Halliday & Matthiessen (2014: 159):
Likewise outside the structure of the Mood and Residue, and occurring in more or less the same places as Vocatives in the clause, are Expletives, whereby the speaker enacts his own current attitude or state of mind. These are perhaps on the fringe of grammatical structure; but since they participate fully in the intonation and rhythm of the clause they do figure in the analysis.