Compared with the "will" future, the "be + to" construction may be less expressive of a prediction, and more of the existence of a plan or duty. Thus "John will go ..." implies a belief on the speaker's part that this will occur, while "John is to go ..." implies knowledge on the speaker's part that there exists a plan or obligation entailing such an occurrence (the latter statement will not be falsified if John ends up not going). The "be + to" construction may therefore resemble a renarrative mood in some ways.