Built In Types: Introduction

This section covers the different built-in types available in Hack.

Primitive Types

Hack has the following primitive types: bool, int, float, string, and null.

Union Types

Hack supports union types, like:

• num, where int and float are subtypes of num, and
• arraykey, where int and string are subtypes of arraykey.

The Super Type

Hack's super type is mixed, which represents any value. All other types are subtypes of mixed.

A few things to know when working with mixed as a type:

• The opposite of mixed is nothing, a special type at the "bottom" of all other types.
• mixed is equivalent to ?nonnull. nonnull is a type that represents any value except null.

Hack Arrays

There are three types of Hack Arrays. They are: vec, keyset, and dict.

Though not built-in as types, other alternatives exist in Hack Collections.

Other Built-In Types

Hack has other built-in types too, like: enum (with enum class and enum class labels), shape, and tuples.

Function Return Types

Other types like noreturn and void are only valid as function return types .

Special Types

These last few types are special in their utility and/or versatility: classname, dynamic, and this.