From: daniel@dan42.com
To: ruby-core@ruby-lang.org
Subject: [ruby-core:98321] [Ruby master Bug#6087] How should inherited methods deal with return values of their own subclass?
Date: Wed, 13 May 2020 15:14:15 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-85560.20200513151415.182@ruby-lang.org> (raw)
In-Reply-To: redmine.issue-6087.20120226060237.182@ruby-lang.org
Issue #6087 has been updated by Dan0042 (Daniel DeLorme).
> * A method that seems to return a new array that is directly related to the receiver, should return an instance of the receiver's class.
> * A method that seems to return a new array that is not directly related to the receiver, should return an Array.
So this is the old thinking?
> I used to think methods should honor subclasses, but I changed my mind that the behavior made things too complex.
And this is the new thinking? In that case +1
If a subclass needs a method to return an instance of the subclass, it can easily and _safely_ opt-in to this behavior (similar to Hash)
```ruby
class A < Array
def select(...)
A.new(super) #or e.g. dup.replace(super) depending on specifics of the subclass
end
end
```
On the other hand returning a subclass by default opens the door to all kinds of complexity and bugs depending on how the subclass is implemented. In particular if it has any state/ivars. `ary.select` is not the same as `ary.dup.select!` in that case.
Is there somewhere a complete list of methods that currently return a subclass?
For Array I think there's only this: drop, drop_while, take, take_while, flatten, uniq, slice
----------------------------------------
Bug #6087: How should inherited methods deal with return values of their own subclass?
https://bugs.ruby-lang.org/issues/6087#change-85560
* Author: marcandre (Marc-Andre Lafortune)
* Status: Assigned
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 3.0
* ruby -v: trunk
----------------------------------------
Just noticed that we still don't have a consistent way to handle return values:
```ruby
class A < Array
end
a = A.new
a.flatten.class # => A
a.rotate.class # => Array
(a * 2).class # => A
(a + a).class # => Array
```
Some methods are even inconsistent depending on their arguments:
```ruby
a.slice!(0, 1).class # => A
a.slice!(0..0).class # => A
a.slice!(0, 0).class # => Array
a.slice!(1, 0).class # => Array
a.slice!(1..0).class # => Array
```
Finally, there is currently no constructor nor hook called when making these new copies, so they are never properly constructed.
Imagine this simplified class that relies on `@foo` holding a hash:
```ruby
class A < Array
def initialize(*args)
super
@foo = {}
end
def initialize_copy(orig)
super
@foo = @foo.dup
end
end
a = A.new.flatten
a.class # => A
a.instance_variable_get(:@foo) # => nil, should never happen
```
I feel this violates object orientation.
One solution is to always return the base class (`Array`/`String`/...).
Another solution is to return the current subclass. To be object oriented, I feel we must do an actual `dup` of the object, including copying the instance variables, if any, and calling `initialize_copy`. Exceptions to this would be (1) explicit documentation, e.g. `Array#to_a`, or (2) methods inherited from a module (like `Enumerable` methods for `Array`).
I'll be glad to fix these once there is a decision made on which way to go.
--
https://bugs.ruby-lang.org/
next prev parent reply other threads:[~2020-05-13 15:14 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <redmine.issue-6087.20120226060237.182@ruby-lang.org>
2020-04-09 3:12 ` [ruby-core:97763] [Ruby master Bug#6087] How should inherited methods deal with return values of their own subclass? mame
2020-04-10 7:17 ` [ruby-core:97779] " matz
2020-04-10 7:18 ` [ruby-core:97780] " matz
2020-04-10 16:44 ` [ruby-core:97811] " eregontp
2020-05-07 7:23 ` [ruby-core:98171] " ko1
2020-05-13 15:14 ` daniel [this message]
2020-10-23 4:10 ` [ruby-core:100511] " merch-redmine
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