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Why Infer Types?

May 26, 2022

Series: Bite Sized Go

Someone at work asked the following question:

Why write code like this:

foo := getFoo()
bar, err := getBar()

instead of this:

var foo Type = getFoo()

var err error
var bar Type

bar, err = getBar(foo)

Isn’t the latter more explicit? Isn’t explicit better? It’ll be easier to review because you’ll be able to see all the types.

Well, yes and no.

For one thing, between the name of the function you’re calling and the name of the variable you’re assigning to, the type is obvious most of the time, at least at the high level.

userID, err := store.FindUser(username)

Maybe you don’t know if userID is a UUID or some custom ID type, or even just a numeric id… but you know what it represents, and the compiler will ensure you don’t send it to some function that doesn’t take that type or call a method on it that doesn’t exist.

In an editor, you’ll be able to hover over it to see the type. In a code review, you may be able to as well, and even if not, you can rely on the compiler to make sure it’s not being used inappropriately.

One big reason to prefer the inferred type version is that it makes refactoring a lot easier.

If you write this code everywhere:

var foo FooType = getFoo()
doSomethingWithFoo(foo)

Now if you want to change getFoo to return a Foo2, and doSomethingWithFoo to take a Foo2, you have to go change every place where these two functions are called and update the explicitly declared variable type.

But if you used inference:

foo := getFoo()
doSomethingWithFoo(foo)

Now when you change both functions to use Foo2, no other code has to change. And because it’s statically typed, we can know this is safe, because the compiler will make sure we can’t use Foo2 inappropriately.

Does this code really care what type getFoo returns, or what type doSomethingWithFoo takes? No, it just wants to pipe the output of one into the other. If this shouldn’t work, the type system will stop it.

So, yes, please use the short variable declaration form. Heck, if you look at it sideways, it even looks kinda like a gopher :=

This is a post in the Bite Sized Go series.
Other posts in this series:

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