Behavioural Modelling
Upon a warm autumn day, Brother Farbold was walking through the Gardens of Abstraction near the recently opened Temple of Rust. He passed many curious and elaborate displays of complex abstractions, when he chanced upon a monk toiling away at her own display. Her robes identified her as a neophyte, though her creased, worn hands spoke of many, many years of experience. Around her were scatted dozens of white stones of all shapes and sizes along with several different rakes. Attached to each was a single, small, paper label with a word written on it.
Intrigued, he walked closer.
"Ho!" he said, "what is your name?"
"I am Neophyte Maran."
"What problem are you working on, if I might enquire?"
"I am attempting to translate a previous design that it might be suitable for the temple," the neophyte replied, concentrating on a large sheet of paper covered with sketches of boxes, connected by many lines and arrows.
"I see many labels; what are those for?"
"They define the class of each object."
Brother Farbold paused. "What are classes?"
The neophyte gave him a look of confusion. "You do not know what classes are?"
"No," he replied. "I have heard such a phrase carried on the wind from far away, but in this place, we do not use it."
"Ah, I see," said Maran. "In the temple where I came from, we use classes to define all the properties and behaviours of a thing, that many may share a single definition."
"A useful device, then. In such a scheme, as a simple gardener, I would have a tend behaviour, then?"
The neophyte nodded and picked up a small rock. "This is a Rock," she said, showing Brother Farbold the label. "It has no methods, for all it does is lie on the ground until operated on, but it has properties such as weight and volume."
"So a rock cannot do anything at all?"
"Nothing. Unlike this Rake, which possesses the ability to rake one or more Rocks. This allows me to model a garden and its evolution in its entirety."
"Indeed? This simple model allows you to capture all possible interactions, then?"
"Yes!" exclaimed Neophyte Maran proudly. "I have used this for many years with great success."
"But what if the rock wishes to roll, or shine, or talk? Classes sound restrictive; I prefer to define smaller behaviours, that each thing may pick and choose what it wishes to do."
The neophyte scoffed. "An object can do only that which it was designed to do, nothing more. You said you were a gardener; are you tasked to instruct new students, or to cook meals?"
"I am not; I am tasked with gardening."
"With classes, such is expressed by your class at design-time. As such, there can never be any confusion! To change behaviours is to change the design of the system!"
"Facinating," said Farbold, nodding. The neophyte turned back to her work.
"I must admit I am having trouble finding how to express this concept in a way that the temple will approve of, but I am su--" the neophyte was interrupted when one of the smaller white rocks struck her head.
Holding her head, Maran turned around. "Did you throw a rock at me?" she demanded.
"Me?" said Brother Farbold. "Of course not. I am but a simple Gardener; I know nothing of throwing Rocks."
"The rock could not have thrown itself!"
"And yet, your design does not allow for such a thing to have happened. Surely, you must be mistaken: no rock was thrown."
Neophyte Maran grumbled and turned back to her sketches.
A moment later, she flinched away as a long wooden pole struck her arm. "Did you...?"
"A Rake can only rake Rocks; it cannot "hit Neophytes". You must be imagining things.
"But you..."
"Ho! Brother Farbold!" another monk cried from the other end of the garden. "Brother Schieb has taken ill; would you be so kind as to cook the evening meal in his stead?"
"I would be happy to, Sister Doure; I am just now finishing my instruction of this neophyte."
At that moment, Maran was enlightened.