In his Practical English Usage, Michael Swan says, "The rules about hyphens are complicated, and rules are not yet clear [...] The situation at present is rather confused,and it is not unusual to find the same expression spelt in three different ways. (e.g. bookshop, book-shop, book shop)"
Swan feels that, perhaps because of the confusion, people are using hyphens less, preferring to write the two parts as one word.
Student or Learner