require "stringio"

# method I want to test
def ask(question)
  puts "#{question} [Y/n]"
  STDIN.read.chomp
end

def STDIN.capture(default, &block)
  STDIN.reopen "/dev/null" # so we don't get the fucking prompt
  STDIN.ungetbyte(default) # so the default value can be get by STDIN.getc & similar methods
  block.call
  # TODO: how I can get back the original STDIN?
end

p STDIN.capture("yes") { ask("Do you hear me?") }
# => "yes"