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leo/docs/rfc/005-countdown-loops.md

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# Leo RFC 005: Countdown Loops
## Authors
- Max Bruce
- Collin Chin
- Alessandro Coglio
- Eric McCarthy
- Jon Pavlik
- Damir Shamanaev
- Damon Sicore
- Howard Wu
## Status
DRAFT
# Summary
This proposal suggests adding countdown loops and inclusive loop ranges into Leo language.
# Motivation
In the current design of the language only incremental ranges are allowed. Though
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in some cases there's a need for loops going in the reverse direction. This example
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demonstrates the shaker sort algorithm where countdown loops are mocked:
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```ts
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function shaker_sort(a: [u32; 10], const rounds: u32) -> [u32; 10] {
for k in 0..rounds {
for i in 0..9 {
if a[i] > a[i + 1] {
let tmp = a[i];
a[i] = a[i + 1];
a[i + 1] = tmp;
}
}
for j in 0..9 { // j goes from 0 to 8
let i = 8 - j; // j is flipped
if a[i] > a[i + 1] {
let tmp = a[i];
a[i] = a[i + 1];
a[i + 1] = tmp;
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}
}
}
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return a;
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}
```
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Having a countdown loop in the example above could improve readability and
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usability of the language by making it more natural to the developer.
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However, if we imagined this example using a countdown loop, we would see that
it wouldn't be possible to count to 0; because the first bound of the range is
inclusive and the second is exclusive, and loops ranges must use only unsigned integers.
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```ts
// loop goes 0,1,2,3,4,5,6,7,8
for i in 0..9 { /* ... */ }
// loop goes 9,8,7,6,5,4,3,2,1
for i in 9..0 { /* ... */ }
```
Hence direct implementation of the coundown loop ranges would create asymmetry (1)
and would not allow loops to count down to 0 (2). To implement coundown loops and
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solve these two problems we suggest adding an inclusive range bounds.
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# Design
## Coundown loops
Countdown ranges do not need any changes to the existing syntax. However their
functionality needs to be implemented in the compiler.
```ts
for i in 5..0 {}
```
## Inclusive ranges
To solve loop asymmetry and to improve loop ranges in general we suggest adding
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inclusive range operator to Leo. Inclusive range would extend the second bound
of the loop making it inclusive (instead of default - exclusive)
therefore allowing countdown loops to reach 0 value.
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```ts
// default loop: 0,1,2,3,4
for i in 0..5 {}
// inclusive range: 0,1,2,3,4,5
for i in 0..=5 {}
```
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## Example
The code example demostrated in the Motivation part of this document
could be extended (or simplified) with suggested syntax:
```ts
function shaker_sort(a: [u32; 10], const rounds: u32) -> [u32; 10] {
for k in 0..rounds {
for i in 0..9 { // i goes from 0 to 8
if a[i] > a[i + 1] {
let tmp = a[i];
a[i] = a[i + 1];
a[i + 1] = tmp;
}
}
for i in 8..=0 { // j goes from 8 to 0
if a[i] > a[i + 1] {
let tmp = a[i];
a[i] = a[i + 1];
a[i + 1] = tmp;
}
}
}
return a;
}
```
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# Drawbacks
-
# Effect on Ecosystem
Suggested change should have no effect on ecosystem because of its backward compatibility.
# Alternatives
Coundown loops can be mocked manually.