What is the difference between "has to be" and "is to be"?

This is the example sentence: In case a diarization is desired, a proper XYZ algorithm is/has to be used.

I would like to point out, that my personal opinion is using the XYZ algorithm.


"Is to be used" is grammatically acceptable but stylistically weak. Better to stick to one of these:

  • "has to do": it is required (Sometimes people use this hyperbolically: "I have to go to the movies, all my friends are."). In a technical setting like you allude to, this often means something that fundamental to the project blows up if you don't: CEO goes to jail, client withdraws funding, people get fired, etc. It is usually too strong to use it for an alternative implementation that is merely a mistake.
  • "should do": there are other valid alternatives, but it has been decided that this is the best one
  • "will do": this usually is reserved for a future certainty: "It will be February soon," "It will be night in 4 hours," "The sun will rise tomorrow," etc.

If the emphasis is on the necessity of using a proper XYZ algorithm, then use has to be used. An alternative is must be used. (Also consider if instead of in case.)

  • What would "is to be used" imply?
    – Lukas
    Jan 23 '14 at 18:00
  • 4
    It depends on the context, but it generally carries a lower sense of necessity, and rather expresses prediction instead. Jan 23 '14 at 18:36

"Has to be used" is stronger than "is to be used."

"Has to" suggests a requirement, meaning that absolutely no other option is acceptable. "Is to" is a matter-of-fact future tense (similar to "will be used").

So it depends. If you feel that:

  1. There is no choice but to use XYZ, and
  2. Readers might think another algorithm is acceptable (such as if you discussed ABC in the previous paragraph),

then use "has to." Otherwise, go with "is to."

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