myjoin.pp

function myjoin(Array $a, String $delimiter) {
  $formats = {
    Array => {
      # no delimiters = '% ', and 'a' means array
      format => '% a',
      separator => $delimiter,
      # format strings without delimiters = '%s'
      string_formats => { String => '%s' }
    }
  }
  String($a, $formats)
}

$test = ['the', 'quick', 'brown', 'fox']
notice myjoin($test, "::")

outputs the::quick::brown::fox