Issue #20404 has been updated by mame (Yusuke Endoh).
Status changed from Open to Rejected
April Fools' Day is over. Closing.
This proposal is half joke, half serious. I can think of some reasons why we should
introduce `2pi`.
### There are some languages that provide not only `pi` but also commonly used values.
@ko1 and @shyouhei told me that Gauche and C (POSIX) defines the following constants. It
seem reasonable to provide frequently used values, even if they can be easily computed
from other values.
* Gauche (Scheme): `pi`, `2pi`, `pi/2`, `pi/4`, `pi/180`, `1/pi`, and `180/pi`
* C (POSIX): `pi`, `pi/2`, `pi/4`, `1/pi`, `2/pi`, and `2/sqrt(pi)` (but not `2pi`, why?)
https://practical-scheme.net/gauche/man/gauche-refe/Mathematical-constants.…
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/math.h.html
### There is no good alternative constant name.
We cannot define `Math::2PI` unfortunately. `Math::PI2` and `Math::PI_2` are in unnatural
word order, and also ambiguous as pi^2 or pi/2. `Math::TWO_PI` does not look so good.
I mentioned Tau in the original proposal, and seriously, I don't believe that
`Math::Tau` is the way to go at the present. As long as most textbooks use 2pi,
programmers will have to replace it with `2pi` in their mind every time they see
`Math::Tau`, which is too inconvenient. We should consider this when most textbooks start
using Tau. (If I had a time machine, I would try to convince the ancient Greeks to
consider the ratio of circumference to radius instead of diameter.)
### `Math::PI` is often the only used one in Math module.
Because Ruby has Complex, it is often not necessary to use `Math.sin` and `cos` directly.
When dealing with 2D graphics, we typically use only Complex multiplication and
`Math::PI`.
### `Math::PI` leads to a bit cryptic code.
For example, to find the coordinates of the vertices of a hexagon, one would write
`Complex.polar(1, 2 * Math::PI / 6 * n)`. However, many people may be bothered by the
redundant multiplication and rewrite it as `Complex.polar(1, Math::PI / 3 * n)`. Then the
original intent of the hexagon disappears from the code. Since math code tends to be
originally cryptic, I think it is better if the intent remains as straightforward as
possible. With `2pi`, we could write `Complex.polar(1, 2pi / 6 * n)`.
... Well, but I wonder if a new literal is impossible?
----------------------------------------
Feature #20404: `2pi`
https://bugs.ruby-lang.org/issues/20404#change-107582
* Author: mame (Yusuke Endoh)
* Status: Rejected
----------------------------------------
I propose a new Float literal: `2pi`.
```ruby
p 2pi #=> 6.283185307179586 == 2 * Math::PI
```
I am not proposing `1pi`, `3pi` or `4pi`. I do only `2pi`. Because in most cases, all you
need is `2pi`. Any other multiple of pi is rarely needed.
I've got the statistics. GitHub code search revealed that more than 80% of the
occurrences of `n * Math::PI` and `Math::PI * n` had n = 2. [1]
| n |`n * Math::PI`|`Math::PI * n`|sum | % |
|--:|-------------:|-------------:|----:|---:|
| 0 | 48| 1| 49|1.4%|
| 1 | 58| 3| 61|1.7%|
| 2 | 2300| 534| 2834|80.4%|
| 3 | 218| 17| 235|6.7%|
| 4 | 200| 23| 223|6.3%|
| 5 | 24| 2| 26|0.7%|
| 6 | 41| 3| 44|1.2%|
| 7 | 14| 0| 14|0.4%|
| 8 | 6| 2| 8|0.2%|
| 9 | 8| 0| 8|0.2%|
|10 | 17| 0| 17|0.5%|
|11 | 2| 0| 2|0.1%|
|12 | 2| 0| 2|0.1%|
Here is the PIe chart
![](https://gist.github.com/assets/21557/1c077af3-f753-4ee1-bc75-5b38a3bd6487)
I know that a new constant name "Tau" is proposed (#4897, #17496). But notice:
"2pi" and "Tau" have the same number of characters. Then, it is
obvious that familiarity is better. In fact, `2 * Math::PI` is 4 times more than `Math::PI
* 2`, indicating that all programmers are copying "2pi" in their textbook.
The idea of 2pi came up regularly in chats among committers. The discussion quickly
shifted to its generalization: should we treat `3e` as `3 * Math::E`, or even `42foo` as
`42 * foo`? However, I noticed such a generalization is unnecessary because all we need is
2pi. Unneeded generalization is evil.
Here is a patch.
```diff
diff --git a/parse.y b/parse.y
index 55619273b8..93b16a16ac 100644
--- a/parse.y
+++ b/parse.y
@@ -10208,6 +10208,13 @@ parse_numeric(struct parser_params *p, int c)
return set_number_literal(p, tINTEGER, suffix, 10, 0);
}
}
+ else if (c == '2' && peek(p, 'p') && peek_n(p,
'i', 1)) {
+ tokadd(p, c);
+ tokadd(p, nextc(p));
+ tokadd(p, nextc(p));
+ tokfix(p);
+ return set_number_literal(p, tFLOAT, 0, 0, 0);
+ }
for (;;) {
switch (c) {
diff --git a/ruby_parser.c b/ruby_parser.c
index 6d85a72c5b..3a6e0b5704 100644
--- a/ruby_parser.c
+++ b/ruby_parser.c
@@ -936,6 +936,9 @@ VALUE
rb_node_float_literal_val(const NODE *n)
{
const rb_node_float_t *node = RNODE_FLOAT(n);
+ if (strcmp(node->val, "2pi") == 0) {
+ return DBL2NUM(2 * M_PI);
+ }
double d = strtod(node->val, 0);
if (node->minus) {
d = -d;
P I
Happy a r l fool
```
[1] I used these queries: [`/[^.]\b2 ?\* ?Math::PI/
language:Ruby`](https://github.com/search?type=code&q=%2F%5B%5E.%5D%5Cb…
and [`/Math::PI *\* *2\b *[^.\/]/
language:Ruby`](https://github.com/search?type=code&q=%2FMath%3A%3API+*…
--
https://bugs.ruby-lang.org/