Dear Junior

Right now I am sitting on train number 928, travelling from home on my way to Arlanda airport and onwards to Explore DDD Conference to present on Domain Primitives together with my friend and colleague Daniel Deogun. Train numer 928 … if I would write a program handling trains I would probably come up with a class named TrainNumber somewhere sooner or later.

Some people might object ”come on Dan, that’s overdoing it, 928 is just an integer” and suggest representing it as a simple int. Me forming a separate class - am I doing it more complex that it is?

For a moment, let us dive into ”it is just an integer”. What does it mean to be ”just an integer” anyway?

Obviously integers are not just a bunch of numbers in a pile. One of the first intellectual things we teach our kids is to count stuff ”one, two, three, four …” pedagogically moving our adult index finger from one toy car to the next. Clearly the fact that ”three” comes immediately after ”four” is something very fundamental for us.

Does that hold for train numbers as well?

Another important property of integers are that we can add them to each other: 4+5=9. This is a really useful property that enable us to take some new-bought apples (five of them) and put them in the fruit bowl where there were some apples since before (four of them). We now know there are nine apples in the bowl. And it neither matters how many apples there were in the bowl, nor how many apples we put in, the result will always be a integer amount of apples.

Of course, integers can be negative. Negatives denote a shortcoming, a lack, or a debt. According to the bank, I own an impressing number of negative Swedish kronor related to my house loan. And when I add Swedish kronor to that negative pile, at time of mortgage, that negative number increase towards zero. Hopefully it will be zero some day.

Does that hold for train numbers as well?

We can also multiply integers: collecting four piles of plates, each six plates high, yields a pile of twenty-four plates.

Does that hold for train numbers as well?

Zooming out, using mathematical notation: the integers form ”an abelian group under addition”. This means for example that there exists an ”identity element” (denoted 0) such that it doesn’t matter if you add it to another integer: 0+a = a. It also means that for every other element (not 0) there exists an ”inverse” which is the ”opposite”; so as 4 is an integer which is not not 0, there must exist an integer (which we name -4) such that 4 + (-4) = 0.

Does that hold for train numbers as well?

Being ”just an integer” does not seem so simple any longer, doesn’t it? It turns out that integers is a very rich type with a lot of properties (and we have not even started talking about prime numbers yet).

Does really train numbers have all these complex properties?

Of course not: There isn’t necessarily any specific meaning in that train number 928 is ”adjacent to” train number 929. Adding train number 928 and 356 does not necessarily yield a train number. Train numbers are very seldom negativ. Multiplying two train numbers doesn’t make sense at all (a train-square?). Train numbers does certainly not form an abelian group.

Train numbers are much simper than integers. It’s basically just a set of distinct elements, without any operations or relations apart from comparing two of them.

Train numbers are simply something that is a primitive in our domain. It’s one of the fundamental concepts that the train domain stand on, its parts carry no meaning. In that regard it fulfils the same kind of role that ”language primitives” fulfil in programming languages, but in the train domain. It is a ”train primitive”.

Turing our heads toward programming, think about all the things you can do with an int - just have a look in the Math-class for inspiration. This means that whenever someone think ”train numbers are just integers” they have to take care not to do any of these things that can be done. The TrainNumber class on the other hand can only be used for what it is intended for, nothing else.

Saying that train numbers is a domain primitive, and giving it a class TrainNumber, is not to make things complex. It’s to make things simple.

Yours

Dan