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Copy file name to clipboardExpand all lines: docs/src/concepts/futures.md
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- they either run to succession and yield a result or they can yield an error
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## Deferring computation
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As mentioned above `Send` and `Sync` are about data. But programs are not only about data, they also talk about *computing* the data. And that's what \[Futures\][futures] do. We are going to have a close look at how that works in the next chapter. Let's look at what Futures allow us to express, in English. Futures go from this plan:
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As mentioned above `Send` and `Sync` are about data. But programs are not only about data, they also talk about *computing* the data. And that's what [`Futures`][futures] do. We are going to have a close look at how that works in the next chapter. Let's look at what Futures allow us to express, in English. Futures go from this plan:
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- Do X
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- If X succeeds, do Y
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Remember the talk about "deferred computation" in the intro? That's all it is. Instead of telling the computer what to execute and decide upon *now*, you tell it what to start doing and how to react on potential events the... well... `Future`.
Let's have a look at a simple function, specifically the return value:
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Ignore `Pin` and `Context` for now, you don't need them for high-level understanding. Looking at it closely, we see the following: it is generic over the `Output`. It provides a function called `poll`, which allows us to check on the state of the current computation.
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Every call to `poll()` can result in one of these two cases:
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1. The future is done, `poll` will return `[Poll::Ready](https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Ready)`
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2. The future has not finished executing, it will return `[Poll::Pending](https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Pending)`
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1. The future is done, `poll` will return [`Poll::Ready`](https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Ready)
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2. The future has not finished executing, it will return [`Poll::Pending`](https://doc.rust-lang.org/std/task/enum.Poll.html#variant.Pending)
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This allows us to externally check if a `Future` has finished doing its work, or is finally done and can give us the value. The most simple way (but not efficient) would be to just constantly poll futures in a loop. There's optimistions here, and this is what a good runtime is does for you.
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Note that calling `poll` after case 1 happened may result in confusing behaviour. See the [futures-docs](https://doc.rust-lang.org/std/future/trait.Future.html) for details.
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