More noodling with #to_proc but, this time, through #===, c.f. http://mudge.name/2014/11/26/data-structures-as-functions.html

threequals.rb

require 'set'

# Let's provide a default to_proc in terms of threequals
class Object
  def to_proc
    method(:===).to_proc
  end
end

class Set
  def ===(element)
    element if member?(element)
  end
end

class Hash
  alias_method :===, :[]
end

class Array
  alias_method :===, :[]
end

[:name, :age].map(&{name: 'Bob', age: 42})
#=> ["Bob", 42]

[0, 2].map(&%w(foo bar baz quux))
#=> ["foo", "baz"]

%w(Alice Bob Crispin).reject(&Set['Bob', 'Daisy'])
#=> ["Alice", "Crispin"]

case "Alice"
when Set["Alice", "Bob"]
  "Welcome!"
end
#=> "Welcome!"

to_proc.rb

# As Proc#=== is already defined to call the proc, we could try the other way around:
require 'forwardable'
require 'set'

class Object
  extend Forwardable
  def_delegator :to_proc, :===

  # We have to provide a default implementation that matches the behaviour of
  # Kernel#=== to prevent a nasty TypeError
  def to_proc
    proc { |other| self.equal?(other) || self == other }
  end
end

class Hash
  def to_proc
    method(:[]).to_proc
  end
end

class Array
  def to_proc
    method(:[]).to_proc
  end
end

class Set
  def to_proc
    proc { |element| element if member?(element) }
  end
end

[:name, :age].map(&{name: 'Bob', age: 42})
#=> ["Bob", 42]

[0, 2].map(&%w(foo bar baz quux))
#=> ["foo", "baz"]

%w(Alice Bob Crispin).reject(&Set['Bob', 'Daisy'])
#=> ["Alice", "Crispin"]

case "Alice"
when Set["Alice", "Bob"]
  "Welcome!"
end
#=> "Welcome!"

Set["Alice", "Bob"] === "Bob"
#=> "Bob"

Set["Alice", "Bob"] === "Carol"
#=> nil