I was working on my twitterstream package the other day and got thinking.

When do you make a new type? When don't you make a new type?

Type System Basics

If you use a programming without a strong static type system, you probably don't have much to worry about. In ruby and python for example, it doesn't really care about what you pass a method or function, as long as it has whatever methods you expect it to have. This is called duck typing

The Go programming language is statically typed. When I define a function, I have to specify that the function takes an int as the argument. I also specify the return type.

func AddOne(int x) int {
    return x + 1
}

Compare this to ruby:

def add_one(x)
    x + 1
end

The ruby version only cares if x implements a + method. You could do something like this:

Ruby doesn't care. The thing implements the + method, so it just works.

Go is pickier. It wants an int, and won't even compile it you try to give it a wrong type.

Isn't that nice.

Can we get to the point?

Let's move on. What I came across was that Tweets have geolocation information, expressed in latitude and longitude. If I didn't want to make a new type for these, I could just use a 64-bit floating point. But that's not right, let's make a new type.

type Latitude float64
type Longitude float64

I just made two distinct types for each, based on the float64 type. Now when I define methods, I can be explicit about which one I want. I don't have to worry about passing a latitude where I should've passed a longitude, and vice versa.

So when do you decide, "yes, this needs a new distinct type" instead of just using something built in? In this particular case, it was honestly the first thing I thought of. My brain immediately went to the "what if you try to do stuff with the incompatible types?" area. Naturally, new types.

But when do you really start making new types? When do you stop?

If you're dealing with currency, do you just do it with an int and call it cents? Do you make a new Cents type aliased to int? Just use a Decimal type? New distinct type aliased to Decimal? Completely new type (struct) not aliased to anything? What do you do!??

I gave an article by Martin Fowler a read, and I still don't know. I suppose it really depends on the situation. With the latitude and longitude example, it seemed pretty obvious.

With currency, well it probably depends on the application. If you're making a little budgeting app for yourself, it's probably not too important. If you're making an important financial application, you probably want a currency type with a good test suite.

Your turn

How do you decide when to make a new type? Does it change if you're using a dynamic language with duck typing versus a compiled statically typed language? Let me know on Twitter