Expressing future time : be going to and will

(a) I am going to leave at nine tomorrow morning
(b) I will leave at nine tomorrow morning
(c) Marie is going to be at the meeting today
(d) Marie will be at the meeting today
Be going to and will are used to express future time.
(a) and (b) have the same meaning
(c) and (d) have the same meaning
Will and be going to often give the same meaning, but sometimes they express different meanings.
(e) I shall leave at nine tomorrow morning
(f) We shall leave at nine tomorrow morning
The use of shall (with I or We) to express future time is possible but infrequent.