Issue #17159 has been updated by shyouhei (Shyouhei Urabe).
Description updated
### The reason I use `#define_method` (4)
I sometimes use it to alias _a part_ of a module, like this:
```ruby
class Foo
%i[sin cos tan].each do |sym|
define_method(sym, Math.instance_method(sym))
end
end
p Foo.new.sin(3.14)
```
There seems be no reason to reject such usages.
### Capturing local variables
C++ since C++11 have had lambdas. In the language you can explicitly specify how you want to capture a variable each time when you create a lambda. As of C++20 there are 12 different specifier. Some of them exist for template metaprogramming (we can ignore such things), but I think there are several interesting cases.
```C++
#include <cstdio>
int main() {
int x, y, z;
x = y = z = 1;
auto f = [x, &y, &z]() mutable {
auto g = [x, y, &z]() mutable {
printf("#1: x, y, z = %d, %d, %d\n", x, y, z);
x = y = z = 4;
};
x = y = z = 3;
g();
};
x = y = z = 2;
f();
printf("#2: x, y, z = %d, %d, %d\n", x, y, z);
}
```
This program outputs
```
#1: x, y, z = 1, 2, 3
#2: x, y, z = 2, 3, 4
```
Compilicated? But the `s: s` proposal is very much like this. You can mix call-by-reference and call-by-value. I agree this gives us maximum freedom, but at a cost of complexity.
C++ also has simpler specifier which has no such headaches:
```C++
#include <cstdio>
int main() {
int x, y, z;
x = y = z = 1;
auto f = [=]() mutable {
auto g = [=]() mutable {
printf("#1: x, y, z = %d, %d, %d\n", x, y, z);
x = y = z = 4;
};
x = y = z = 3;
g();
};
x = y = z = 2;
f();
printf("#2: x, y, z = %d, %d, %d\n", x, y, z);
}
```
The above should print:
```
#1: x, y, z = 1, 1, 1
#2: x, y, z = 2, 2, 2
```
And I think this behaviour is much more understandable. The `ractorise: true` proposal is on this line. I'd push this way.
----------------------------------------
Bug #17159: extend `define_method` for Ractor
https://bugs.ruby-lang.org/issues/17159#change-87553
* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Ractor prohibits use of non-isolated `Proc`s.
Non-isolated example is here:
```ruby
s = "foo"
pr = Proc.new{ p s }
```
This Proc `pr` can not be shared among ractors because outer variable `s` can contain an unshareable object. Also outer binding is a mutable object. Sharing it can lead race conditions.
Because of these reasons, `define_method` is also a problem on a multi-Ractor program.
(current implementation allows it just because check is not implemented, and it leads BUG).
I think there are several patterns when `define_method` is needed.
(1) To choose method names on-the-fly
```ruby
name = ...
define_method(name){ nil }
```
(2) To embed variables to the code
```ruby
10.times{|i|
define_method("foo{i}"){ i }
}
```
(3) To use global state by local variables
```ruby
cnt = 0
define_method("inc"){ cnt += 1 }
```
(4) Others I can't imagine
----
(1) is easy. We can allow `define_method(name, &Proc{nil}.isoplate)`.
(3) can never be OK. It introduces data races/race conditions. For this purpose one need to use shared hash.
```ruby
STATE = SharedHash.new(cnt: 0)
define_method("inc"){ STATE.transaction{ STATE[:cnt] += 1 }}
```
I think there are many (2) patterns that should be saved.
To help (2) pattern, the easiest way is to use `eval`.
```ruby
10.times{|i|
eval("def foo#{i} #{i}; end")
}
```
However, `eval` has several issues (it has huge freedom to explode the program, editor's syntax highlighting and so on).
Another approach is to embed the current value to the code, like this:
```ruby
i = 0
define_method("foo", ractorise: true){ i }
#=> equivalent to:
# define_method("foo"){ 0 }
# so that if outer scope's i changed, not affected.
i = 1
foo #=> 0
s = ""
define_method("bar", ractorise: true){ s }
#=> equivalent to:
# define_method("bar"){ "" }
# so that if outer scope's s or s's value, it doesn't affect
s << "x"
bar #=> ""
```
However, it is very differenct from current Proc semantics.
Another idea is to specify embedding value like this:
```ruby
i = 0
define_method("foo", i: i){ i }
#=> equivalent to:
# define_method("foo"){ 0 }
# so that if outer scope's i changed, not affected.
i = 1
foo #=> 0
s = ""
define_method("bar", s: s){ s }
#=> equivalent to:
# define_method("bar"){ "" }
# so that if outer scope's s or s's value, it doesn't affect
s << "x"
bar #=> ""
```
`i: i` and `s: s` are redundant. However, if there are no outer variable `i` or `s`, the `i` and `s` in blocks are compiled to `send(:i)` or `send(:s)`. But I agree these method invocation should be replaced is another idea.
Thoughts?
Thanks,
Koichi
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