The name makes it sound scary and difficult but it's actually a simple and powerful trick that can help make your code safer!
So let's see how it can be applied!
But first, I have a big thank you to a developer tool who's the sponsor of this email: Stream
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Can you guess what's the problem with this code?
I've defined a struct called Person, inside which I store a name and an id.
You can see that I'm using a UUID to represent the id, which guarantees uniqueness:
Now let's introduce a second struct, called Location:
I'm following the same approach and I also use a UUID to represent the id of that new struct.
For now, this code seems pretty reasonable, doesn't it?
Now let's introduce a function that operates over an array of Location:
You can notice that there is nothing preventing me from filtering the array of Location by incorrectly using the id of a Person:
And that's a problem, because such code doesn't make any sense and is guaranteed to lead to a bug!
However, there's an easy way to improve!
We can introduce a new struct called ID:
In this struct, we'll store a UUID.
And we're also going to add a generic argument to this struct:
This might seem weird, because we're not using this generic argument anywhere, but trust me: it will all make sense in a second!
Because now, we're going to remove the UUID from both Person and Location…
…and replace them with our new ID type:
If we look at the type of each id property, we can see that, thanks to the extra generic argument, the types are no longer the same!
This means that if by mistake we attempt to compare the id of a Person with the id of a Location, we'll get a compile-time error and it will be impossible to ship this incorrect code!
So that's what a Phantom Type is: it's a generic parameter that isn't used at runtime but that allows us to implement constraints that will be automatically checked at compile time 👌
That's all for this email, thanks for reading it!
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