Mon Sep 21 09:13:15 EDT 2015

Pattern: maps over lists that change contents

- server with a couple of connections
- connections come and go

- take a snapshot "map", e.g. list, + apply function
  need to do this in 1 step, as list might change between ops

possible solution:
- parallel map
- failure during evaluation means no result is returned
- device appearing after initial list means no result is returned
- how do you know there was a failure?  use exceptions