Halliday & Matthiessen (2014: 196):
Exclamations are the limiting case of an exchange; they are verbal gestures of the speaker addressed to no one in particular. Some of them are in fact not language but protolanguage, such as Wow!, Yuck!, Aha!, and Ouch!. Others are made of language, with recognisable words and sometimes even traces of structure … They can be analysed as nominal groups or as clauses in terms of transitivity, if desired.