What do you call 'be-to' constructions and are they acceptable English?

Question

Consider the following examples:

You have to be really patient if you are to go shopping in the afternoon.
  It must be active if it is to record the film.

What is the construction in bold typeface called and should it be used in formal English texts &mdash; is it good or bad style?

John Lawler · Accepted Answer

It's the Be + Infinitive Construction, if you need a name. The construction is not limited to infinitive use; i.e, it's not just to be to + Infinitive. It can also be used in the present or past tense.

He is to leave tomorrow.
He was to leave yesterday, but the flight was cancelled.

A bare infinitive in a strange place, like after an auxiliary verb (e.g, be or have), is bound to be an idiom, and usually has a modal meaning:

He has to leave. (He must leave)
He is to leave  (He will leave ~ He is expected/scheduled to leave)

just like relative infinitives (infinitive clauses modifying nouns) always have a modal meaning:

He is the man to see about this.  (He is the man that one should see)
He is the man to do the job.  (He is the man who should do it)

Martin · Answer

If "He has to leave" is idiomatic, what is "He doesn't have to leave"? "Have to" plus bare infinitive is perfectly grammatical, I believe, especially after defective verbs.

What do you call 'be-to' constructions and are they acceptable English?

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