Feature #19061
Updated by nobu (Nobuyoshi Nakada) about 2 years ago
**The problem**
Let's imagine this synthetic data:
```ruby
lines = [
"--EMAIL--",
"From: zverok.offline@gmail.com",
"To; bugs@ruby-lang.org",
"Subject: Consuming Enumerators",
"",
"Here, I am presenting the following proposal.",
"Let's talk about consuming enumerators..."
]
```
The logic of parsing it is more or less clear:
* skip the first line
* take lines until meet empty, to read the header
* take the rest of the lines to read the body
It can be easily translated into Ruby code, almost literally:
```ruby
def parse(enumerator)
puts "Testing: #{enumerator.inspect}"
enumerator.next
p enumerator.take_while { !_1.empty? }
p enumerator.to_a
end
```
Now, let's try this code with two different enumerators on those lines:
```ruby
require 'stringio'
enumerator1 = lines.each
enumerator2 = StringIO.new(lines.join("\n")).each_line(chomp: true)
puts "Array#each"
parse(enumerator1) test(enumerator1)
puts
puts "StringIO#each_line"
parse(enumerator2) test(enumerator2)
```
Output (as you probably already guessed):
```
Array#each
Testing: #<Enumerator: [...]:each>
["--EMAIL--", "From: zverok.offline@gmail.com", "To; bugs@ruby-lang.org", "Subject: Consuming Enumerators"]
["--EMAIL--", "From: zverok.offline@gmail.com", "To; bugs@ruby-lang.org", "Subject: Consuming Enumerators", "", "Here, I am presenting the following proposal.", "Let's talk about consuming enumerators..."]
StringIO#each_line
Testing: #<Enumerator: #<StringIO:0x00005581018c50a0>:each_line(chomp: true)>
["From: zverok.offline@gmail.com", "To; bugs@ruby-lang.org", "Subject: Consuming Enumerators"]
["Here, I am presenting the following proposal.", "Let's talk about consuming enumerators..."]
```
Only the second enumerator behaves the way we wanted it to.
Things to notice here:
1. Both enumerators are of the same class, "just enumerator," but they behave differently: one of them is **consuming** data on each iteration method, the other does not; but there is no programmatic way to tell whether some enumerator instance is consuming
2. There is no easy way to **make a non-consuming enumerator behave in a consuming way**, to open a possibility of a sequence of processing "skip this, take that, take the rest"
**Concrete proposal**
1. Introduce an `Enumerator#consuming?` method that will allow telling one of the other (and make core enumerators like `#each_line` properly report they are consuming).
2. Introduce `consuming: true` parameter for `Enumerator.new` so it would be easy for user's code to specify the flag
3. Introduce `Enumerator#consuming` method to produce a consuming enumerator from a non-consuming one:
```ruby
# reference implementation is trivial:
class Enumerator
def consuming
source = self
Enumerator.new { |y| loop { y << source.next } }
end
end
enumerator3 = lines.each.consuming
parse(enumerator3)
```
Output:
```
["From: zverok.offline@gmail.com", "To; bugs@ruby-lang.org", "Subject: Consuming Enumerators"]
["Here, I am presenting the following proposal.", "Let's talk about consuming enumerators..."]
```