hashicorp/terraform — pull request #680 by svanharmelen (2014-12-16 12:16:40+00:00)
Title: provider/aws: updating the resource to use a set instead of a list

By using a set for the availability zones, you can use things like `availability_zones = ["${aws_instance.web.*.availability_zone}"]` where is very likely multiple of the same zones will be added to the
set. If you use a list here, the list will say it’s changed (even if you add the same zone) which will force a new resource.