Issue #12901 has been updated by Charles Nutter.


In https://github.com/rails/sprockets/pull/417 I showed that JRuby can be significantly faster if the proc is stuffed into a `define_method` method. This is because we optimize `define_method` blocks to behave exactly like a plain method (with no closure state, etc) if we can statically determine it does not access its captured scope.

This feature could be generalized in any implementation as a `function` kernel method. When creating a `function` a Ruby impl would check if any captured state was accessed. This includes variables like `$~`. If that state is accessed, it could either be a warning or an error. If it is not accessed, then the closure body would be "unrooted" from its captured scope and turned into a flat method. Much of the overhead shown here would go away.

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Feature #12901: Anonymous functions without scope lookup overhead
https://bugs.ruby-lang.org/issues/12901#change-61402

* Author: Richard Schneeman
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
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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