logging-and-monads.rb

# fyi: I have no idea what I'm doing :)

Log = Struct.new :buffer do
  def call method
    buffer << "Log: before #{method.name}\n"
    method.call buffer
    buffer << "Log: after #{method.name}\n\n"
    self
  end
end

# unit :: a -> M a
unit = -> buffer do Log[buffer] end
# bind :: M a -> (a -> M a) -> M a
bind = -> log, method do log.call method; log end.curry

def foo buffer
  buffer << "hello from foo\n"
end
def bar buffer
  buffer << "hola from bar\n"
end
foo, bar = method(:foo), method(:bar)

buffer = []

# THE MAGIC!
with_logs = bind.(unit.(buffer))

with_logs
  .(foo)
  .(bar)
  .(foo)

puts buffer.join
__END__
Log: before foo
hello from foo
Log: after foo

Log: before bar
hola from bar
Log: after bar

Log: before foo
hello from foo
Log: after foo