2021-06-27 20:18:07 +03:00
|
|
|
# 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
|
2021-07-05 15:47:01 +03:00
|
|
|
in some cases there's a need for loops going in the reverse direction. This example
|
2021-07-05 16:28:50 +03:00
|
|
|
demonstrates the shaker sort algorithm where countdown loops are mocked:
|
2021-06-27 20:18:07 +03:00
|
|
|
|
|
|
|
```ts
|
2021-07-05 16:28:50 +03:00
|
|
|
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;
|
2021-06-27 20:18:07 +03:00
|
|
|
}
|
|
|
|
}
|
|
|
|
}
|
2021-07-05 15:58:10 +03:00
|
|
|
return a;
|
2021-06-27 20:18:07 +03:00
|
|
|
}
|
|
|
|
```
|
|
|
|
|
2021-07-05 15:47:01 +03:00
|
|
|
Having a countdown loop in the example above could improve readability and
|
2021-06-27 20:18:07 +03:00
|
|
|
usability of the language by making it more natural to the developer.
|
|
|
|
|
2021-07-05 15:47:01 +03:00
|
|
|
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.
|
2021-06-27 20:18:07 +03:00
|
|
|
|
|
|
|
```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
|
2021-07-05 15:47:01 +03:00
|
|
|
solve these two problems we suggest adding an inclusive range bounds.
|
2021-06-27 20:18:07 +03:00
|
|
|
|
|
|
|
# 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
|
2021-07-05 15:47:01 +03:00
|
|
|
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.
|
2021-06-27 20:18:07 +03:00
|
|
|
|
|
|
|
```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 {}
|
|
|
|
```
|
|
|
|
|
2021-07-05 16:28:50 +03:00
|
|
|
## 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;
|
|
|
|
}
|
|
|
|
```
|
|
|
|
|
2021-06-27 20:18:07 +03:00
|
|
|
# Drawbacks
|
|
|
|
|
|
|
|
-
|
|
|
|
|
|
|
|
# Effect on Ecosystem
|
|
|
|
|
|
|
|
Suggested change should have no effect on ecosystem because of its backward compatibility.
|
|
|
|
|
|
|
|
# Alternatives
|
|
|
|
|
|
|
|
Coundown loops can be mocked manually.
|