From: shyouhei@...
Date: 2017-11-30T00:53:12+00:00
Subject: [ruby-core:83989] [Ruby trunk Bug#12834] `prepend` getting prepended even if it already exists in the ancestors chain

Issue #12834 has been updated by shyouhei (Shyouhei Urabe).


marcandre (Marc-Andre Lafortune) wrote:
> Could you clarify?
> 
> For inclusion/prepending, there are two questions in my mind:
> 1) what happens if a module is included/prepended multiple times at different places in the hierarchy (e.g. in A and B, in the example above)
> 2) what happens if it is included/prepended multiple times at the same level (e.g. twice in A)
> 
> Was the discussion about (b) allowing multiple times at the same level?

I don't remember there were discussions about inclusion in this topic.  There were ones for prepending only.  From what I understand the intention of prepending a module more than once is:

#### `1)`'s situation: ####

   ```ruby
module X              def x; "X<%s>" % super end end
module Y prepend ::X; def x; "Y<%s>" % super end end
module Z prepend ::Y; def x; "Z<%s>" % super end end
class W
  prepend ::X
  prepend ::Z
  def x
    'W'
  end
end
    
W.new.x # => "X<Y<Z<X<W>>>>"
   ```

#### `2)`'s situation: ####

   ```ruby
module X              def x; "X<%s>" % super end end
module Y prepend ::X; def x; "Y<%s>" % super end end
module Z prepend ::X; def x; "Z<%s>" % super end end
class W
  prepend ::Y
  prepend ::Z
  def x
    'W'
  end
end
    
W.new.x # => "X<Z<X<Y<W>>>>"
   ```

Note: current trunk does not behave this way.

----------------------------------------
Bug #12834: `prepend` getting prepended even if it already exists in the ancestors chain
https://bugs.ruby-lang.org/issues/12834#change-68068

* Author: ndn (Nikola Nenkov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.3.1
* Backport: 2.3: UNKNOWN
----------------------------------------
```
module M; end
class A; prepend M; end
class B < A; prepend M; end

B.ancestors # => [M, B, M, A, Object, Kernel, BasicObject]
```

Even though I find this behaviour to be more intuitive, it is inconsistent with `Module#include` and is potentially breaking.
I didn't see a mention on the [[release notes]](https://www.ruby-lang.org/en/news/2015/12/25/ruby-2-3-0-released/) and the [[documentation]](https://ruby-doc.org/core-2.3.1/Module.html#method-i-prepend_features) is now outdated. 



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