Ruby "type" checking

check.rb

def foo(a : String) 
  # the method prologue is injected with
  raise TypeError unless String === a
end

# Allows for other interesting ones, like:

def use_as_number(a : /\d+/)
  
end

# and more interesting

class RespondTo
  def initialize(method)
    @method = method
  end
  
  def ===(o)
    o.respond_to? @method
  end
end

ToS = RespondTo.new(:to_s)

def stringy(s : ToS)
  # same prologue as before, now doing a respond_to check.
end