(resp.body)
  else:
    echo "error: " & resp.status
  num_running -= 1

# how can i limit this to x concurrent connections?
# does sleep() block the async "crawl"?
# if so, how to do this?
var concurrency = 3
for line in lines(stdin):
  # sleep for 1 second if all connections busy
  while num_running > concurrency: sleep(1000)

  discard crawl("http://" & line & "/")