Lets say that you have an array of NFS clients to allow access to an export. You want to expand this into a list of resources, which you’d normally do via something like:

$allow = hiera_array('some_variable', undef)
nfs::export { $allow:
  ...
}

This works fine until you want to have multiple exports to the same name/IP, at which point you have two nfs::export resources with the same name. But you need the expansion of the array to generate multiple calls to nfs::export, so you need to prefix each string in the array:

$allow = hiera_array('some_variable', undef)
$hacked_allow = prefix($allow, "${fqdn} export of ${path} to client ")
nfs::export { $hacked_allow:
  ...
}

..then in the called define you extract the client fqdn like so:

define nfs::export ( ... ) {
  $client = regsubst($name, "^.* export of .* to client (.*)$", '\1')
  validate_string($client)

  ... [use $client as fqdn in here]
}

This seems like a terrible hack, but it’s a reasonable workaround for the need to expand arrays and keep the names unique to avoid duplicate resources.