In Go, I'm going to avoid using 'any' as an actual type

January 25, 2024

As modern Go programmers know, when Go introduced generics it also introduced a new 'any' type. This is officially documented as:

For convenience, the predeclared type any is an alias for the empty interface.

The 'any' type (alias) exists because it's extremely common in code that's specifying generic types to want to be able to say 'any type', and the way this is done in generics is 'interface{}', the empty interface. This makes generic code clearly easier to read and follow. Consider these two versions of the signature of reflect.TypeFor

func TypeFor[T any]() Type
func TypeFor[T interface{}]() Type

These are semantically equivalent but the first is clearer, because you don't have to remember this special case of what 'interface{}' means. Instead, it's right in the name 'any' (and there's less syntactic noise).

But after Go generics became a thing, there's been a trend of using this new 'any' alias outside of generic types, instead of writing out 'interface{}'. I don't think this is a good idea. To show why, consider the following two function signatures, both of which use 'any':

func One[T any](v T) bool
func Two(v any) bool

These two function signatures look almost the same, but they have wildly different meanings, even if (or when) they're invoked with the same argument. The effects of 'One(10)' are rather different from 'Two(10)', since 'One' is a generic function while 'Two' is a regular one. Now consider them written this way:

func One[T any](v T) bool
func Two(v interface{}) bool

Now we see clearly what Two() is doing differently than One(); it's obvious that it isn't taking 'any type' as such, but instead it's taking a generic interface as the argument type. This makes it obvious that a non-interface value will be converted to an interface value (and will tell some people that an interface value will lose its interface type).

This increased immediate clarity without needing to remember what 'any' is why I'm planning to use 'interface{}' in my code in the future, and why I think you should too. Yes, 'any' is shorter and it has a well defined meaning in the specification and we can probably remember the special meaning all of the time. But why give ourselves that extra cognitive burden when we can be explicit?

(In generics, the argument goes the other way; 'any' really does mean 'any type', and the 'any' name is clearer than writing 'interface{}' and then needing to remember that that's how generics do it.)

In a sense the 'any' name is a misnomer when used as a type. It's true that 'interface{}' will accept any type, but used as a type, it doesn't mean 'any type'; it means specifically the type 'an empty interface', which is to say an interface that has no methods, which implies interface type conversion (unless you already have an 'interface{}' value). Since 'any' does mean 'any type' in the context of generics, I think it's better to use a different name for each thing, even if Go formally makes the names equivalent. The names of things are fundamentally for people.

Written on 25 January 2024.
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Last modified: Thu Jan 25 23:03:50 2024
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