Issue #10394 has been reported by Tsuyoshi Sawada.

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Feature #10394: An instance method on Enumerator that evaluates the block under with self being the block variable.
https://bugs.ruby-lang.org/issues/10394

* Author: Tsuyoshi Sawada
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
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**Background**

There has been desire to omit the `| |` and the explicit receiver in a block used with an enumerator or an enumerable. Currently, when the content of the block is a single method that takes no argument, symbol-to-proc is used with the `&` syntax so that:

~~~ruby
["foo", "bar"].map{|s| s.upcase}
~~~

can be written as:

~~~ruby
["foo", "bar"].map(&:upcase)
~~~

There has repeated been proposals (#8987, #9076, #10318) that express this desire to do this even when the block involves a method chain or a method with arguments like the following:

~~~ruby
["foo", "bar"].map{|s| s.concat("ber")}
["  foo ", "\tbar\n"].map{|s| s.strip.upcase}
~~~

Focus has been on modifying how a block is passed to the enumerable/enumerator, and there has not been consensus on how the syntax should be.


**Proposal**

Unlike the earlier proposals, I suggest that there should be an instance method on `Enumerator`, let's say `Enumerator#as_self`, that evaluates the block each time with `self` being the block variable that would be passed otherwise. With such method, the cases above would be written like this:

~~~ruby
["foo", "bar"].map.as_self{concat("ber")}
["  foo ", "\tbar\n"].map.as_self{strip.upcase}
~~~

This adds no modification to the syntax, it just requires a new method `Enumerator#as_self` to be implemented. I consider this method being along the lines of `Enumerator#with_index`, `Enumerator#with_object`; it intervenes between an enumerator (related to a block-taking method) and a block, and let the block-taking method work in a modified way.

It resembles `instance_eval`, but is different in that it assigns to `self` what would be a block variable (which changes for each iteration), instead of assigning the receiver.



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