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# Syntax ## Types ```markdown _type_ ::= _class-name_ _type-arguments_ (Class instance type) | _interface-name_ _type-arguments_ (Interface type) | `singleton(` _class-name_ `)` (Class singleton type) | _alias-name_ (Alias type) | _literal_ (Literal type) | _type_ `|` _type_ (Union type) | _type_ `&` _type_ (Intersection type) | _type_ `?` (Optional type) | `{` _record-name_ `:` _type_ `,` ... `}` (Record type) | `[]` | `[` _type_ `,` ... `]` (Tuples) | _type-variable_ (Type variables) | `^(` _parameters_ `) ->` _type_ (Proc type) | `self` | `instance` | `class` | `bool` | `untyped` | `nil` | `top` | `bot` | `void` _class-name_ ::= _namespace_ /[A-Z]\w*/ _interface-name_ ::= _namespace_ /_[A-Z]\w*/ _alias-name_ ::= _namespace_ /[a-z]\w*/ _type-variable_ ::= /[A-Z]\w*/ _namespace_ ::= (Empty namespace) | `::` (Root) | _namespace_ /[A-Z]\w*/ `::` (Namespace) _type-arguments_ ::= (No application) | `[` _type_ `,` ... `]` (Type application) _literal_ ::= _string-literal_ | _symbol-literal_ | _integer-literal_ | `true` | `false` ``` ### Class instance type Class instance type denotes _an instance of a class_. ``` Integer # Instance of Integer class ::Integer # Instance of ::Integer class Hash[Symbol, String] # Instance of Hash class with type application of Symbol and String ``` ### Interface type Interface type denotes _type of a value which can be a subtype of the interface_. ``` _ToS # _ToS interface ::MyApp::_Each[String] # Interface name with namespace and type application ``` ### Class singleton type Class singleton type denotes _the type of a singleton object of a class_. ``` singleton(String) singleton(::Hash) # Class singleton type cannot be parametrized. ``` ### Alias type Alias type denotes an alias declared with _alias declaration_. The name of type aliases starts with lowercase `[a-z]`. ``` name ::JSON::t # Alias name with namespace ``` ### Literal type Literal type denotes _a type with only one value of the literal_. ``` 123 # Integer "hello world" # A string :to_s # A symbol true # true or false ``` ### Union type Union type denotes _a type of one of the given types_. ``` Integer | String # Integer or String Array[Integer | String] # Array of Integer or String ``` ### Intersection type Intersection type denotes _a type of all of the given types_. ``` Integer & String # Integer and String ``` Note that `&` has higher precedence than `|` that `Integer & String | Symbol` is `(Integer & String) | Symbol`. ### Optional type Optional type denotes _a type of value or nil_. ``` Integer? Array[Integer?] ``` ### Record type Records are `Hash` objects, fixed set of keys, and heterogeneous. ``` { id: Integer, name: String } # Hash object like `{ id: 31, name: String }` ``` ### Tuple type Tuples are `Array` objects, fixed size and heterogeneous. ``` [ ] # Empty like `[]` [String] # Single string like `["hi"]` [Integer, Integer] # Pair of integers like `[1, 2]` [Symbol, Integer, Integer] # Tuple of Symbol, Integer, and Integer like `[:pair, 30, 22]` ``` *Empty tuple* or *1-tuple* sound strange, but RBS allows these types. ### Type variable ``` U T S Elem ``` Type variables cannot be distinguished from _class instance types_. They are scoped in _class/module/interface declaration_ or _generic method types_. ``` class Ref[T] # Object is scoped in the class declaration. @value: T # Type variable `T` def map: [X] { (T) -> X } -> Ref[X] # X is a type variable scoped in the method type. end ``` ### Proc type Proc type denotes type of procedures, `Proc` instances. ``` ^(Integer) -> String # A procedure with an `Integer` parameter and returns `String` ^(?String, size: Integer) -> bool # A procedure with `String` optional parameter, `size` keyword of `Integer`, and returns `bool` ``` ### Base types `self` denotes the type of receiver. The type is used to model the open recursion via `self`. `instance` denotes the type of instance of the class. `class` is the singleton of the class. `bool` is an alias of `true | false`. `untyped` is for _a type without type checking_. It is `?` in gradual typing, _dynamic_ in some languages like C#, and _any_ in TypeScript. It is both subtype _and_ supertype of all of the types. (The type was `any` but renamed to `untyped`.) `nil` is for _nil_. `top` is a supertype of all of the types. `bot` is a subtype of all of the types. `void` is a supertype of all of the types. #### `nil` or `NilClass`? We recommend using `nil`. #### `bool` or `boolish` We have a builtin type alias called `boolish`. It is an alias of `top` type, and you can use `boolish` if we want to allow any object of any type. We can see an example at the definition of `Enumerable#find`: ``` module Enumerable[Elem, Return] def find: () { (Elem) -> boolish } -> Elem? ... end ``` We want to write something like: ``` array.find {|x| x && x.some_test? } # The block will return (bool | nil) ``` We recommend using `boolish` for method arguments and block return values, if you only use the values for conditions. You can write `bool` if you strictly want `true | false`. #### `void`, `boolish`, or `top`? They are all equivalent for the type system; they are all _top type_. `void` tells developers a hint that _the value should not be used_. `boolish` implies the value is used as a truth value. `top` is anything else. ## Method Types ```markdown _method-type_ ::= `(` _parameters_ `) ->` _type_ # Method without block | `(` _parameters_ `) { (` _parameters_ `) -> ` _type_ `} ->` _type_ # Method with required block | `(` _parameters_ `) ?{ (` _parameters_ `) -> ` _type_ `} ->` _type_ # Method with optional block _parameters_ ::= _required-positionals_ _optional-positionals_ _rest-positional_ _trailing-positionals_ _keywords_ _parameter_ ::= _type_ _var-name_ # Parameter with var name | _type_ # Parameter without var name _required-positionals_ ::= _parameter_ `,` ... _optional-positionals_ ::= `?` _parameter_ `,` ... _rest-positional_ ::= # Empty | `*` _parameter_ _trailing-positionals_ ::= _parameter_ `,` ... _keywords_ ::= # Empty | `**` _parameter_ # Rest keyword | _keyword_ `:` _parameter_ `,` _keywords_ # Required keyword | `?` _keyword_ `:` _parameter_ `,` _keywords_ # Optional keyword _var-name_ ::= /[a-z]\w*/ ``` ### Parameters A parameter can be a type or a pair of type and variable name. Variable name can be used for documentation. ### Examples ``` # Two required positional `Integer` parameters, and returns `String` (Integer, Integer) -> String # Two optional parameters `size` and `name`. # `name` is a optional parameter with optional type so that developer can omit, pass a string, or pass `nil`. (?Integer size, ?String? name) -> String # Method type with a rest parameter (*Integer, Integer) -> void # `size` is a required keyword, with variable name of `sz`. # `name` is a optional keyword. # `created_at` is a optional keyword, and the value can be `nil`. (size: Integer sz, ?name: String, ?created_at: Time?) -> void ``` ## Members ```markdown _member_ ::= _ivar-member_ # Ivar definition | _method-member_ # Method definition | _attribute-member_ # Attribute definition | _include-member_ # Mixin (include) | _extend-member_ # Mixin (extend) | _prepend-member_ # Mixin (prepend) | _alias-member_ # Alias | `public` # Public | `private` # Private _ivar-member_ ::= _ivar-name_ `:` _type_ _method-member_ ::= `def` _method-name_ `:` _method-types_ # Instance method | `def self.` _method-name_ `:` _method-types_ # Singleton method | `def self?.` _method-name_ `:` _method-types_ # Singleton and instance method _method-types_ ::= # Empty | `super` # `super` overloading | _type-parameters_ _method-type_ `|` _method-types_ # Overloading types _type-parameters_ ::= # Empty | `[` _type-variable_ `,` ... `]` _attribute-member_ ::= _attribute-type_ _method-name_ `:` _type_ # Attribute | _attribute-type_ _method-name_ `(` _ivar-name_ `) :` _type_ # Attribute with variable name specification | _attribute-type_ _method-name_ `() :` _type_ # Attribute without variable _attribute-type_ ::= `attr_reader` | `attr_writer` | `attr_accessor` _include-member_ ::= `include` _class-name_ _type-arguments_ | `include` _interface-name_ _type-arguments_ _extend-member_ ::= `extend` _class-name_ _type-arguments_ | `extend` _interface-name_ _type-arguments_ _prepend-member_ ::= `prepend` _class-name_ _type-arguments_ _alias-member_ ::= `alias` _method-name_ _method-name_ | `alias self.` _method-name_ `self.` _method-name_ _ivar-name_ ::= /@\w+/ _method-name_ ::= ... | /`[^`]+`/ ``` ### Ivar definition An instance variable definition consists of the name of an instance variable and its type. ``` @name: String @value: Hash[Symbol, Key] ``` ### Method definition Method definition has several syntax variations. You can write `self.` or `self?.` before the name of the method to specify the kind of method: instance, singleton, or both instance and singleton. ``` def to_s: () -> String # Defines a instance method def self.new: () -> AnObject # Defines singleton method def self?.sqrt: (Numeric) -> Numeric # self? is for `module_function`s ``` The method type can be connected with `|`s to define an overloaded method. ``` def +: (Float) -> Float | (Integer) -> Integer | (Numeric) -> Numeric ``` You need extra parentheses on return type to avoid ambiguity. ``` def +: (Float | Integer) -> (Float | Integer) | (Numeric) -> Numeric ``` Method types can end with `super` which means the methods from existing definitions. This is useful to define an _extension_, which adds a new variation to the existing method preserving the original behavior. ### Attribute definition Attribute definitions help to define methods and instance variables based on the convention of `attr_reader`, `attr_writer` and `attr_accessor` methods in Ruby. You can specify the name of instance variable using `(@some_name)` syntax and also omit the instance variable definition by specifying `()`. ``` # Defines `id` method and `@id` instance variable. attr_reader id: Integer # @id: Integer # def id: () -> Integer # Defines `name=` method and `@raw_name` instance variable. attr_writer name (@raw_name) : String # @raw_name: String # def name=: (String) -> String # Defines `people` and `people=` methods, but no instance variable. attr_accessor people (): Array[Person] # def people: () -> Array[Person] # def people=: (Array[Person]) -> Array[Person] ``` ### Mixin (include), Mixin (extend), Mixin (prepend) You can define mixins between class and modules. ``` include Kernel include Enumerable[String, void] extend ActiveSupport::Concern ``` You can also `include` or `extend` an interface. ``` include _Hashing extend _LikeString ``` This allows importing `def`s from the interface to help developer implementing a set of methods. ### Alias You can define an alias between methods. ``` def map: [X] () { (String) -> X } -> Array[X] alias collect map # `#collect` has the same type with `map` ``` ### `public`, `private` `public` and `private` allows specifying the visibility of methods. These work only as _statements_, not per-method specifier. ## Declarations ```markdown _decl_ ::= _class-decl_ # Class declaration | _module-decl_ # Module declaration | _interface-decl_ # Interface declaration | _extension-decl_ # Extension declaration | _type-alias-decl_ # Type alias declaration | _const-decl_ # Constant declaration | _global-decl_ # Global declaration _class-decl_ ::= `class` _class-name_ _module-type-parameters_ _members_ `end` | `class` _class-name_ _module-type-parameters_ `<` _class-name_ _type-arguments_ _members_ `end` _module-decl_ ::= `module` _module-name_ _module-type-parameters_ _members_ `end` | `module` _module-name_ _module-type-parameters_ `:` _module-self-types_ _members_ `end` _module-self-types_ ::= _class-name_ _type-arguments_ `,` _module-self-types_ (Class instance) | _interface-name_ _type-arguments_ `,` _module-self-types_ (Interface) _interface-decl_ ::= `interface` _interface-name_ _module-type-parameters_ _interface-members_ `end` _interface-members_ ::= _method-member_ # Method | _include-member_ # Mixin (include) | _alias-member_ # Alias _extension-decl_ ::= `extension` _class-name_ _type-parameters_ `(` _extension-name_ `)` _members_ `end` _type-alias-decl_ ::= `type` _alias-name_ `=` _type_ _const-decl_ ::= _const-name_ `:` _type_ _global-decl_ ::= _global-name_ `:` _type_ _const-name_ ::= _namespace_ /[A-Z]\w*/ _global-name_ ::= /$[a-zA-Z]\w+/ | ... _module-type-parameters_ ::= # Empty | `[` _module-type-parameter_ `,` ... `]` _module-type-parameter_ ::= _check_ _variance_ _type-variable_ _variance_ ::= `out` | `in` _check_ ::= # Empty | `unchecked` ``` ### Class declaration Class declaration can have type parameters and superclass. When you omit superclass, `::Object` is assumed. ``` class Ref[A] < Object attr_reader value: A def initialize: (value: A) -> void end ``` For classes with type parameters, you may specify if they are "invariant" (default), "covariant" (`out`) or "contravariant" (`in`). See [this definition of covariance and contravariance](https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)). For example, an `Array` of `String` can almost be considered to be an `Array` of `Object`, but not the reverse, so we can think of: ``` class Array[out T] # ... end ``` There's a limitation with this is for mutable objects (like arrays): a mutation could invalidate this. If an array of String is passed to a method as an array of Objects, and that method adds an Integer to the array, the promise is broken. In those cases, one must use the `unchecked` keyword: ``` class Array[unchecked out T] # ... end ``` This is how `Array` is actually defined in RBS. ### Module declaration Module declaration takes optional _self type_ parameter, which defines a constraint about a class when the module is mixed. ``` interface _Each[A, B] def each: { (A) -> void } -> B end module Enumerable[A, B] : _Each[A, B] def count: () -> Integer end ``` The `Enumerable` module above requires `each` method for enumerating objects. ### Interface declaration Interface declaration can have parameters but allows only a few of the members. ``` interface _Hashing def hash: () -> Integer def eql?: (any) -> bool end ``` There are several limitations which are not described in the grammar. 1. Interface cannot `include` modules 2. Interface cannot have singleton method definitions ``` interface _Foo include Bar # Error: cannot include modules def self.new: () -> Foo # Error: cannot include singleton method definitions end ``` ### Type alias declaration You can declare an alias of types. ``` type subject = Attendee | Speaker type JSON::t = Integer | TrueClass | FalseClass | String | Hash[Symbol, t] | Array[t] ``` ### Constant type declaration You can declare a constant. ``` Person::DefaultEmailAddress: String ``` ### Global type declaration You can declare a global variable. ``` $LOAD_PATH: Array[String] ``` ### Comments You can write single line comments. Comments must be on their own line. Comments can lead with whitespace. ``` # This if interface Foo # Usage of Foo is bar interface _Foo # New foo is a method # it will return foo. def new: () -> Foo end ```