Go's duck typing: implicitly satisfied contracts.

The Empty Interface (any)

The interface with zero methods is satisfied by every type. It used to be spelled interface{}; since Go 1.18 you can write the alias any instead. They're identical.

var x any = 42
x = "now a string"
x = []int{1, 2, 3}

Use any when you genuinely don't know the type at compile time — JSON decoding, generic containers (before generics existed), fmt.Println(x ...any), etc.

Getting the value back out

An any value is a box containing both a type and a value. To use it, you need to "open the box" — with a type assertion or a type switch.

When to reach for any

Use any sparingly. If you find yourself with any everywhere:

• For "either A or B" you usually want a struct or interface, not any.
• For "any printable thing" you want fmt.Stringer.
• For "container of T" you want generics (chapter 13).
• For "decode unknown JSON" you want map[string]any — that's fine, but try to unmarshal into a struct as soon as you know the shape.