Specification for Hack

Facebook has dedicated all copyright to this specification to the public domain worldwide under the CC0 Public Domain Dedication located at http://creativecommons.org/publicdomain/zero/1.0/. This specification is distributed without any warranty.

(Initially written by Facebook, Inc., February 2015)

Table of Contents

• Introduction
• Conformance
• Terms and Definitions
• Basic Concepts
  • Program Structure
  • Program Start-Up
  • Program Termination
  • The Memory Model
    • General
    • Reclamation and Automatic Memory Management
    • Assignment
      • General
      • Value Assignment of Scalar Types to a Local Variable
      • Value Assignment of Object and Resource Types to a Local Variable
      • ByRef Assignment for Scalar Types with Local Variables
      • Byref Assignment of Non-Scalar Types with Local Variables
      • Value Assignment of Array Types to Local Variables
      • Deferred Array Copying
      • General Value Assignment
      • General ByRef Assignment
    • Argument Passing
    • Value Returning
    • Cloning objects
  • Scope
  • Storage Duration
• Types
  • General
  • The Boolean Type
  • The Integer Type
  • The Floating-Point Type
  • The Numeric Type
  • The String Type
  • The Array Key Type
  • The Null Type
  • Enumerated Types
  • The Void Type
  • Array Types
  • Class Types
  • Interface Types
  • Trait Types
  • The this Type
  • Tuple Types
  • Shape Types
  • Closure Types
  • Resource Types
  • Nullable Types
  • Generic Types
  • The Classname Type
  • Type Aliases
  • Supertypes and Subtypes
  • Type Side Effects
  • Type Inferencing
• Constants
  • General
  • Context-Dependent Constants
  • Core Predefined Constants
  • User-Defined Constants
• Variables
  • General
  • Kinds of Variables
    • Local Variables
    • Array Elements
    • Function Statics
    • Instance Properties
    • Static Properties
    • Class and Interface Constants
• Conversions
  • General
  • Converting to Boolean Type
  • Converting to Integer Type
  • Converting to Floating-Point Type
  • Converting to Number Type
  • Converting to String Type
  • Converting to Array Key Type
  • Converting to Array Type
  • Converting to Object Type
  • Converting to Interface Type
  • Converting to Resource Type
  • Converting to Mixed Type
• Lexical Structure
  • Scripts
  • Grammars
  • Grammar ambiguities
  • Lexical Analysis
    • General
    • Comments
    • White Space
    • Tokens
      • General
      • Names
      • Keywords
      • Literals
        • General
        • Boolean Literals
        • Integer Literals
        • Floating-Point Literals
        • String Literals
          • Single-Quoted String Literals
          • Double-Quoted String Literals
          • Heredoc String Literals
          • Nowdoc String Literals
        • The Null Literal
      • Operators and Punctuators
• Expressions
  • General
  • Restrictions on Arithmetic Operations
  • Operations on Operands Having One or More Subtypes
  • Primary Expressions
    • General
    • Intrinsics
      • General
      • array
      • echo
      • exit
      • invariant
      • list
    • Collection Literals
    • Tuple Literals
    • Shape Literals
    • Anonymous Function-Creation
    • Async Blocks
  • Postfix Operators
    • General
    • The clone Operator
    • The new Operator
    • Array Creation Operator
    • Subscript Operator
    • Function Call Operator
    • Member-Selection Operator
    • Null-Safe Member-Selection Operator
    • Postfix Increment and Decrement Operators
    • Scope-Resolution Operator
    • Exponentiation Operator
  • Unary Operators
    • General
    • Prefix Increment and Decrement Operators
    • Unary Arithmetic Operators
    • Error Control Operator
    • Cast Operator
    • Await Operator
  • instanceof Operator
  • Multiplicative Operators
  • Additive Operators
  • Bitwise Shift Operators
  • Relational Operators
  • Equality Operators
  • Bitwise AND Operator
  • Bitwise Exclusive OR Operator
  • Bitwise Inclusive OR Operator
  • Logical AND Operator
  • Logical Inclusive OR Operator
  • Conditional Operator
  • Coalesce Operator
  • Pipe Operator
  • Lambda Expressions
  • Assignment Operators
    • General
    • Simple Assignment
    • Compound Assignment
  • yield Operator
  • Constant Expressions
• Statements
  • General
  • Compound Statements
  • Labeled Statements
  • Expression Statements
  • Selection Statements
    • General
    • The if Statement
    • The switch Statement
  • Iteration Statements
    • General
    • The while Statement
    • The do Statement
    • The for Statement
    • The foreach Statement
  • Jump Statements
    • General
    • The continue Statement
    • The break Statement
    • The return Statement
    • The throw Statement
  • The try Statement
• Script Inclusion Operators
  • General
  • The require Directive
  • The require_once Directive
• Enums
  • General
  • Enum Declarations
  • The Predefined Enumerated Type Methods
• Generic Types, Methods, and Functions
  • General
  • Type Parameters
  • Type Constraints
  • Type Arguments
  • Open and Closed Generic Types
  • Type Inferencing Revisited
• Functions
  • General
  • Function Calls
  • Function Definitions
  • Anonymous Functions
  • Asynchronous Functions
• Classes
  • General
  • Class Declarations
  • Class Members
  • Dynamic Methods
  • Constants
  • Properties
  • Methods
  • Constructors
  • Destructors
  • Type Constants
  • Methods with Special Semantics
    • General
    • Method __call
    • Method __callStatic
    • Method __clone
    • Method __sleep
    • Method __toString
    • Method __wakeup
  • Serialization
  • Predefined Classes
    • Class AsyncGenerator
    • Class Generator
    • Class __PHP_Incomplete_Class
    • Class Shapes
    • Class stdClass
• Interfaces
  • General
  • Interface Declarations
  • Interface Members
  • Constants
  • Methods
  • Predefined Interfaces
    • Interface ArrayAccess
    • Interface AsyncIterator
    • Interface AsyncKeyedIterator
    • Interface Awaitable
    • Interface Container
    • Interface IMemoizeParam
    • Interface Iterator
    • Interface IteratorAggregate
    • Interface KeyedContainer
    • Interface KeyedTraversable
    • Interface Serializable
    • Interface Stringish
    • Interface Traversable
• Traits
  • General
  • Trait Declarations
  • Trait Members
• Exception Handling
  • General
  • Class Exception
  • Tracing Exceptions
  • User-Defined Exception Classes
• Namespaces
  • General
  • Name Lookup
  • Defining Namespaces
  • Namespace Use Declarations**
• Attributes
  • General
  • Attribute Specification
  • Predefined Attributes
    • General
    • Attribute __ConsistentConstruct
    • Attribute __Memoize
    • Attribute __Override
• Grammar
  • General
  • Lexical Grammar
    • General
    • Comments
    • White Space
    • Tokens
      • General
      • Names
    • Keywords
    • Literals
      • General
      • Boolean Literals
      • Integer Literals
      • Floating-Point Literals
      • String Literals
      • The Null Literal
    • Operators and Punctuators
  • Syntactic Grammar
    • Program Structure
    • Types
      • General
      • Array Types
      • Tuple Types
      • Shape Types
      • Closure Types
      • Nullable Types
      • The Classname Type
      • Type Aliases
    • Variables
    • Expressions
      • Primary Expressions
      • Postfix Operators
      • Unary Operators
      • instanceof Operator
      • Multiplicative Operators
      • Additive Operators
      • Bitwise Shift Operators
      • Relational Operators
      • Equality Operators
      • Bitwise Logical Operators
      • Logical Operators
      • Conditional Operator
      • Pipe Operator
      • Lambda Expressions
      • Assignment Operators
      • yield Operator
      • Constant Expressions
    • Statements
      • General
      • Compound Statements
      • Labeled Statements
      • Expression Statements
      • Iteration Statements
      • Jump Statements
      • The try Statement
    • Script Inclusion
    • Enums
    • Generic Types, Methods and Functions
    • Functions
    • Classes
    • Interfaces
    • Traits
    • Namespaces
    • Attributes
• Differences from PHP
  • General
  • Program Start-Up
  • Constants
  • Variables
  • Conversions
  • Lexical Structure
    • Comments
    • Names
    • Keywords
  • Expressions
    • Primary Expressions
      • General
      • Intrinsics
      • Anonymous Function-Creation
    • Postfix Operators
      • The new Operator
      • Array Creation Operator
      • Function call operator
      • Member-Selection Operator
      • Postfix Increment and Decrement Operators
      • Exponentiation Operator
    • Unary Operators
      • Prefix Increment and Decrement Operators
      • Unary Arithmetic Operators
      • Shell Command Operator
      • Cast Operator
      • Variable-Name Creation Operator
    • instanceof Operator
    • Multiplicative Operators
    • Bitwise Shift Operators
    • Bitwise AND Operator
    • Bitwise Exclusive OR Operator
    • Bitwise Inclusive OR Operator
    • Assignment Operators
      • byRef Assignment
    • Logical AND, OR, XOR Operators (Alternate Forms)
    • String Literals
  • Statements
    • General
    • Labeled Statements
    • The if Statement
    • The switch Statement
    • The while Statement
    • The for Statement
    • The foreach Statement
    • The goto Statement
    • The continue Statement
    • The break Statement
    • The return Statement
    • The declare Statement
  • Script Inclusion
  • Functions
  • Classes
    • Class Members
    • Dynamic Members
    • Properties
    • Methods
    • Constructors
    • Methods with Special Semantics
      • General
      • Method __call
      • Method __get
      • Method __invoke
      • Method __isset
      • Method __set
      • Method __set_state
      • Method __unset
    • Predefined Classes
      • Class Closure
  • Interfaces
    • Interface Members
    • Methods
    • Predefined Interfaces
  • Traits
    • Trait Declarations
  • Namespaces
    • Namespace Use Declarations
• Bibliography