List comprehension look-alike in Ruby

list_compr.rb

class Hash
  # http://stackoverflow.com/a/14881438/3551701
  def product
    product = values[0].product(*values[1..-1])
    product.map {|p| Hash[keys.zip(p)] }
  end
  
end

class Proc
  # http://stackoverflow.com/a/10059209/3551701
  def eval_with_hash(h)
    Struct.new(*h.keys).new(*h.values).instance_exec(&self)
  end
  
  def for(params, &predicate)
    eager_params = Hash[params.map {|k, v| [k, v.to_a] }]
    lines = eager_params.product
    lines = lines.select {|h| predicate.eval_with_hash(h) } if predicate
    lines.map {|h| eval_with_hash(h) }
  end
  
end

list_compr_example_1.rb

# Python for comparison:
# [ (x,y) for x in range(3) for y in range(3) if x != y ] #=> [(0, 1), (0, 2), (1, 0), (1, 2), (2, 0), (2, 1)]

->{ [x, y] }.for(x: 0..2, y: 0..2) { x != y } #=> [[0, 1], [0, 2], [1, 0], [1, 2], [2, 0], [2, 1]]

list_compr_example_2.rb

Person = Struct.new(:name, :age)

people = [
  Person.new("Alice", 42),
  Person.new("Bob", 24),
  Person.new("Eve", 16)
]

# Idiomatic Ruby:

people.select {|person| person.age > 18 }.map(&:name) #=> ["Alice", "Bob"]

# With this method:

->{ person.name }.for(person: people) { person.age > 18 } #=> ["Alice", "Bob"]