[ruby-core:91181] [Ruby trunk Feature#15549] Enumerable#to_reader (or anything enumerable, Enumerator, lazy enums, enum_for results)

Issue #15549 has been reported by chucke (Tiago Cardoso).

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Feature #15549: Enumerable#to_reader (or anything enumerable, Enumerator, lazy enums, enum_for results)
https://bugs.ruby-lang.org/issues/15549

* Author: chucke (Tiago Cardoso)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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This is a feature proposal for something I've had to implement before multiple times.

For a lot of IO-related APIs, there is this unspoken (because ruby doesn't have official interfaces) notion of a reader/writer protocol, that is, you pass arguments to certain functions where they either must implement "#read(nsize, buffer)" or "#write(data)". An example would be "IO.copy_stream".

It happened to me multiple times in the past that I started implementing some data-generator using "#each" in a specific format (CSV data, JSON...) to be lazy and memory conservative, but end up rewriting it because I can't read from an enumerable into a socket/file handle directly.

Lately I've been adopting the pattern of "injecting" a "#read" method to these objects, so that I can indeed use these APIs to my benefit. Sadly, I have to reimplement this in every project. This is the gist:

https://gist.github.com/HoneyryderChuck/625c7b873a00a18d12b1a08695551510

I think such an API would be very benefitial to the common user. In most projects I've worked in, writing data to a tempfile, S3 bucket, FTP server, is very common, and I've lost the count to the number of implementations which write the whole data in memory **then** write to the handle, which obviously gives the impression that ruby consumes a lot of memory.

Now, I also understand that this is only beneficial to particular case of enums (those which yield strings/"to_s"-ables). But since there's a precedent for "#sum", so maybe I can make a case.

This is an example that works if you load the gist code":

```ruby
enum = %w(a) * 65536
puts "size: #{enum.size}"
reader = enum.to_reader
IO.copy_stream(reader, $stderr)
```



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