[#104004] [Ruby master Feature#17883] Load bundler/setup earlier to make `bundle exec ruby -r` respect Gemfile — mame@...
Issue #17883 has been reported by mame (Yusuke Endoh).
21 messages
2021/05/24
[ruby-core:104037] [Ruby master Feature#16038] Provide a public WeakMap that compares by equality rather than by identity
From:
jean.boussier@...
Date:
2021-05-25 13:51:30 UTC
List:
ruby-core #104037
Issue #16038 has been updated by byroot (Jean Boussier).
> Instead of exploiting WeakMap, a dedicated cache library makes more sense.
I'm not sure how one could write such library today without a WeakMap (or other Weak* datastructure) like I'm requesting.
----------------------------------------
Feature #16038: Provide a public WeakMap that compares by equality rather than by identity
https://bugs.ruby-lang.org/issues/16038#change-92175
* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
I know `ObjectSpace::WeakMap` isn't really supposed to be used, and that the blessed interface is `WeakRef`. However, I'd like to make a case for a better public WeakMap.
### Usage
As described in [Feature #16035], `WeakMap` is useful for deduplicating "value objects". A typical use case is as follows:
```ruby
class Position
REGISTRY = {}
private_constant :REGISTRY
class << self
def new(*)
instance = super
REGISTRY[instance] ||= instance
end
end
attr_reader :x, :y, :z
def initialize(x, y, z)
@x = x
@y = y
@z = z
freeze
end
def hash
self.class.hash ^
x.hash >> 1 ^
y.hash >> 2 ^
y.hash >> 3
end
def ==(other)
other.is_a?(Position) &&
other.x == x &&
other.y == y &&
other.z == z
end
alias_method :eql?, :==
end
p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```
That's pretty much the pattern [I used in Rails to deduplicate database metadata and save lots of memory](https://github.com/rails/rails/blob/f3c68c59ed57302ca54f4dfad0e91dbff426962d/activerecord/lib/active_record/connection_adapters/deduplicable.rb).
The big downside here is that these value objects can't be GCed anymore, so this pattern is not viable in many case.
### Why not use WeakRef
A couple of reasons. First, when using this pattern, the goal is to reduce memory usage, so having one extra `WeakRef` for every single value object is a bit counter productive.
Then it's a bit annoying to work with, as you have to constantly check wether the reference is still alive, and/or rescue `WeakRef::RefError`.
Often, these two complications make the tradeoff not worth it.
### Ruby 2.7
Since [Feature #13498] `WeakMap` is a bit more usable as you can now use an interned string as the unique key, e.g.
```ruby
class Position
REGISTRY = ObjectSpace::WeakMap.new
private_constant :REGISTRY
class << self
def new(*)
instance = super
REGISTRY[instance.unique_id] ||= instance
end
end
attr_reader :x, :y, :z, :unique_id
def initialize(x, y, z)
@x = x
@y = y
@z = z
@unique_id = -"#{self.class}-#{x},#{y},#{z}"
freeze
end
def hash
self.class.hash ^
x.hash >> 1 ^
y.hash >> 2 ^
y.hash >> 3
end
def ==(other)
other.is_a?(Position) &&
other.x == x &&
other.y == y &&
other.z == z
end
alias_method :eql?, :==
end
p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```
That makes the pattern much easier to work with than dealing with `WeakRef`, but there is still that an extra instance.
### Proposal
What would be ideal would be a `WeakMap` that works by equality, so that the first snippet could simply replace `{}` by `WeakMap.new`.
Changing `ObjectSpace::WeakMap`'s behavior would cause issues, and I see two possibilities:
- The best IMO would be to have a new top level `::WeakMap` be the equality based map, and have `ObjectSpace::WeakMap` remain as a semi-private interface for backing up `WeakRef`.
- Or alternatively, `ObjectSpace::WeakMap` could have a `compare_by_equality` method (inverse of `Hash#compare_by_identity`) to change its behavior post instantiation.
I personally prefer the first one.
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