[ruby-core:106945] [Ruby master Feature#12901] Anonymous functions without scope lookup overhead

Issue #12901 has been updated by cvss (Kirill Vechera).


Another reason for having an isolated Proc is using it with an implicit block argument, for example, for `define_method` or for some other metaprogramming. For example, the `yield` on an implicit block argument in this code is incorrect now:

```ruby
class A
  B = proc do
    yield 'b'
  end
  define_method :a, &B
end
```

In earlier Ruby versions `yield` produced a runtime error, now it is a parsing error. So `Proc.new(isolate: true){ ... }` approach would require to switch back to the runtime checking

----------------------------------------
Feature #12901: Anonymous functions without scope lookup overhead
https://bugs.ruby-lang.org/issues/12901#change-95769

* Author: schneems (Richard Schneeman)
* Status: Open
* Priority: Normal
----------------------------------------
Right now if you are writing performance sensitive code you can get a performance bump by not using anonymous functions (procs, lambdas, blocks) due to the scope lookup overhead.

https://twitter.com/apotonick/status/791167782532493312?lang=en

I would like to be able to create an anonymous function and specify I don't need it to perform the lookup when called.

I am thinking that this code:

~~~
Proc.new(scope: false) {|var| puts var }
~~~

Would be the equivalent of 

~~~
def anon(var)
  puts var
end
~~~

If you call it while trying to access something from outside of the scope it would error

~~~
var = "hello"
Proc.new(scope: false) { puts var }.call
# => NameError: undefined local variable or method `var' for main:Object
~~~

An example of a case where this could be used is found in https://github.com/rails/sprockets/pull/417. In this case we are getting rid of anonymous functions in favor of a method created with refinements. This solution works but it was not an obvious optimization. It would be convenient to have a syntax for defining anonymous functions that do not need access to caller scope.






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