Copying Clojure's IFn interface for Set#to_proc and Hash#to_proc. More details at http://mudge.name/2014/11/26/data-structures-as-functions.html

hash.rb

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

%w(a b c d).map(&{"a" => 1, "b" => 2, "c" => 3})
# => [1, 2, 3, nil]

# Or, a more readable example:

person = { name: "Robert Paulson", age: 43 }

name, age = %i(name age).map(&person)
# => ["Robert Paulson", 43]

set.rb

class Set
  def to_proc
    method(:include?).to_proc
  end
end

(1..10).select(&Set[1, 5, 7])
# => [1, 5, 7]

# Or to be closer to Clojure's behaviour...

class Set
  def to_proc
    ->(x) { x if include?(x) }
  end
end

# A more readable example:

acceptable_values = Set[2, 4, 6, 8]
user_submission = [1, 3, 4, 5, 7, 8]

user_submission.select(&acceptable_values)
# => [4, 8]