cond.rb

class Cond
  IDENTITY = -> v { v }

  def initialize(if_cond: IDENTITY, then_branch: IDENTITY, else_branch: IDENTITY)
    @if_cond = if_cond
    @then_branch = then_branch
    @else_branch = else_branch
  end

  def call(v)
    @if_cond.call(v) ? @then_branch.call(v) : @else_branch.call(v)
  end

  alias_method :===, :call

  def to_proc
    -> v { call(v) }
  end

  define_method(:if) do |&block|
    @if_cond = block
    self
  end

  define_method(:then) do |&block|
    @then_branch = block
    self
  end

  define_method(:else) do |&block|
    @else_branch = block
    self
  end

  class << self
    define_method(:if) do |&block|
      new(if_cond: block)
    end

    define_method(:then) do |&block|
      new(then_branch: block)
    end

    define_method(:else) do |&block|
      new(else_branch: block)
    end
  end
end

(1..5).map(&Cond.if(&:even?).then { _1 * 2 }.else { _1 * 3 })
# => [3, 4, 9, 8, 15]