Tya v0.6 Specification

This document is the specification for Tya v0.6 after v0.5 minimal classes and objects.

Theme

Tya v0.6 is about class-level state and behavior.

v0.5 adds instance fields and instance methods. v0.6 extends the same class model with class variables and class methods, using the @@field syntax that v0.5 reserved for future class-level members.

Goals

Add class variables.
Add class methods.
Add instance field defaults in class bodies.
Keep @field and @@field visually distinct.
Keep class-level members explicit and easy to compile to C.
Leave inheritance and richer object-oriented features for later versions.

Included in v0.6

v0.6 includes all v0.5 class behavior and adds:

@@field class variable syntax
instance field defaults declared with field = value
class variables declared in class bodies
class variable read/write inside instance methods
class variable read/write inside class methods
public class variable read/write with ClassName.field
class methods declared with @@method = args ->
public class method calls with ClassName.method(args...)
class methods as module class members with module_name.ClassName.method(...)

Not Included in v0.6

v0.6 does not include:

inheritance
super
interfaces
visibility modifiers
private fields
private class variables
private class methods
method overloading
operator overloading
decorators
metaclasses
dictionary member access with dict.key
package manager
native-backed stdlib

Instance and Class Fields

@field is an instance field.

class User
  init = name ->
    @name = name

@@field is a class variable.

class User
  @@count = 0

The distinction is lexical:

@name belongs to the current object.
@@count belongs to the current class.

Class Variables

Class variables are declared directly in the class body with @@field = value.

class User
  @@count = 0

  init = name ->
    @name = name
    @@count = @@count + 1

The initializer expression is evaluated once when the class is defined.

Class variables are shared by all instances of the class.

User("komagata")
User("tya")

print User.count

Class variables are public in v0.6. They can be read or assigned through the class name.

User.count = 0
print User.count

Reading a missing class variable is an error.

Instance Field Defaults

Instance field defaults are declared directly in the class body with field = value.

class Counter
  value = 0

  increment = ->
    @value = @value + 1

The default value is copied into each new instance before init runs.

counter = Counter()
print counter.value

init may overwrite a default field.

class User
  name = ""
  active = true

  init = name ->
    @name = name

Field defaults define instance fields, not class variables. Use @@field for class-level state.

class User
  name = ""   # instance field default
  @@count = 0 # class variable

Field default names use snake_case. A field default and an instance method may not share the same name in the same class.

Class Methods

Class methods are declared in the class body with @@method = args ->.

class User
  @@count = 0

  @@build = name ->
    User(name)

  @@count_users = ->
    @@count

Call class methods through the class name.

user = User.build("komagata")
print User.count_users()

Class methods do not have an instance receiver. @field is invalid inside a class method. Use @@field for class-level state.

class User
  @@count = 0

  @@reset = ->
    @@count = 0

Reading a class method as a first-class value without calling it is not part of v0.6.

Instance Methods and Class Variables

Instance methods may read and write class variables with @@field.

class User
  @@count = 0

  init = name ->
    @name = name
    @@count = @@count + 1

  count = ->
    @@count

@field and @@field may appear in the same instance method.

class User
  @@prefix = "user"

  init = name ->
    @name = name

  label = ->
    "{@@prefix}:{@name}"

Module Classes

A class declared inside a module exposes class variables and class methods through the module namespace.

# user.tya
module user
  class User
    @@count = 0

    init = name ->
      @name = name
      @@count = @@count + 1

    @@count_users = ->
      @@count

Use the class-level members through the module class.

import user

user.User("komagata")
print user.User.count
print user.User.count_users()

v0.6 does not import module classes directly into the local namespace.

Member Namespaces

A class has two member namespaces:

instance members, used through objects
class members, used through the class name

An instance member and a class member may use the same name because they are called through different receivers.

class User
  init = name ->
    @name = name

  name = ->
    @name

  @@name = ->
    "User"

user = User("komagata")
print user.name()
print User.name()

Within the class member namespace, a class variable and a class method may not share the same name.

class User
  @@name = "User"
  @@name = ->
    "User"

The second @@name is an error.

Within the instance member namespace, a field default and an instance method may not share the same name.

class User
  name = ""
  name = ->
    @name

The second name is an error.

Construction and `init`

v0.6 keeps the v0.5 construction rules.

class User
  init = name ->
    @name = name

user = User("komagata")

init is still an instance initializer. v0.6 does not add class constructors or factory constructors. Use a class method when factory-like construction is needed.

class User
  @@build = name ->
    User(name)

Dot Access Boundary

Dot access has these specified meanings in v0.6:

module member access: module_name.member
object field access: object.field
object method calls: object.method(args...)
class variable access: ClassName.field
class method calls: ClassName.method(args...)

Dictionaries continue to use bracket access.

profile = {"name": "komagata"}
print profile["name"]

Dictionary member access with profile.name is not part of v0.6.

Naming

Class names use PascalCase.

class User
class HttpClient
class CsvRow

Variables, functions, methods, fields, class variables, modules, files, and dictionary keys keep using snake_case.

Diagnostics

v0.6 implementations should report source-oriented errors for:

non-PascalCase class names
@field outside an instance method
@field inside a class method
@@field outside a class body, instance method, or class method
duplicate class members in the same class
duplicate instance members in the same class
constructor arity mismatch
missing object fields
missing object methods
missing class variables
missing class methods
dictionary member access with dot syntax