Interface Type Lists

When generics were released, a new way of writing interfaces was also released at the same time!

Traditional interfaces in Go are method-based: a type satisfies an interface if it has the required methods.

With generics, we also got type-set (type-list) interfaces. Instead of listing methods, they list the concrete (or underlying) types that are allowed. These are mostly used as constraints on type parameters.

For example, to use < and > on a type parameter T, the compiler must know T is ordered. A type-list interface spells out exactly which types count as ordered:

// Ordered matches any type that supports <, <=, >, and >=.
type Ordered interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64 |
        ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
        ~float32 | ~float64 |
        ~string
}

// Because T is constrained by Ordered, the compiler knows
// that < is valid for any T used with this function.
func Min[T Ordered](a, b T) T {
    if a < b {
        return a
    }
    return b
}