Halliday & Matthiessen (2004: 500):
The time-phase system has split into two. The original opposition is doing/is to do (meaning, in modern terms, ‘keeps doing’ and ‘will do’) has disappeared, since both have turned into grammatical categories of the verbal group. The former has evolved into tense, defined along the lines of future/present/past. … The latter … similarly turned into a secondary future; but here there has been a further change: is to has now turned into a modal form, and its function as secondary tense has been taken over by is going to.