Puppet Exec what “<| |>” means?

问题

I'm learning puppet language and noticed one very intriguing line of code: Exec["apt-update"] -> Package <| |> on following context:

class manifest::module {

  exec { "apt-update":
      command => "/usr/bin/apt-get -y update",
      timeout => 3600;
  }

  package {
    ["alien", "bc", "libaio1", "unixodbc", "unzip", "rlwrap"]:
      ensure => installed;
  }

  Exec["apt-update"] -> Package <| |>
}

Why Exec is followed by the ->? And most important, what's the meaning of <| |> ???

回答1:

This expression essentially instructs Puppet to have any package resource require the "apt-update" exec resource. In other words Puppet will be sure to execute apt-get update before installing/purging/... a package.

回答2:

It matches any package, see http://docs.puppetlabs.com/puppet/latest/reference/lang_collectors.html for more details

来源：https://stackoverflow.com/questions/20593368/puppet-exec-what-means