SiB Example

sib_example.rb

# Something I thought of at 25:06 in this talk: https://www.youtube.com/watch?time_continue=1&v=FDs-sSxo2iY
class If
  def initialize(condition)
    @condition = condition
  end
  def then(result)
    @condition.then result
  end
  def inspect
    "<if #{@condition.inspect}>"
  end
end

class ReturnThen
  def initialize(result)
    @result = result
  end
  def else(_)
    @result
  end
  def inspect
    "<returnThen>"
  end
end
class ReturnElse
  def else(result)
    result
  end
  def inspect
    "<returnElse>"
  end
end

class True
  def then(result)
    ReturnThen.new result
  end
  def inspect
    "<true>"
  end
end
class False
  def then(result)
    ReturnElse.new
  end
  def inspect
    "<false>"
  end
end

class Zero
  def succ
    Number.new self
  end
  def inspect
    "<zero>"
  end
end
class Number
  def initialize(pred)
    @pred = pred
  end
  def succ
    Number.new self
  end
  def inspect
    @pred.inspect + "+" # I like a plus here better, since it's the successor
  end
end

# we'll ignore that constants are cheating ;)
TRUE  = True.new
FALSE = False.new
ZERO  = Zero.new
ONE   = ZERO.succ
TWO   = ONE.succ

# you don't have to assign the result to a variable,
# it's smart enough to look at the previous line
If.new(TRUE)
  .then(ONE)
  .else(TWO)
  # => <zero>+

# you can also look at the intermediate values, which might be useful
If.new(FALSE) # => <if <false>>
  .then(ONE)  # => <returnElse>
  .else(TWO)  # => <zero>++