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Merge pull request #1171 from AleoHQ/rfc-countdown-loops

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[RFC] Add discussion of loop step and direction.
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Alessandro Coglio 2021-07-20 09:16:15 -07:00 committed by GitHub
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docs/rfc/005-countdown-loops.md
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@ -94,6 +94,26 @@ for i in 0..5 {}
for i in 0..=5 {}
```
## Step and Direction
We remark that the step of both counting-up and counting-down loops is implicitly 1;
that is, the loop variable is incremented or decremented by 1.
Whether the loop counts up or down is determined by how the starting and ending bounds compare.
Note that the bounds are not necessarily literals;
they may be more complex `const` expressions, and thus in general their values are resolved at code flattening time.
Because of the type restrictions on bounds, their values are always non-negative integers.
If `S` is the integer value of the starting bound and `E` is the integer value of the ending bound,
there are several cases to consider:
1. If `S == E` and the ending bound is exclusive, there is no actual loop; the range is empty.
2. If `S == E` and the ending bound is inclusive, the loop consists of just one iteration; the loop counts neither up nor down.
3. If `S < E` and the ending bound is exclusive, the loop counts up, from `S` to `E-1`.
4. If `S < E` and the ending bound is inclusive, the loop counts up, from `S` to `E`.
5. If `S > E` and the ending bound is exclusive, the loop counts down, from `S` to `E+1`.
6. If `S > E` and the ending bound is inclusive, the loop counts down, from `S` to `E`.
Cases 3 and 5 consist of one or more iterations; cases 4 and 6 consist of two or more iterations.
## Example
The code example demostrated in the Motivation part of this document