"It is to be discussed", what is the infinitive doing in this sentence?

It is to be discussed.

Is be + infinitive forming the future tense here?

You are to be dressed and ready by 8:00.

I was thinking it's almost commanding (or speaking of a command) but this doesn't seem to be the case, as sentences like this are commanding without this structure:

You will be dressed and ready by 8:00.

or

You are going to be dressed and ready by 8:00.

And there's sentences like this that don't imply commandment in any way:

I am to go home tomorrow!

or

I am to be home tomorrow!

(Slightly different meaning, same general sentiment)

So what's going on with the be + infinitive structure?

Solution 1:

No, this is not usually a way to indicate the future: it’s more like must than it is like will.

The OED cover this as senses 16 and 17 for be:

• 16. With the dative infinitive, making a future of appointment or arrangement; hence of necessity, obligation, or duty; in which sense have is now commonly substituted.
• † 16a. with infinitive active. Obs.
  • 1742 Richardson Pamela III. 264, ― I am to thank you, my dear Miss, for your kind Letter.
• 16b. with infinitive passive
  • 1869 Freeman Norm. Conq. III. xii. 145 ― Normandy was to be invaded on each side.
• 17. The same construction is used in the sense of ‘to be proper or fit (to).’ a. with infinitive active. arch. and now commonly expressed by b.
  • Mod. ― Is this house to let? They are not to compare with these.
• 17b. with infinitive passive
  • 1798 Malthus Popul. (1817) II. 194 ― It must be to be depended upon.

That means this is a deontic periphrastic construction. In other words, be+to+infinitive is simply another way of expressing obligation, just as have+to+infinitive is.

So these are pretty much the same thing:

• You are to turn in your assignment on time — or not at all.
• You have to turn in your assignment on time — or not at all.
• You shall turn in your assignment on time — or not at all.
• You must turn in your assignment on time — or not at all.

And by the same token, so too are these also essentially equivalent:

• You are to be dressed and ready by 8 o’clock.
• You have to be dressed and ready by 8 o’clock.
• You shall be dressed and ready by 8 o’clock.
• You must be dressed and ready by 8 o’clock.

One advantage of a finite be or have here is that it can express time (and other things), whereas the modals for the most part cannot.

• Yesterday, Mr Johnson told me that I was to turn in my assignment on time — or not at all.
• Yesterday, Mr Johnson told us that we were to turn in our assignment on time — or not at all.

Note that this is not the same as the were+to+infinitive construction uses for hypotheticals as given by OED sense 18:

• 18. The past subjunctive were with the infinitive makes an emphatically hypothetical condition: cf. the degrees of uncertainty in If I went, If I should go, If I were to go.

So this is completely different from the two previous examples:

• If I were to turn my assignment in late, would it still count?

There is one more be+to+infinitive, but it does not apply here. It is OED sense 6:

• 6. Idiomatically, in past, now only in perfect and pluperfect tenses, with to, and a substantive, or infinitive of purpose: To have been (at the proper place) in order to, or for the purpose of. Cf. Sp. and Pg. fué ‘I was’ in sense of ‘I went.’
  

  • 1760 Goldsmith Cit. W. (1840) 158, ― I was this morning to buy silk for a nightcap.
  • Mod. ― Have you been to the Crystal Palace? I had been to see Irving that night.

Solution 2:

Modal use of the verbs to be and to have is very common in the English language. They are then followed by the infinitive form (active or passive) of the verb that denotes the action (or state) itself.

In your question, the infinitive means just that: that the first verb to be is used as a modal verb. This use implies that the action/state is one or more of the following:

• Scheduled: The work is to be completed by Monday.
• Planned: The database application is ready; now you are to provide the data.
• Very probable or desirable: You are to wear a tie at the wedding.

So, your It is to be discussed means We plan to discuss it or It's a good idea to discuss it, and the infinitive indicates modal use of to be.

And, to your question: yes, in most cases it is about a future action. However, it may also refer to a «general truth»: You are to arrive on time for work – meaning you always need to do that, and should have done when you didn't in the past.

Similarly, the verb to have followed by the infinitive of a verb means that the action/state must occur because of external conditions or a necessity:

It has to be discussed later: certain conditions prevents this to be discussed now, so we are forced to postpone the discussion.

This also refer to a future action.in most cases, but also to general truths sometimes:

Whenever he uses my computer I have to re-install Windows.