From: "AMomchilov (Alexander Momchilov) via ruby-core" <ruby-core@...>
Date: 2023-06-03T00:23:29+00:00
Subject: [ruby-core:113744] [Ruby master Feature#19708] Support `attr_reader :foo?`

Issue #19708 has been updated by AMomchilov (Alexander Momchilov).


You can workaround this by using `attr_reader` to create the optimized method, `alias_method` to give it the nice `active?` name, and then `remove_method` to delete the original non-`?` name:

```ruby
module PredicateHelper # Or just monkey-patch Module, I won't sue :D
  def define_predicate(predicate_name)
    ivar_name = predicate_name.to_s.delete_suffix("?")
    attr_reader(ivar_name)
    alias_method(predicate_name, ivar_name)
    remove_method(ivar_name)
  end
end
```

Usage:

```ruby
class Person  
  extend PredicateHelper

  attr_reader :name, :age
  define_predicate :active?
  
  def initialize
    @name = "Bob"
    @age = 30
    @active = true
  end
end

bob = Person.new

p [bob.name, bob.age, bob.active?] # ["Bob", 30, true]

# Shows that `#active?` is just aliasing the now-deleted `#active`
p Person.instance_method(active?) # => #<UnboundMethod: Person#active?(active)() Untitled 2.rb:6>
```

----------------------------------------
Feature #19708: Support `attr_reader :foo?`
https://bugs.ruby-lang.org/issues/19708#change-103391

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* Priority: Normal
----------------------------------------
Creating reader methods with `attr_reader` is preferable over defining them by hand, not only because it's more convenient, but also because it hits a fast path in MRI (see `VM_METHOD_TYPE_IVAR`).

Since local and instance variables can't end with `?`, you can't use `attr_reader` to define predicate-style methods for boolean values, like:

```ruby
class Person
  attr_reader(:name, :age, :active?) # invalid attribute name `active?' (NameError)
  
  def initialize
    @name = "Bob"
    @age = 30
    @active = true
  end
end
```

It would be nice if `attr_reader` (and friends) could behave like:

```ruby
def active? = @active
```

(`attr_writer` and `attr_accessor` couldn't do the same, since `def active?=(new_value)` isn't valid, so they'd probably stick with `def active=(new_value)`)



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