Classes and Interfaces

Classes and interfaces are Verse's object-oriented building blocks that enable rich type hierarchies with inheritance, polymorphism, and interface-based contracts. Classes provide object-oriented programming with fields, methods, and single inheritance, enabling you to model complex hierarchies of game entities with shared behavior and specialized implementations. Interfaces define contracts that classes must fulfill, promoting loose coupling and enabling multiple inheritance of behavior specifications.

Together, classes and interfaces form a powerful system for modeling game entities, components, and systems with both is-a relationships (through class inheritance) and can-do contracts (through interface implementation).

Let's explore classes first, then delve into interfaces and how they complement each other.

Classes

Classes form the backbone of object-oriented programming in Verse. A class serves as a blueprint for creating objects that share common properties and behaviors. When you define a class, you're creating a new type that bundles data (fields) with operations on that data (methods), encapsulating related functionality into a cohesive unit.

Class definitions occur at module scope. You cannot define a class inside another class, struct, interface, or function. Classes are top-level type definitions that establish the type system's structure:

# Valid: class at module scope
MyModule := module:
    entity := class:
        ID:int

# Invalid: class inside another class
# outer := class:
#     inner := class:  # ERROR: classes must be at module scope
#         Value:int

The simplest form of a class groups related data together. Consider modeling a character in your game:

character := class:
    Name : string
    var Health : int = 100
    var Level : int = 1
    MaxHealth : int = 100

This class definition establishes several important concepts. Fields without the var modifier are immutable after construction—once you create a character with a specific name, that name cannot change. Fields marked with var are mutable and can be modified after the object is created (see Mutability for details on var and set). Default values provide sensible starting points, making object construction more convenient while ensuring objects start in valid states.

Object Construction

Creating instances of a class involves specifying values for its fields through an archetype expression:

<!--versetest character := class: Name : string var Health : int = 100 var Level : int = 1 MaxHealth : int = 100

Ignore:int=1 -->

Hero := character{Name := "Aldric", Health := 100, Level := 5}
Villager := character{Name := "Martha"}  # default values for unspecified fields

The archetype syntax uses named parameters, making the construction explicit and self-documenting. Any field with a default value can be omitted from the archetype, and the default will be used. Fields without defaults must be specified, ensuring objects are always fully initialized. Fields can be passed to an archetype in any order.

Methods

Classes become truly powerful when you add methods that operate on the class's data:

character := class:
    Name : string
    var Health : int = 100
    var Level : int = 1
    var MaxHealth : int = 100

    TakeDamage(Amount : int) : void =
        set Health = Max(0, Health - Amount)

    Heal(Amount : int) : void =
        set Health = Min(MaxHealth, Health + Amount)

    IsAlive()<decides>:void= Health > 0

    LevelUp() : void =
        set Level += 1
        set MaxHealth = 100 + (Level * 10)
        set Health = MaxHealth  # Full heal on level up

Methods have access to all fields of the class and can modify mutable fields. They encapsulate the logic for how objects of the class should behave, ensuring that state changes happen in controlled, predictable ways.

All methods in non-abstract classes must have implementations. Unlike interfaces (which can declare abstract methods), a concrete class method declaration without an implementation is an error:

# Valid: method with implementation
valid_class := class:
    Compute():int = 42

# Invalid: method without implementation in concrete class
# invalid_class := class:
#     Compute():int  # ERROR: needs implementation

Blocks for Initialization

Classes can include block clauses in their body, which execute when an instance is created. These blocks run initialization code that goes beyond simple field assignment, allowing you to perform setup logic, validation, or side effects during construction:

<!--versetest GetCurrentTime()<computes>:float=0.0

logged_entity := class: ID:int var CreationTime:float = 0.0

block:
    # This executes when an instance is created
    Print("Creating entity with ID: {ID}")
    set CreationTime = GetCurrentTime()

M()<transacts>:void = Entity := logged_entity{ID := 42} # Prints: "Creating entity with ID: 42" <# -->

logged_entity := class:
    ID:int
    var CreationTime:float = 0.0

    block:
        # This executes when an instance is created
        Print("Creating entity with ID: {ID}")
        set CreationTime = GetCurrentTime()

# Entity := logged_entity{ID := 42}
# Prints: "Creating entity with ID: 42"

Block clauses have access to all fields of the class, including Self, and can modify mutable fields. They execute in the order they appear in the class definition:

multi_step_init := class:
    var Step1:int = 0
    var Step2:int = 0

    block:
        set Step1 = 10

    var Step3:int = 0

    block:
        set Step2 = Step1 + 5  # Can access earlier fields
        set Step3 = Step2 * 2

# Instance := multi_step_init{}
# Instance.Step1 = 10, Step2 = 15, Step3 = 30

Execution order with inheritance: When a class inherits from another class, the Verse VM executes blocks in subclass-before-superclass order, while the BP VM uses superclass-before-subclass order. For portable code, avoid depending on the execution order of blocks across inheritance hierarchies.

Why blocks instead of constructors? Block clauses have access to Self and all fields of the class, while constructor functions do not have access to Self. This makes blocks the natural place for initialization logic that needs to reference the object being constructed — such as registering Self with a global system or computing derived values from multiple fields.

Additionally, field default values cannot use divergent calls — calls that might not complete. This means you cannot write:

# ERROR V3582: Divergent calls cannot be used to define data-members
bar := class:
    Foo:foo = MakeFoo()

Instead, you give the field a simple default and move the initialization logic into a block:

bar := class:
    var Foo:foo = foo{}

    block:
        set Foo = MakeFoo()  # Block can call divergent functions

Constraints on block clauses:

Blocks cannot contain failure (<decides>) operations
Blocks cannot call suspending (<suspends>) functions
Blocks can use defer statements, which execute when the block exits
Block clauses are only allowed in classes, not in interfaces, structs, or modules

Block clauses are particularly useful for:

Logging object creation
Computing derived values during initialization
Registering objects with global systems
Performing initialization that requires Self or divergent calls

Let Clauses in Archetypes

Archetype expressions (used to construct class and struct instances) can include let clauses that introduce local variable bindings. These are useful for computing intermediate values used by multiple field initializers, avoiding repetition:

MkWord8<constructor>(I:int)<decides><transacts> := Word8:
    let:
        MaxU8:int = Int[Pow(2.0, 8.0)] - 1 or Impossible("MkWord8")
    B := 0 <= I and I <= MaxU8

The let clause introduces bindings (MaxU8 in the example above) that are visible to subsequent field initializers in the same archetype. Unlike block clauses, let clauses are restricted to variable declarations only — standalone expressions are not permitted inside let.

Self

Within class methods, Self is a special keyword that refers to the current instance of the class. Each method invocation has its own Self that refers to the specific object the method was called on.

You can use Self in multiple ways within method bodies:

access fields of the instance
calling methods of the instance
pass the instance to other functions
return the instance

character := class:
    var Name : string
    var Config:[string]string = map{}
	
    Announce() : void =
        # Using Self to pass the whole object
        LogCharacterAction(Self, "announced")


    SetOption(Key:string, Value:string):character =
        set Config[Key] = Value
        Self  # Return this instance for method chaining


    SetName(NewName:string):void =
       set Self.Name = NewName	  # Set the name of this instance
	   Self.Announce()            # Call a method of this instance

You can capture Self when creating nested objects:

container := class:
    ID:int

    CreateChild():child_with_parent =
        child_with_parent{Parent := Self}  # Capture this instance

child_with_parent := class:
    Parent:container

# C := container{ID := 42}
# Child := C.CreateChild()
# Child.Parent.ID = 42  # Child stores reference to C

Inheritance

Classes support single inheritance, allowing you to create specialized versions of existing classes. This creates an "is-a" relationship where the subclass is a more specific type of the superclass:

<!--versetest vector3:=struct{}

entity := class: var Position : vector3 = vector3{} var IsActive : logic = true

Activate() : void = set IsActive = true
Deactivate() : void = set IsActive = false

character := class(entity): # character inherits from entity Name : string var Health : int = 100

TakeDamage(Amount : int) : void =
    set Health = Max(0, Health - Amount)
    if (Health = 0):
        Deactivate()  # Can call inherited methods

player := class(character): # player inherits from character var Score : int = 0 var Lives : int = 3

AddScore(Points : int) : void =
    set Score += Points

<# -->

entity := class:
    var Position : vector3 = vector3{}
    var IsActive : logic = true

    Activate() : void = set IsActive = true
    Deactivate() : void = set IsActive = false

character := class(entity):  # character inherits from entity
    Name : string
    var Health : int = 100

    TakeDamage(Amount : int) : void =
        set Health = Max(0, Health - Amount)
        if (Health = 0):
            Deactivate()  # Can call inherited methods

player := class(character):  # player inherits from character
    var Score : int = 0
    var Lives : int = 3

    AddScore(Points : int) : void =
        set Score += Points

Inheritance creates a type hierarchy where a player is also a character, and a character is also an entity. This means you can use a player object anywhere a character or entity is expected, enabling polymorphic behavior.

Important constraints on inheritance:

Single class inheritance only: A class can inherit from at most one other class, though it can implement multiple interfaces. Multiple class inheritance is not supported:

base1 := class:
    Value1:int

base2 := class:
    Value2:int

# Valid: inherit from one class and multiple interfaces
interface1 := interface:
    Method1():void

interface2 := interface:
    Method2():void

derived := class<abstract>(base1, interface1, interface2):
    # Valid: one class, multiple interfaces
    Method1<override>():void = {}
    Method2<override>():void = {}

# Invalid: cannot inherit from multiple classes
# invalid := class(base1, base2):  # ERROR

No shadowing of data members: Subclasses cannot declare fields with the same name as fields in their superclass. This prevents ambiguity and ensures clear data ownership:

base := class:
    Value:int

# Invalid: cannot shadow parent's field
# derived := class(base):
#     Value:int  # ERROR: shadowing base.Value

No method signature changes: When overriding a method, you must use the exact same signature. Changing parameter types or return types creates a shadowing error:

base := class:
    Compute():int = 42

# Invalid: different return type
# derived := class(base):
#     Compute():float = 3.14  # ERROR: signature doesn't match

To override a method, use the <override> specifier with the matching signature.

Super

Within a subclass, you can use the super keyword to refer to the superclass type. This is primarily used to access the superclass's implementation or to construct a superclass instance:

entity := class:
    ID:int
    Name:string

    Display():void =
        Print("Entity {ID}: {Name}")

character := class(entity):
    Health:int

    Display<override>():void =
        # Create a superclass instance to call its method
        super{ID := ID, Name := Name}.Display()
        Print("Health: {Health}")

The super keyword represents the superclass type itself. When you write super{...}, you're creating an instance of the superclass with the specified field values. This allows you to delegate to superclass behavior while adding subclass-specific functionality.

Within an overriding method, you can call the parent class's implementation using the (super:) syntax. This is the primary way to invoke parent method implementations while adding or modifying behavior:

base := class:
    Method():void =
        Print("Base implementation")

derived := class(base):
    Method<override>():void =
        # Call parent implementation first
        (super:)Method()
        Print("Derived implementation")

# Creates instance and calls Method()
# derived{}.Method()
# Output:
# Base implementation
# Derived implementation

The (super:) syntax explicitly calls the parent class's version of the current method. This is cleaner and more efficient than constructing a parent instance with super{...} when you only need to call parent methods.

Basic Usage:

<!--versetest ToString(:vector3)<computes>:string="" vector3:=class<final>{ X:float=0.0; Y:float=0.0; Z:float=0.0 }

entity := class: Position:vector3

Move(Delta:vector3):void =
    Print("Entity moving by {Delta}")
    # Update position logic here

character := class(entity): var Stamina:float = 100.0

Move<override>(Delta:vector3):void =
    # Call parent movement logic
    (super:)Move(Delta)
    # Add character-specific behavior
    set Stamina -= 1.0

<# -->

entity := class:
    Position:vector3

    Move(Delta:vector3):void =
        Print("Entity moving by {Delta}")
        # Update position logic here

character := class(entity):
    var Stamina:float = 100.0

    Move<override>(Delta:vector3):void =
        # Call parent movement logic
        (super:)Move(Delta)
        # Add character-specific behavior
        set Stamina -= 1.0

With Effect Specifiers:

The (super:) syntax works seamlessly with all effect specifiers:

<!--versetest async_base := class: Process()<suspends>:void = Sleep(1.0) Print("Base processing")

async_derived := class(async_base): Process<override>()<suspends>:void = # Parent method suspends, so this suspends too (super:)Process() Print("Derived processing")

transactional_base := class: var Value:int = 0

Update()<transacts>:void =
    set Value += 1

transactional_derived := class(transactional_base): var Counter:int = 0

Update<override>()<transacts>:void =
    (super:)Update()
    set Counter += 1

<# -->

async_base := class:
    Process()<suspends>:void =
        Sleep(1.0)
        Print("Base processing")

async_derived := class(async_base):
    Process<override>()<suspends>:void =
        # Parent method suspends, so this suspends too
        (super:)Process()
        Print("Derived processing")

transactional_base := class:
    var Value:int = 0

    Update()<transacts>:void =
        set Value += 1

transactional_derived := class(transactional_base):
    var Counter:int = 0

    Update<override>()<transacts>:void =
        (super:)Update()
        set Counter += 1

Virtual Dispatch Through Parent Methods:

When parent methods call other methods, virtual dispatch still applies based on the actual object type. This means Self binds to the derived instance even when calling through (super:):

base := class:
    # Virtual method that can be overridden
    GetValue()<computes>:int = 10

    # Parent method that uses GetValue
    ComputeDouble()<computes>:int =
        2 * GetValue()  # Calls derived GetValue if overridden

derived := class(base):
    # Override GetValue to return different value
    GetValue<override>()<computes>:int = 20

    # Override ComputeDouble to call parent, but GetValue dispatch is virtual
    ComputeDouble<override>()<computes>:int =
        # Calls base.ComputeDouble, which calls derived.GetValue!
        (super:)ComputeDouble()

# derived{}.ComputeDouble()  # Returns 40, not 20

In this example, even though ComputeDouble calls the parent implementation, the GetValue() call inside the parent uses virtual dispatch and calls the derived version.

With Overloaded Methods:

The (super:) syntax works with overloaded methods, calling the parent's version of the same overload:

base := class:
    Process(X:int):void =
        Print("Base int: {X}")

    Process(S:string):void =
        Print("Base string: {S}")

derived := class(base):
    Process<override>(X:int):void =
        (super:)Process(X)  # Calls parent's int overload
        Print("Derived int: {X}")

    Process<override>(S:string):void =
        (super:)Process(S)  # Calls parent's string overload
        Print("Derived string: {S}")

Return Type Covariance:

When overriding methods with (super:), the return type can be a subtype of the parent's return type (covariant return types):

base_type := class:
    Name:string

derived_type := class(base_type):
    Value:int

base := class:
    Create():base_type =
        base_type{Name := "base"}

derived := class(base):
    # Override with more specific return type
    Create<override>():derived_type =
        # Can still call parent even with different return type
        Parent := (super:)Create()
        derived_type{Name := Parent.Name, Value := 42}

Method Overriding

Subclasses can override methods defined in their superclasses to provide specialized behavior:

entity := class:
    OnUpdate<public>() : void = {}  # Default no-op implementation

enemy := class(entity):
    var Target : ?character = false

    OnUpdate<override>()<transacts> : void =
        if (Target?.IsAlive[]):
            MoveToward(Target)
        else:
            Patrol()

turret := class(entity):
    var Rotation:int= 0

    OnUpdate<override>()<transacts>: void =
        if (V:= Mod[Rotation, 360]):
            set Rotation = V
        ScanForTargets()

The override mechanism ensures that the correct method implementation is called based on the actual type of the object, not the type of the variable holding it. This is the foundation of polymorphic behavior in object-oriented programming.

Constructor Functions

Classes don't have traditional constructor methods like you might find in other object-oriented languages. Instead, Verse provides three approaches to object construction, each suited to different needs:

Archetype expressions — direct field initialization for simple cases. Straightforward and requires no extra definitions.
Block clauses — initialization code in the class body that runs on every construction. Has access to Self and all fields, making it ideal for registering the object, computing derived values, or calling divergent functions that can't appear in field defaults.
Constructor functions — annotated with <constructor>, these are first-class functions that can validate inputs, delegate to other constructors (including parent class constructors), be overloaded, and be passed around as values. They are the most powerful option and essential for inheritance hierarchies where subclass constructors need to initialize superclass fields.

These approaches compose: a constructor function returns an archetype expression, which can contain let and block clauses, and the class body can also have its own block clauses that execute regardless of which constructor was used.

For simple cases where you just need to set field values, use archetype expressions directly:

player := class:
    Name:string
    var Health:int = 100
    Level:int = 1

# Direct construction with archetype
# Hero := player{Name := "Aldric", Health := 150, Level := 5}

When you need validation, computation, or complex initialization logic, use constructor functions annotated with <constructor>:

<!--versetest player := class: Name:string var Health:int = 100 Level:int = 1

MaxLevel:int = 99 -->

MakePlayer<constructor>(InName:string, InLevel:int)<transacts> := player:
    Name := InName
    Level := InLevel
    Health := InLevel * 100

Here's an example of calling this constructor:

Hero := MakePlayer("Aldric", 5) # Call constructor function

Constructor functions are regular functions that return class instances, but the <constructor> annotation enables special capabilities like delegating to other constructors. When calling a constructor function from normal code, use just the function name—the <constructor> annotation only appears in the definition.

Constructor functions can have effects that control their behavior. Common effects include <computes>, <allocates>, and <transacts>. A particularly useful effect is <decides>, which allows constructors to fail if preconditions aren't met:

<!--versetest player := class: Name:string var Health:int = 100 Level:int = 1

MaxLevel:int = 99 -->

MakeValidPlayer<constructor>(InName:string, InLevel:int)<transacts><decides> := 
    player:
         Name := InName
         Level := block:
                 InLevel > 0
                 InLevel <= MaxLevel
                 InLevel
         Health := InLevel * 100

Here's an example using the validated constructor with failure handling:

# Constructor can fail - use with failure syntax
if (Player := MakeValidPlayer["Hero", 5]):
    # Construction succeeded
    AddPlayer(Player)
else:
    # Construction failed - level out of range

Constructor functions cannot use the <suspends> effect. Construction must complete synchronously to maintain object consistency.

Overloading Constructors

You can provide multiple constructor functions with different parameter signatures, allowing flexible object creation:

entity := class:
    Name:string
    var Health:int = 100
    Position:vector3

# Constructor with all parameters
MakeEntity<constructor>(Name:string, Health:int, Position:vector3) := entity:
    Name := Name
    Health := Health
    Position := Position

# Constructor with defaults
MakeEntity<constructor>(Name:string, Position:vector3) := entity:
    Name := Name
    Health := 100
    Position := Position

# Constructor for origin placement
MakeEntity<constructor>(Name:string) := entity:
    Name := Name
    Health := 100
    Position := vector3{X := 0.0, Y := 0.0, Z := 0.0}

# Each overload can be called based on arguments
# Enemy1 := MakeEntity("Goblin", 50, SpawnPoint)
# Enemy2 := MakeEntity("Guard", PatrolPoint)
# NPC := MakeEntity("Shopkeeper")

Delegating Constructors

Constructor functions can delegate to other constructors, enabling code reuse and constructor chaining. This is particularly important for inheritance hierarchies where subclass constructors need to initialize superclass fields.

When delegating to a parent class constructor from a subclass, you must initialize the subclass fields first, then call the parent constructor using the qualified <constructor> syntax within the archetype:

<!--versetest entity := class: Name:string var Health:int

character := class(entity): Class:string Level:int

MakeEntity<constructor>(Name:string, Health:int) := entity: Name := Name Health := Health

Subclass constructor delegates to parent constructor

MakeCharacter<constructor>(Name:string, Class:string, Level:int) := character: # Initialize subclass fields first Class := Class Level := Level # Then delegate to parent constructor MakeEntity<constructor>(Name, Level * 100) <# -->

entity := class:
    Name:string
    var Health:int

MakeEntity<constructor>(Name:string, Health:int) := entity:
    Name := Name
    Health := Health

character := class(entity):
    Class:string
    Level:int

# Subclass constructor delegates to parent constructor
MakeCharacter<constructor>(Name:string, Class:string, Level:int) := character:
    # Initialize subclass fields first
    Class := Class
    Level := Level
    # Then delegate to parent constructor
    MakeEntity<constructor>(Name, Level * 100)

Hero := MakeCharacter("Aldric", "Warrior", 5)

Constructor functions can also forward to other constructors of the same class:

<!--versetest player := class: Name:string var Score:int

Primary constructor

MakePlayer<constructor>(Name:string, Score:int) := player: Name := Name Score := Score

Convenience constructor forwards to primary

MakeNewPlayer<constructor>(Name:string) := player: # Delegate to another constructor of the same class MakePlayer<constructor>(Name, 0) <# -->

player := class:
    Name:string
    var Score:int

# Primary constructor
MakePlayer<constructor>(Name:string, Score:int) := player:
    Name := Name
    Score := Score

# Convenience constructor forwards to primary
MakeNewPlayer<constructor>(Name:string) := player:
    # Delegate to another constructor of the same class
    MakePlayer<constructor>(Name, 0)

Here's an example of calling the constructor:

<!--versetest player := class: Name:string var Score:int

Primary constructor

MakePlayer<constructor>(Name:string, Score:int) := player: Name := Name Score := Score

Convenience constructor forwards to primary

MakeNewPlayer<constructor>(Name:string) := player: # Delegate to another constructor of the same class MakePlayer<constructor>(Name, 0) -->

NewPlayer := MakeNewPlayer("Alice")

When delegating to a constructor of the same class, the delegation replaces all field initialization—any fields you initialize before the delegation are ignored. When delegating to a parent class constructor, your subclass field initializations are preserved, and the parent constructor initializes the parent fields.

Order of Execution

Understanding execution order is crucial for correct initialization:

Archetype expression: Field initializers execute in the order they're written in the archetype
Delegating constructor: Subclass fields are initialized first, then the parent constructor runs
Class body blocks: When using direct archetype construction, blocks in the class definition execute before field initialization

For delegating constructors to parent classes:

<!--versetest base := class: BaseValue:int

derived := class(base): DerivedValue:int

MakeBase<constructor>(Value:int) := base: block: Print("Base constructor") BaseValue := Value

MakeDerived<constructor>(Base:int, Derived:int) := derived: # This executes first DerivedValue := Derived # Then parent constructor executes MakeBase<constructor>(Base) <# -->

base := class:
    BaseValue:int

MakeBase<constructor>(Value:int) := base:
    block:
        Print("Base constructor")
    BaseValue := Value

derived := class(base):
    DerivedValue:int

MakeDerived<constructor>(Base:int, Derived:int) := derived:
    # This executes first
    DerivedValue := Derived
    # Then parent constructor executes
    MakeBase<constructor>(Base)

Here's an example showing execution order:

<!--versetest base := class: BaseValue:int

MakeBase<constructor>(Value:int) := base: block: Print("Base constructor") BaseValue := Value

derived := class(base): DerivedValue:int

MakeDerived<constructor>(Base:int, Derived:int) := derived: # This executes first DerivedValue := Derived # Then parent constructor executes MakeBase<constructor>(Base) -->

# Prints: "Base constructor"
# Results in: derived{BaseValue := 10, DerivedValue := 20}
Instance := MakeDerived(10, 20)

For classes with mutable fields, initialization sets starting values that can change during the object's lifetime. Immutable fields must be initialized during construction and cannot be modified afterward. This distinction makes the construction phase critical for establishing invariants that will hold throughout the object's existence.

Shadowing and Qualification

Verse has strict rules about name shadowing to prevent ambiguity and maintain code clarity. Understanding these rules and the qualification syntax is essential for working with inheritance hierarchies, multiple interfaces, and nested modules.

In most contexts, you cannot redefine names that already exist in an enclosing scope. This applies to functions, variables, classes, interfaces, and modules:

# ERROR: Function at module level shadows class method
# F(X:int):int = X + 1
# c := class:
#     F(X:int):int = X + 2  # ERROR - shadows outer F

This prohibition extends across various contexts:

# ERROR: Cannot shadow classes
something := class {}

M := module:
    something := class {}  # ERROR

# ERROR: Cannot shadow variables
Value:int = 1

M := module:
     Value:int = 2        # ERROR

# ERROR: Cannot shadow data members
c := class { A:int }

A():void = {}             # ERROR - order doesn't matter

# ERROR: Module and function cannot share name

Id():void = {}
Id := module {}           # ERROR

The shadowing prohibition exists regardless of definition order - it doesn't matter whether the outer name is defined before or after the inner scope.

To define methods with the same name in different contexts, use qualified names with the syntax (ClassName:)MethodName:

# Class with qualified method of same name
c := class:
   (c:)F(X:int):int = X + 2

# Module-level function
F(X:int):int = X + 1

# Call the module-level function
F(10)  # Returns 11

# Call the class method
c{}.F(10)  # Returns 12

# Explicit qualification (optional here)
c{}.(c:)F(10)  # Returns 12

The (c:) qualifier indicates this F is defined specifically in the c class context, distinguishing it from the module-level F. This allows the same name to coexist without shadowing errors.

Methods with Same Name

Using qualifiers, you can define new methods with the same name as inherited methods, creating multiple distinct methods in the same class:

c := class<abstract> { F(X:int):int }

d := class(c):
    F<override>(X:int):int = X + 1

e := class(d):
    (e:)F(X:int):int = X + 2 # NEW method with same name, not an override

# e now contains BOTH methods:
#    - (d:)F inherited from d
#    - (e:)F newly defined in e

Using the above:

E := e{}
E.(c:)F(10)  # Returns 11 (inherited from d's override)
E.(e:)F(10)  # Returns 12 (new method in e)

Key distinction:

F<override> without qualifier: Overrides the inherited F
(e:)F without <override>: Defines a new F specific to e

This allows a class to have multiple methods with the same name, differentiated by their qualifiers, each serving different purposes in the class hierarchy.

`(super:)` Qualified

The (super:) qualifier works with qualified method names to call the parent class's implementation:

i := interface { F(X:int):int }

ci := class(i):
    (i:)F<override>(X:int):int = X + 1
    (ci:)F(X:int):int = X + 2

dci := class(ci):
    # Override both inherited methods, calling super implementations
    (i:)F<override>(X:int):int = 100 + (super:)F(X)
    (ci:)F<override>(X:int):int = 200 + (super:)F(X)

And a use case:

<!--versetest i := interface { F(X:int):int }

ci := class(i): (i:)F<override>(X:int):int = X + 1 (ci:)F(X:int):int = X + 2

dci := class(ci): # Override both inherited methods, calling super implementations (i:)F<override>(X:int):int = 100 + (super:)F(X) (ci:)F<override>(X:int):int = 200 + (super:)F(X) -->

DCI := dci{}
DCI.(i:)F(10)  # Returns 111 (100 + ci's 11)
DCI.(ci:)F(10)  # Returns 212 (200 + ci's 12)

(super:)F(X) within the qualified method calls the parent class's implementation of that same qualified method. This enables you to extend behavior for multiple method variants independently.

Interface Collisions

When implementing multiple interfaces with methods of the same name, qualifiers disambiguate which interface's method you're implementing:

i := interface:
    B(X:int):int

j := interface:
    B(X:int):int

collision := class(i, j):
    # Implement both B methods separately
    (i:)B<override>(X:int):int = 20 + X
    (j:)B<override>(X:int):int = 30 + X

And a use case:

Obj := collision{}
Obj.(i:)B(1)  # Returns 21
Obj.(j:)B(1)  # Returns 31

Without qualifiers, the compiler cannot determine which interface's method you're implementing, resulting in an error. The qualification makes your intent explicit.

Complex interface hierarchies:

i := interface:
    C(X:int):int

j := interface(i):
    A(X:int):int

k := interface(i):
    B(X:int):int
    (k:)C(X:int):int  # k redefines C

multi := class(j, k):
    A<override>(X:int):int = 10 + X
    B<override>(X:int):int = 20 + X
    # Must implement C from both inheritance paths
    (i:)C<override>(X:int):int = 30 + X
    (k:)C<override>(X:int):int = 40 + X

A use case:

<!--versetest i := interface: C(X:int):int

j := interface(i): A(X:int):int

k := interface(i): B(X:int):int (k:)C(X:int):int # k redefines C

multi := class(j, k): A<override>(X:int):int = 10 + X B<override>(X:int):int = 20 + X # Must implement C from both inheritance paths (i:)C<override>(X:int):int = 30 + X (k:)C<override>(X:int):int = 40 + X -->

Obj := multi{}
Obj.(i:)C(1)  # Returns 31
Obj.(k:)C(1)  # Returns 41

When an interface redefines a method from a parent interface using qualification (k:)C, implementing classes must provide separate implementations for both variants.

Nested Module Qualification

Modules can be nested, and deeply qualified names reference members through the entire hierarchy:

Top := module:
    (Top:)M<public> := module:
        (Top.M:)Value<public>:int = 1
        (Top.M:)F<public>(X:int):int = X + 10

        (Top.M:)M<public> := module:
            (Top.M.M:)Value<public>:int = 3
            (Top.M.M:)F<public>(X:int):int = X + 100

And a use case:

<!--versetest Top := module: (Top:)M<public> := module: (Top.M:)Value<public>:int = 1 (Top.M:)F<public>(X:int):int = X + 10

    (Top.M:)M<public> := module:
        (Top.M.M:)Value<public>:int = 3
        (Top.M.M:)F<public>(X:int):int = X + 100

using { Top.M } using { Top.M.M }

-->

# using { Top.M }
# using { Top.M.M }

# Access with full qualification
(Top.M:)F(0)          # Returns 10
(Top.M.M:)F(0)        # Returns 100

# Access via path
Top.M.F(1)            # Returns 11
Top.M.M.F(1)          # Returns 101

Nested modules can have the same simple name (e.g., both M) when qualified with their full path, allowing hierarchical organization without naming conflicts.

Restrictions

Qualifiers can only be used in appropriate contexts. You cannot use class qualifiers for local variables:

C := class:
    f():void =
        (C:)X:int = 0  # ERROR - wrong context

Certain qualifiers are not supported. Function qualifiers for local variables are not allowed:

C := class:
    f():void =
        (C.f:)X:int = 0  # ERROR - unsupported pattern

Similarly, using module function paths as qualifiers is not supported:

M := module:
    f():void =
        (M.f:)X:int = 0  # ERROR

Local variables cannot shadow class members:

A := class:
    I:int
    F(X:int):void =
        I:int = 5  # ERROR - shadows member I

Currently, there is no (local:) qualifier to disambiguate, so this pattern is not supported. You must use different names for local variables and members.

Parametric Classes

Parametric classes, also known as generic classes, allow you to define classes that work with any type. Rather than writing separate container classes for integers, strings, players, and every other type, you write one parametric class that accepts a type parameter.

A parametric class takes one or more type parameters in its definition:

<!--versetest

Simple container that holds a single value

container(t:type) := class: Value:t <# -->

# Simple container that holds a single value
container(t:type) := class:
    Value:t

Here are examples of instantiating this parametric class with different types:

<!--versetest container(t:type) := class: Value:t

player := class: Name:string var Health:int = 100 -->

# Can be instantiated with any type
IntContainer := container(int){Value := 42}
StringContainer := container(string){Value := "hello"}
PlayerContainer := container(player){Value := player{Name := "Hero", Health := 100}}

The syntax container(t:type) defines a class that is parameterized by type t. Within the class definition, t can be used anywhere a concrete type would appear—in field declarations, method signatures, or return types.

Multiple type parameters:

Classes can accept multiple type parameters:

pair(t:type, u:type) := class:
    First:t
    Second:u

Here are examples of using the parametric pair class:

# Different types for each parameter
Coordinate := pair(int, int){First := 10, Second := 20}
NamedValue := pair(string, float){First := "score", Second := 99.5}

Type parameters in methods:

Type parameters are available throughout the class, including in methods:

<!--versetest optional_container(t:type) := class: var MaybeValue:?t = false

Set(Value:t):void =
    set MaybeValue = option{Value}

Get()<decides>:t =
    MaybeValue?

Clear():void =
    set MaybeValue = false

<# -->

optional_container(t:type) := class:
    var MaybeValue:?t = false

    Set(Value:t):void =
        set MaybeValue = option{Value}

    Get()<decides>:t =
        MaybeValue?

    Clear():void =
        set MaybeValue = false

Methods automatically know about the type parameter from the class definition—you don't redeclare it in method signatures.

Instantiation and Identity

When you instantiate a parametric class with specific type arguments, Verse creates a concrete type. Critically, multiple instantiations with the same type arguments produce the same type:

<!--versetest container(t:type) := class: Value:t

These are the same type

Type1 := container(int) Type2 := container(int) Type3 := container(int)

All three are equal - they're the same type

<# -->

container(t:type) := class:
    Value:t

# These are the same type
Type1 := container(int)
Type2 := container(int)
Type3 := container(int)

# All three are equal - they're the same type

This type identity is guaranteed across the program:

# Create instances
C1 := container(int){Value := 1}
C2 := container(int){Value := 2}

# Both have the same type: container(int)
# Type checking treats them identically

The instantiation process is deterministic and memoized. The first time you write container(int), Verse generates a concrete type. Every subsequent use of container(int) refers to that same type, not a new copy.

This matters for:

Type compatibility: Two values of container(int) can be used interchangeably
Memory efficiency: Not creating duplicate type definitions
Semantic correctness: Same type arguments always mean the same type

While the same type arguments always produce the same type, different type arguments produce distinct, incompatible types:

container(t:type) := class:
    Value:t

Here's an example showing that different instantiations create distinct types:

IntContainer := container(int){Value := 42}
StringContainer := container(string){Value := "text"}

# These are different types and cannot be mixed
# IntContainer = StringContainer  # Type error!

container(int) and container(string) are completely different types, with no subtype relationship. They happen to share the same structure (both defined from container), but that doesn't make them compatible.

While different instantiations of a parametric class are distinct types, Verse allows certain instantiations to be used in place of others based on variance. Variance determines when parametric_class(subtype) can be used where parametric_class(supertype) is expected (or vice versa).

The variance of a parametric type depends on how the type parameter is used within the class definition:

Covariant

When a type parameter appears only in return positions (method return types, field types being read), the parametric class is covariant in that parameter (see Types for details on variance). This means instantiations follow the same subtyping direction as their type arguments:

# Base class hierarchy
entity := class:
    ID:int

player := class(entity):
    Name:string

# Covariant class - type parameter only in return position
producer(t:type) := class:
    Value:t

    Get():t = Value  # Returns t - covariant position

# Can use producer(player) where producer(entity) expected
ProcessProducer(P:producer(entity)):int = P.Get().ID

Here's an example demonstrating covariance:

<!--versetest

Base class hierarchy

entity := class: ID:int

player := class(entity): Name:string

Covariant class - type parameter only in return position

producer(t:type) := class: Value:t

Get():t = Value  # Returns t - covariant position

Can use producer(player) where producer(entity) expected

ProcessProducer(P:producer(entity)):int = P.Get().ID -->

# Covariance allows subtype → supertype
PlayerProducer:producer(player) = producer(player){Value := player{ID := 1, Name := "Alice"}}
EntityProducer:producer(entity) = PlayerProducer  # Valid!

Result := ProcessProducer(PlayerProducer)  # Works!

Why this is safe: If you expect to get an entity from a producer, receiving a player (which is a subtype of entity) is always valid—a player has all the properties of an entity.

Direction: producer(player) → producer(entity) ✓ (follows subtype direction)

Contravariant

When a type parameter appears only in parameter positions (method parameters being consumed), the parametric class is contravariant in that parameter (see Types for details on variance). This means instantiations follow the opposite subtyping direction:

entity := class:
    ID:int

player := class(entity):
    Name:string

# Contravariant class - type parameter only in parameter position
consumer(t:type) := class:
    Process(Item:t):void = {}  # Accepts t - contravariant position

And a use case:

# Contravariance allows supertype → subtype
EntityConsumer:consumer(entity) = consumer(entity){}
PlayerConsumer:consumer(player) = EntityConsumer  # Valid!

# Can use consumer(entity) where consumer(player) expected
ProcessPlayers(C:consumer(player)):void =
    C.Process(player{ID := 1, Name := "Bob"})

ProcessPlayers(EntityConsumer)                    # Works!

Why this is safe: If you have a function that accepts any entity, it can certainly handle the more specific player type. A consumer(entity) can consume anything a consumer(player) can consume, plus more.

Direction: consumer(entity) → consumer(player) ✓ (opposite of subtype direction)

Invariant

When a type parameter appears in both parameter and return positions, the parametric class is invariant in that parameter. No subtyping relationship exists between different instantiations:

<!--versetest entity := class: ID:int

player := class(entity): Name:string

Invariant class - type parameter in both positions

transformer(t:type) := class: Transform(Input:t):t = Input # Both parameter and return <# -->

entity := class:
    ID:int

player := class(entity):
    Name:string

# Invariant class - type parameter in both positions
transformer(t:type) := class:
    Transform(Input:t):t = Input  # Both parameter and return

Here's an example showing that no variance exists between different instantiations:

<!--versetest entity := class: ID:int

player := class(entity): Name:string

Invariant class - type parameter in both positions

transformer(t:type) := class: Transform(Input:t):t = Input # Both parameter and return -->

# No variance - cannot convert in either direction
EntityTransformer:transformer(entity) = transformer(entity){}
PlayerTransformer:transformer(player) = transformer(player){}

# Invalid: Cannot use one where the other is expected
# X:transformer(entity) = PlayerTransformer  # ERROR 3509
# Y:transformer(player) = EntityTransformer  # ERROR 3509

Why this is necessary: If a transformer(player) could be used as a transformer(entity), you could pass any entity to its Transform method, which expects specifically a player. This would be unsafe.

Direction: No conversion allowed in either direction

Bivariant

When a type parameter is not used in any method signatures (only in private implementation details or not at all), the parametric class is bivariant. Any instantiation can be converted to any other:

<!--versetest entity := class: ID:int

player := class(entity): Name:string

Bivariant class - type parameter not used in public interface

container(t:type) := class: DoSomething():void = {} # Doesn't use t at all <# -->

entity := class:
    ID:int

player := class(entity):
    Name:string

# Bivariant class - type parameter not used in public interface
container(t:type) := class:
    DoSomething():void = {}  # Doesn't use t at all

Here's an example showing that bivariant classes allow conversion in both directions:

<!--versetest entity := class: ID:int

player := class(entity): Name:string

Bivariant class - type parameter not used in public interface

container(t:type) := class: DoSomething():void = {} # Doesn't use t at all -->

# Bivariant allows conversion in both directions
EntityContainer:container(entity) = container(entity){}
PlayerContainer:container(player) = container(player){}

# Both directions work
X:container(entity) = PlayerContainer  # Valid
Y:container(player) = EntityContainer  # Also valid

Why this works: Since the type parameter doesn't affect the observable behavior, the instantiations are interchangeable.

Recursive Parametric Types

Parametric classes can reference themselves in their field types, enabling recursive generic data structures like linked lists, trees, and graphs. The key requirement is that the self-reference uses the same type parameter — this is the only form of recursion Verse allows. It works because the compiler can resolve the type structure in a single pass: list_node(int) contains a ?list_node(int), which contains a ?list_node(int), and so on. The optional (?) provides the base case that terminates the recursion at runtime.

Here is a generic linked list built as a recursive parametric class:

<!--versetest

Linked list node

list_node(t:type) := class: Value:t Next:?list_node(t) # Same type parameter 't'

Helper to create lists

Cons(Head:t, Tail:?list_node(t) where t:type):list_node(t) = list_node(t){Value := Head, Next := Tail}

Sum a linked list

SumList(List:?list_node(int)):int = if (Head := List?): Head.Value + SumList(Head.Next) else: 0 <# -->

# Linked list node
list_node(t:type) := class:
    Value:t
    Next:?list_node(t)  # Same type parameter 't'

# Helper to create lists
Cons(Head:t, Tail:?list_node(t) where t:type):list_node(t) =
    list_node(t){Value := Head, Next := Tail}

# Sum a linked list
SumList(List:?list_node(int)):int =
    if (Head := List?):
        Head.Value + SumList(Head.Next)
    else:
        0

Here's an example of using the linked list:

<!--versetest

Linked list node

list_node(t:type) := class: Value:t Next:?list_node(t) # Same type parameter 't'

Helper to create lists

Cons(Head:t, Tail:?list_node(t) where t:type):list_node(t) = list_node(t){Value := Head, Next := Tail}

Sum a linked list

SumList(List:?list_node(int)):int = if (Head := List?): Head.Value + SumList(Head.Next) else: 0 -->

# Usage
IntList := list_node(int){
    Value := 1
    Next := option{list_node(int){
        Value := 2
        Next := false
    }}
}

Disallowed: Direct Type Alias Recursion

You cannot define a parametric type that directly aliases to a structural type containing itself:

# Invalid: Direct array recursion
# t(u:type) := []t(u)  # ERROR 3502

# Invalid: Direct map recursion
# t(u:type) := [int]t(u)  # ERROR 3502

# Invalid: Direct optional recursion
# t(u:type) := ?t(u)  # ERROR 3502

# Invalid: Direct function recursion
# t(u:type) := u->t(u)  # ERROR 3502
# t(u:type) := t(u)->u  # ERROR 3502

These fail because they create infinite type expansion—the compiler cannot determine the actual structure of the type.

Valid alternative: Wrap the recursive reference in a class. For example, a tree where each node holds a list of children is a recursive parametric type — each nested_list(t) contains an array of nested_list(t):

# Valid: Indirect recursion through class
nested_list(t:type) := class:
    Items:[]nested_list(t)  # OK - wrapped in class

Here's an example of constructing a tree with two children:

<!--versetest

Valid: Indirect recursion through class

nested_list(t:type) := class: Items:[]nested_list(t) # OK - wrapped in class -->

Tree := nested_list(int){
    Items := array{
        nested_list(int){Items := array{}},
        nested_list(int){Items := array{}}
    }
}

Disallowed: Polymorphic Recursion

Polymorphic recursion occurs when a parametric type references itself with a different type argument:

# Invalid: Type parameter changes
# my_type(t:type) := class:
#     Next:my_type(?t)  # ERROR 3509 - ?t is different from t

# Invalid: Alternating type parameters
# bi_list(t:type, u:type) := class:
#     Value:t
#     Next:?bi_list(u, t)  # ERROR 3509 - parameters swapped

Why this is disallowed: Polymorphic recursion makes type inference undecidable and can create infinitely complex types. When you instantiate my_type(int), it would need my_type(?int), which needs my_type(??int), and so on forever.

Current limitation: While polymorphic recursion is theoretically sound in some type systems, Verse currently does not support it to keep type checking tractable.

Disallowed: Mutual Recursion

Mutual recursion between multiple parametric types is not supported:

# Invalid: Mutual recursion
# t1(t:type) := class:
#     Next:?t2(t)  # References t2
#
# t2(t:type) := class:
#     Next:?t1(t)  # References t1

Why this is disallowed: Similar to polymorphic recursion, mutual recursion complicates type inference and can create circular dependencies that are difficult for the compiler to resolve.

Workaround: Combine into a single type:

# Valid: Single type with multiple cases
node_type := enum:
    TypeA
    TypeB

combined_node(t:type) := class:
    Type:node_type
    Value:t
    Next:?combined_node(t)

Disallowed: Inheritance Recursion

You cannot inherit from a type variable or create recursive inheritance through parametric types:

# Invalid: Inheriting from parametric self
# t(u:type) := class(t(u)){}  # ERROR 3590

# Invalid: Inheriting from type variable
# inherits_from_variable(t:type) := class(t){}  # ERROR 3590

Why this is disallowed: Inheritance requires knowing the parent's structure,but with parametric recursion, this structure would be self-referential before being defined.

Parametric Interfaces

While parametric classes get most of the attention, interfaces can also be parametric, enabling abstract contracts that work with any type:

<!--versetest equivalence(t:type, u:type) := interface: Equal(Left:t, Right:u)<transacts><decides>:t

Generic collection interface

collection_ifc(t:type) := interface: AddItem(Item:t)<transacts>:void RemoveItem(Item:t)<transacts><decides>:void Has(Item:t)<reads>:logic <# -->

# Generic equality interface
equivalence(t:type, u:type) := interface:
    Equal(Left:t, Right:u)<transacts><decides>:t

# Generic collection interface
collection_ifc(t:type) := interface:
    Add(Item:t)<transacts>:void
    Remove(Item:t)<transacts><decides>:void
    Has(Item:t)<reads>:logic

Classes implement parametric interfaces by providing concrete types for the parameters:

<!-- versetest equivalence(t:type, u:type) := interface: Equal(Left:t, Right:u)<transacts><decides>:t

int_equivalence := class(equivalence(int, comparable)): Equal<override>(Left:int, Right:comparable)<transacts><decides>:int = Left = Right

Or with type parameters matching the class

comparable_equivalence(t:subtype(comparable)) := class(equivalence(t, comparable)): Equal<override>(Left:t, Right:comparable)<transacts><decides>:t = Left = Right <# -->

equivalence(t:type, u:type) := interface:
    Equal(Left:t, Right:u)<transacts><decides>:t

# Implement with specific types
int_equivalence := class(equivalence(int, comparable)):
    Equal<override>(Left:int, Right:comparable)<transacts><decides>:int =
        Left = Right

# Or with type parameters matching the class
comparable_equivalence(t:subtype(comparable)) := class(equivalence(t, comparable)):
    Equal<override>(Left:t, Right:comparable)<transacts><decides>:t =
        Left = Right

Here's an example of using the parametric interface:

<!--versetest equivalence(t:type, u:type) := interface: Equal(Left:t, Right:u)<transacts><decides>:t

Implement with specific types

int_equivalence := class(equivalence(int, comparable)): Equal<override>(Left:int, Right:comparable)<transacts><decides>:int = Left = Right

Or with type parameters matching the class

comparable_equivalence(t:subtype(comparable)) := class(equivalence(t, comparable)): Equal<override>(Left:t, Right:comparable)<transacts><decides>:t = Left = Right -->

# Usage
Eq := comparable_equivalence(int){}
Eq.Equal[5, 5]  # Succeeds

Parametric interfaces follow the same variance rules as parametric classes:

entity := class:
    ID:int

player := class(entity):
    Name:string

# Covariant interface - returns t
producer_interface(t:type) := interface:
    Produce():t

player_producer := class(producer_interface(player)):
    Produce<override>():player = player{ID := 1, Name := "Test"}

Here's an example of covariant subtyping:

<!--versetest entity := class: ID:int

player := class(entity): Name:string

Covariant interface - returns t

producer_interface(t:type) := interface: Produce():t

player_producer := class(producer_interface(player)): Produce<override>():player = player{ID := 1, Name := "Test"} -->

# Covariant subtyping works
EntityProducer:producer_interface(entity) = player_producer{}

You can create specialized (non-parametric) interfaces from parametric ones:

generic_handler(t:type) := interface:
    Handle(Item:t):void

# Specialize to a concrete type
int_handler := interface(generic_handler(int)):
    # Inherits Handle(Item:int):void
    # Can add more methods here

int_processor := class(int_handler):
    Handle<override>(Item:int):void =
        Print("Handling: {Item}")

Here's an example of using specialized interfaces in casts:

<!--versetest generic_handler(t:type) := interface: Handle(Item:t):void

Specialize to a concrete type

int_handler := interface(generic_handler(int)): # Inherits Handle(Item:int):void # Can add more methods here

int_processor := class(int_handler): Handle<override>(Item:int):void = Print("Handling: {Item}") -->

# Can use in casts now (specialized interfaces are non-parametric)
Base := int_processor{}
if (Handler := int_handler[Base]):
    Handler.Handle(42)

Multiple Type Parameters

Interfaces can have multiple type parameters with independent variance:

converter_interface(input:type, output:type) := interface:
    Convert(In:input):output
    # input is contravariant, output is covariant

entity := class:
    ID:int

player := class(entity):
    Name:string

# Implement with specific types
player_to_entity := class(converter_interface(player, entity)):
    Convert<override>(In:player):entity = entity{ID := In.ID}

Is used here:

<!--versetest converter_interface(input:type, output:type) := interface: Convert(In:input):output # input is contravariant, output is covariant

entity := class: ID:int

player := class(entity): Name:string

Implement with specific types

player_to_entity := class(converter_interface(player, entity)): Convert<override>(In:player):entity = entity{ID := In.ID}

-->

# Variance allows flexible usage
C:converter_interface(player, entity) = player_to_entity{}

Advanced Parametric Types

Effects

Parametric types can have effect specifiers that apply to all instantiations:

<!-- versetest

Parametric class with effects

async_container(t:type) := class<computes>: Property:t

All instantiations inherit the effect

X:async_container(int) = async_container(int){Property := 1} # <computes> effect

Multiple effects

transactional_container(t:type) := class<transacts>: Property:t

assert: Y:transactional_container(int) = transactional_container(int){Property := 2} <# -->

# Parametric class with effects
async_container(t:type) := class<computes>:
    Property:t

# All instantiations inherit the effect
X:async_container(int) = async_container(int){Property := 1}  # <computes> effect

# Multiple effects
transactional_container(t:type) := class<transacts>:
    Property:t

# Constructor inherits effects
# Y:transactional_container(int) = transactional_container(int){Property := 2}

Allowed effects:

<computes> - Allows non-terminating computation
<transacts> - Participates in transactions
<reads> - Reads mutable state
<writes> - Writes mutable state
<allocates> - Allocates resources

Not allowed:

<decides> - Can fail
<suspends> - Can suspend execution
<converges> - The <converges> effect guarantees that a function terminates (see the Effects chapter). Parametric classes cannot use it because instantiating a parametric type may involve arbitrary computation — the compiler cannot guarantee that constructing my_type(t) for all possible t will terminate.

Effect propagation:

<!-- versetest my_type(t:type) := class<computes>: Property:t

This requires <computes> in the context

CreateInstance()<computes>:my_type(int) = my_type(int){Property := 1} <# -->

# Effect on parametric type propagates to constructor
my_type(t:type) := class<computes>:
    Property:t

# This requires <computes> in the context
CreateInstance()<computes>:my_type(int) =
    my_type(int){Property := 1}

The effect becomes part of the type's contract—all code constructing or working with instances must account for these effects.

Aliases

You can create type aliases that simplify complex parametric type expressions:

# Alias for map type
string_map(t:type) := [string]t

# Use the alias
PlayerScores:string_map(int) = map{
    "Alice" => 100,
    "Bob" => 95
}

# Alias for optional array
optional_array(t:type) := []?t

# Simplifies type signatures
FilterValid(Items:optional_array(int)):[]int =
    for (Item : Items; Value := Item?):
        Value

Structural type aliases:

# Function type aliases
transformer(input:type, output:type) := input -> output
predicate(t:type) := t -> logic

# Tuple type aliases
pair(t:type, u:type) := tuple(t, u)
triple(t:type) := tuple(t, t, t)

# Use in signatures
ApplyTransform(T:transformer(int, string), Value:int):string =
    T(Value)

CheckCondition(P:predicate(int), Value:int):logic =
    P(Value)

Type aliases improve readability and maintainability for complex generic types.

Advanced Type Constraints

Beyond basic subtype constraints, parametric types support specialized constraints:

Subtype constraints:

<!--versetest entity:=class{ID:int=0} player:=class(entity){}

Constrain to subtype of a class

bounded_container(t:subtype(entity)) := class: Value:t

GetID():int = Value.ID  # Can access entity members

Valid: player is subtype of entity

PlayerContainer := bounded_container(player){}

Invalid: int is not subtype of entity

IntContainer := bounded_container(int){} # Type error

<# -->

# Constrain to subtype of a class
bounded_container(t:subtype(entity)) := class:
    Value:t

    GetID():int = Value.ID  # Can access entity members

# Valid: player is subtype of entity
# PlayerContainer := bounded_container(player){}

# Invalid: int is not subtype of entity
# IntContainer := bounded_container(int){}  # Type error

Castable subtype constraints:

<!--versetest component:=class<castable>{} ProcessTyped(:component)<computes>:void={}

Requires castable subtype

dynamic_handler(t:castable_subtype(component)) := class: Handle(Item:component):void = if (Typed := t[Item]): # Typed has the specific subtype ProcessTyped(Typed)

<# -->

# Requires castable subtype
dynamic_handler(t:castable_subtype(component)) := class:
    Handle(Item:component):void =
        if (Typed := t[Item]):
            # Typed has the specific subtype
            ProcessTyped(Typed)

Constraint propagation:

<!--versetest

Constraints propagate through function calls

wrapper(t:subtype(comparable)) := class: Data:t

Process(W:wrapper(t) where t:subtype(comparable))<computes><decides>:void = # Compiler knows t is comparable here W.Data = W.Data <# -->

# Constraints propagate through function calls
wrapper(t:subtype(comparable)) := class:
    Data:t

Process(W:wrapper(t) where t:subtype(comparable))<computes><decides>:void =
    # Compiler knows t is comparable here
    W.Data = W.Data

When defining parametric functions that work with parametric types, the constraints must be compatible:

base_class := class:
    ID:int

constrained(t:subtype(base_class)) := class:
    Data:t

# Valid: Constraint matches
UseConstrained(C:constrained(t) where t:subtype(base_class)):int =
    C.Data.ID

# Invalid: Missing or incompatible constraint
UseConstrained(C:constrained(t) where t:type):int =  # ERROR 
    C.Data.ID

Access Specifiers

Classes support fine-grained control over member visibility through access specifiers:

game_state := class:
    Score<public> : int = 0                    # Anyone can read
    var Lives<private> : int = 3               # Only this class can access
    var Shield<protected> : float = 100.0      # This class and subclasses
    DebugInfo<internal> : string = ""          # Same module only

    # Public method - anyone can call
    GetLives<public>() : int = Lives

    # Protected method - subclasses can override
    OnLifeLost<protected>() : void = {}

    # Private helper - only this class
    ValidateState<private>() : void = {}

Access specifiers apply to both fields and methods, controlling who can read fields and call methods. The default visibility is internal, restricting access to the same module. This encapsulation is crucial for maintaining class invariants and hiding implementation details.

Concrete

The <concrete> specifier enforces that all fields have default values, allowing construction with an empty archetype:

config := class<concrete>:
    MaxPlayers : int = 8
    TimeLimit : float = 300.0
    FriendlyFire : logic = false

# Can construct with empty archetype
DefaultConfig := config{}

This is particularly useful for configuration classes where reasonable defaults exist for all values.

A concrete class C can be constructed by writing C{}, that is to say with the empty archetype.

A concrete class may have non-concrete subclasses.

Unique

The <unique> specifier creates classes and interfaces with reference semantics where each instance has a distinct identity. When a class or interface is marked as <unique>, instances become comparable using the equality operators (= and <>), with equality based on object identity rather than field values.

Classes marked with <unique> compare by identity, not by value:

<!-- versetest vector3:=struct{X:float,Y:float,Z:float} entity := class<unique>: Name : string Position : vector3 F()<decides>:void={ E1 := entity{Name := "Guard", Position := vector3{X := 0.0, Y := 0.0, Z := 0.0}} E2 := entity{Name := "Guard", Position := vector3{X := 0.0, Y := 0.0, Z := 0.0}} E3 := E1

not(E1 = E2 ) # Fails - different instances despite identical field values E1 = E3 # Succeeds - same instance } <# -->

entity := class<unique>:
   Name : string
   Position : vector3

E1 := entity{Name := "Guard", Position := vector3{X := 0.0, Y := 0.0, Z := 0.0}}
E2 := entity{Name := "Guard", Position := vector3{X := 0.0, Y := 0.0, Z := 0.0}}
E3 := E1

E1 = E2  # Fails - different instances despite identical field values
E1 = E3  # Succeeds - same instance

Without <unique>, class instances cannot be compared for equality at all—the language prevents meaningless comparisons. With <unique>, you gain the ability to use instances as map keys, store them in sets, and perform identity checks, essential for tracking specific objects throughout their lifetime.

Interfaces

Interfaces can also be marked with <unique>, which makes all instances of classes implementing that interface comparable by identity:

component := interface<unique>:
    Update():void
    Render():void

physics_component := class(component):
    Update<override>():void = {}
    Render<override>():void = {}

And a use case:

<!--versetest component := interface<unique>: Update():void Render():void

physics_component := class(component): Update<override>():void = {} Render<override>():void = {} -->

# Instances are comparable because component is unique
P1 := physics_component{}
P2 := physics_component{}

P1 <> P2  # true - different instances
P1 = P1   # true - same instance

The <unique> property propagates through interface inheritance. If a parent interface is marked <unique>, all child interfaces and classes implementing those interfaces automatically become comparable:

base_component := interface<unique>:
    Update():void

# Child interface inherits <unique> from parent
advanced_component := interface(base_component):
    AdvancedUpdate():void

# Classes implementing any interface in the hierarchy become comparable
player_component := class(advanced_component):
    Update<override>():void = {}
    AdvancedUpdate<override>():void = {}

And a use case:

<!--versetest base_component := interface<unique>: Update():void

Child interface inherits <unique> from parent

advanced_component := interface(base_component): AdvancedUpdate():void

Classes implementing any interface in the hierarchy become comparable

player_component := class(advanced_component): Update<override>():void = {} AdvancedUpdate<override>():void = {} -->

C1 := player_component{}
C2 := player_component{}
C1 <> C2  # true - comparable due to base_component being unique

When a class implements multiple interfaces, comparability is determined by whether ANY of the inherited interfaces is <unique>:

updateable := interface:  # Not unique
    Update():void

renderable := interface<unique>:  # Unique
    Render():void

game_object := class(updateable, renderable):
    Update<override>():void = {}
    Render<override>():void = {}

And a use case:

<!--versetest updateable := interface: # Not unique Update():void

renderable := interface<unique>: # Unique Render():void

game_object := class(updateable, renderable): Update<override>():void = {} Render<override>():void = {} -->

# game_object is comparable because renderable is unique
G1 := game_object{}
G2 := game_object{}
G1 <> G2  # true - comparable due to renderable interface

Even if most interfaces are non-unique, a single <unique> interface in the hierarchy makes the entire class comparable.

Unique in Default Values

When a <unique> class appears in a field's default value, each containing object receives its own distinct instance. This guarantee applies even when the unique class is nested within complex parametric types:

token := class<unique>:
    ID:int = 0

container := class:
    MyToken:token = token{}

And a use case:

<!--versetest token := class<unique>: ID:int = 0

container := class: MyToken:token = token{} -->

C1 := container{}
C2 := container{}
C1.MyToken <> C2.MyToken  # true - each container has its own unique token

This behavior extends to <unique> instances within arrays, optionals, tuples, and maps:

item := class<unique>{}

# Each class instantiation creates fresh unique instances in default values
with_array := class:
    Items:[]item = array{item{}}

with_optional := class:
    MaybeItem:?item = option{item{}}

with_map := class:
    ItemMap:[int]item = map{0 => item{}}

And a use case:

<!--versetest item := class<unique>{}

Each class instantiation creates fresh unique instances in default values

with_array := class: Items:[]item = array{item{}}

with_optional := class: MaybeItem:?item = option{item{}}

with_map := class: ItemMap:[int]item = map{0 => item{}} -->

A := with_array{}
B := with_array{}
A.Items[0] <> B.Items[0]  # true - different unique instances

C := with_optional{}
D := with_optional{}
if (ItemC := C.MaybeItem?, ItemD := D.MaybeItem?):
    ItemC <> ItemD  # true - different unique instances

The same principle applies when parametric classes contain unique instances in their fields:

<!--versetest entity := class<unique>{}

registry(t:type) := class: DefaultEntity:entity = entity{} Data:t <# -->

entity := class<unique>{}

registry(t:type) := class:
    DefaultEntity:entity = entity{}
    Data:t

<!--versetest entity := class<unique>{}

registry(t:type) := class: DefaultEntity:entity = entity{} Data:t -->

R1 := registry(int){Data:=1}
R2 := registry(int){Data:=2}
R1.DefaultEntity <> R2.DefaultEntity  # true

R3 := registry(string){Data:="hi"}
R3.DefaultEntity <> R1.DefaultEntity  # true - even across different type parameters

This guarantee ensures that identity-based operations remain reliable. If you store objects in maps keyed by unique instances, or maintain sets of unique objects, each container genuinely owns distinct instances rather than sharing references. The language prevents subtle bugs where multiple objects might unexpectedly share the same identity.

Overload Resolution

Types marked with <unique> are subtypes of the built-in comparable type. This can create overload ambiguity:

<!--versetest

Valid: non-unique interface doesn't conflict with comparable

regular_interface := interface: Method():void

Process(A:comparable, B:comparable):void = {} Process(A:regular_interface, B:regular_interface):void = {} # OK - no conflict

Invalid: unique interface conflicts with comparable

assert_semantic_error(3532): my_unique_interface := interface<unique>: Method():void

Handle(A:comparable, B:comparable):void = {}
Handle(A:my_unique_interface, B:my_unique_interface):void = {}  # ERROR - ambiguous!

<# -->

# Valid: non-unique interface doesn't conflict with comparable
regular_interface := interface:
    Method():void

Process(A:comparable, B:comparable):void = {}
Process(A:regular_interface, B:regular_interface):void = {}  # OK - no conflict

# Invalid: unique interface conflicts with comparable
unique_interface := interface<unique>:
    Method():void

Handle(A:comparable, B:comparable):void = {}
# Handle(A:unique_interface, B:unique_interface):void = {}  # ERROR - ambiguous!

Since unique_interface is a subtype of comparable, both overloads could match when called with unique_interface arguments, causing a compilation error. When designing overloaded functions, be aware that <unique> types participate in the comparable type hierarchy.

Use Cases

The <unique> specifier is ideal for:

Game Entities: Where each entity in the world must be distinguishable regardless of current state

#entity := class<unique>:
#    var Health:int = 100
#    var Position:vector3

# Can track specific entities in collections
var ActiveEntities:[entity]logic = map{}

Component Interfaces: Where you need identity-based equality for interface types

<!--versetest entity:=class:

component := interface<unique>: Owner:entity Update():void -->

#component := interface<unique>:
#    Owner:entity

# Can use interface references as map keys
var ComponentRegistry:[component]string = map{}

Session Objects: Where identity matters more than current property values

<!--versetest connection_info := class:

player_session := class<unique>: PlayerID:string var ConnectionTime:float -->

#player_session := class<unique>:
#    PlayerID:string
#    var ConnectionTime:float

# Track specific sessions
var ActiveSessions:[player_session]connection_info = map{}

Resource Handles: Where you need to track specific instances rather than equivalent values

<!--versetest gpu_resource:=class:

texture_handle := class<unique>: ResourceID:int FilePath:string -->

#texture_handle := class<unique>:
#    ResourceID:int
#    FilePath:string

# Manage resource lifecycle
var LoadedTextures:[texture_handle]gpu_resource = map{}

The <unique> specifier enables these patterns by providing identity-based equality semantics, making it possible to use instances as map keys, maintain sets of unique objects, and distinguish between different instances even when their data is identical.

Abstract

The <abstract> specifier marks classes that cannot be instantiated directly — they exist solely as base classes for inheritance. When you declare a class with <abstract>, you're creating a template that defines structure and behavior for subclasses to inherit and implement.

Abstract classes serve as architectural foundations in a type hierarchy. They define contracts through abstract methods that subclasses must implement, while potentially providing concrete methods and fields that subclasses inherit. This creates a powerful pattern for code reuse and polymorphic behavior.

vehicle := class<abstract>:
      Speed():float             # Abstract method
      MaxPassengers:int = 1

      # Concrete method all vehicles share
      CanTransport(Count:int)<decides>:void =
          Count <= MaxPassengers

car := class(vehicle):
      Speed<override>():float = 60.0
      MaxPassengers<override>:int = 4

bicycle := class(vehicle):
      Speed<override>():float = 15.0

Abstract methods within abstract classes have no implementation — they're pure declarations that establish what subclasses must provide. An abstract method creates a contract: any non-abstract subclass must override all abstract methods or the code won't compile.

Castable

The <castable> specifier enables runtime type checking and safe downcasting for classes. When a class is marked with <castable>, you can use dynamic type tests and casts to determine if an object is an instance of that class or its subclasses at runtime.

Without <castable>, Verse's type system operates purely at compile time. The <castable> specifier adds runtime type information, allowing code to inspect and react to actual object types during execution. This bridges the gap between static type safety and dynamic polymorphism.

Verse provides two forms of type casting: fallible casts (which can fail at runtime) and infallible casts (which are verified at compile time).

Fallible casts use bracket syntax Type[Value] and return an optional result. These are runtime checks that succeed only if the value is actually an instance of the target type:

component := class<abstract><castable><allocates>:
    Name:string

physics_component := class<allocates>(component):
    Name<override>:string = "Physics"
    Velocity:vector3

render_component := class<allocates>(component):
    Name<override>:string = "Render"
    Material:string

ProcessComponent(Comp:component):void =
    # Attempt to cast to physics_component
    if (PhysicsComp := physics_component[Comp]):
        # Cast succeeded - PhysicsComp has type physics_component
        Print("Physics component with velocity: {PhysicsComp.Velocity}")
    else if (RenderComp := render_component[Comp]):
        # Cast succeeded - RenderComp has type render_component
        Print("Render component with material: {RenderComp.Material}")
    else:
        # Neither cast succeeded
        Print("Unknown component type")

The cast expression has the <decides> effect—it fails if the object is not an instance of the target type. This integrates naturally with Verse's failure handling:

<!--versetest vector3:=class<final><allocates>{ X:float=0.0; Y:float=0.0; Z:float=0.0 } component := class<abstract><castable><allocates>: Name:string

physics_component := class<allocates>(component): Name<override>:string = "Physics" Velocity:vector3=vector3{}

SomeComponent:component=physics_component{} UpdatePhysics(:physics_component)<computes>:void={} -->

GetPhysicsComponent(Comp:component)<computes><decides>:physics_component =
    # Returns physics_component or fails
    physics_component[Comp]

# Use with failure handling
if (Physics := GetPhysicsComponent[SomeComponent]):
    UpdatePhysics(Physics)

Infallible casts use parenthesis syntax Type(Value) and only work when the compiler can verify the cast is safe—that is, when the value type is a subtype of the target type:

base := class:
    ID:int

derived := class(base):
    Name:string

GetDerived():derived = derived{ID := 1, Name := "Test"}

Use case:

<!--versetest base := class: ID:int

derived := class(base): Name:string

GetDerived():derived = derived{ID := 1, Name := "Test"} -->

# Infallible upcast - derived is a subtype of base
BaseRef:base = base(GetDerived())  # Always safe

Attempting an infallible downcast (from supertype to subtype) is a compile error, as the compiler cannot guarantee safety:

DerivedRef := derived(BaseRef)  # ERROR: not a subtype relationship

Castable and Inheritance

The <castable> property is inherited by all subclasses. When you mark a class as <castable>, every class that inherits from it automatically becomes castable as well:

base := class<castable>:
    Value:int

child := class(base):
    # Automatically castable - inherits from castable base
    Name:string

grandchild := class(child):
    # Also automatically castable
    Extra:string

# Can cast through the hierarchy
ProcessBase(Instance:base):void =
    if (AsChild := child[Instance]):
        Print("It's a child: {AsChild.Name}")
    if (AsGrandchild := grandchild[Instance]):
        Print("It's a grandchild: {AsGrandchild.Extra}")

Important constraint: Parametric types cannot be <castable>. The <castable> specifier enables runtime type checks (dynamic casts), but Verse erases type parameters at runtime — only the concrete class exists, not the specific parametric instantiation. This means the runtime cannot distinguish between container(int) and container(string), so allowing dynamic casts on parametric types would be unsound:

# Valid: non-parametric castable class
valid_castable := class<castable>:
    Data:int

# Invalid: parametric classes cannot be castable
# invalid_castable(t:type) := class<castable>:  # ERROR
#     Data:t

However, a non-parametric class can be <castable> even if it inherits from or contains parametric types:

container(t:type) := class:
    Value:t

# Valid: concrete instantiation of parametric type
int_container := class<castable>(container(int)):
    Extra:string

Using castable_subtype

The castable_subtype type constructor works with <castable> classes to enable type-safe filtered queries and dynamic type dispatch:

<!--versetest component<public> := class<abstract><unique><castable>: Parent<public>:entity

entity<public> := class<concrete><unique><transacts><castable>: FindDescendantEntities(entity_type:castable_subtype(entity)):[]entity_type = array{} <# -->

component<public> := class<abstract><unique><castable>:
      Parent<public>:entity

  entity<public> := class<concrete><unique><transacts><castable>:
      FindDescendantEntities(entity_type:castable_subtype(entity)):[]entity_type

When you call FindDescendantEntities(player), the function returns only entities that are actually player instances or subclasses thereof, verified at runtime through the castable mechanism. The type parameter ensures type safety—the returned values have the specific subtype you requested.

Permanence of Castable

Once a class is published with <castable>, this decision becomes permanent. You cannot add or remove the <castable> specifier after publication because doing so would break existing code that relies on runtime type checking. Code that performs casts would suddenly fail or behave incorrectly if the castable property changed.

This permanence is enforced through the versioning system—attempting to change the <castable> status of a published class will result in a compatibility error.

Final

The <final> specifier prevents inheritance, creating a terminal point in a class hierarchy. When you mark a class with <final>, no other class can inherit from it. For methods, <final> prevents overriding in subclasses, locking the implementation at that level of the hierarchy.

Classes marked with <final> serve as concrete implementations that cannot be extended. This is particularly important for persistable classes, which require <final> to ensure their structure remains stable for serialization:

<!--versetest player_stats:=struct<persistable>{}

player_profile := class<final><persistable>: Username:string = "Player" Level:int = 1 Gold:int = 0

player_data := class<final><persistable>: Version:int = 1 LastLogin:string = "" Statistics:player_stats = player_stats{} <# -->

player_profile := class<final><persistable>:
      Username:string = "Player"
      Level:int = 1
      Gold:int = 0

  player_data := class<final><persistable>:
      Version:int = 1
      LastLogin:string = ""
      Statistics:player_stats = player_stats{}

The <final> requirement for persistable classes prevents schema evolution problems. If subclasses could extend persistable classes, the serialization system would face ambiguity about which fields to persist and how to handle polymorphic deserialization.

For methods, <final> locks behavior at a specific point in the inheritance chain:

<!--versetest base_entity := class: GetName():string = "Entity"

game_object := class(base_entity): GetName<override><final>():string = "GameObject" # Any subclass of game_object cannot override GetName <# -->

base_entity := class:
      GetName():string = "Entity"

  game_object := class(base_entity):
      GetName<override><final>():string = "GameObject"
      # Any subclass of game_object cannot override GetName

For fields, <final> prevents modification through archetype construction. When a field is marked <final> and has a default value, that value is locked and cannot be changed when creating instances:

foo := class<computes>:
    Val<final>:int = 0
    X:int = 5

# Valid: X can be changed during construction
ValidFoo := foo{X := 10}

# COMPILE ERROR: Cannot override final field Val
# InvalidFoo := foo{Val := 10}

This restriction ensures that final fields maintain their guaranteed values throughout the object's lifetime. Final fields with default values act as immutable constants for each instance. If you need a field to be customizable during construction, don't mark it as <final>. Final fields must also provide a default value — you cannot declare a final field without initializing it.

The related <final_super> specifier does not prevent further subclassing. Instead, it guarantees that all subclasses of this class will always directly inherit from it — there will be no intermediate classes inserted between the <final_super> class and its descendants in the inheritance chain. Subclasses can themselves be further subclassed:

component := class<abstract><unique><castable><final_super_base>:
      Parent:entity

physics_component := class<final_super>(component):
      Mass:float = 1.0

# Valid: further subclassing is allowed
gravity_component := class(physics_component):
      GravityScale:float = 1.0

<final_super_base> marks the root of a restricted inheritance tree. Its purpose is to work with GetCastableFinalSuperClass, which finds the <final_super> class in the hierarchy for a given instance. This enables component architectures where you need to identify the "category" of a component at runtime:

#            base_type<castable>
#               /         \
#  a_class<final_super>   w_class
#         |                  |
#      b_class            x_class<final_super>
#         |                  |
#      c_class            y_class

# GetCastableFinalSuperClass[base_type, c_class{}]
# returns a_class — the <final_super> ancestor under base_type

This design is particularly valuable in component architectures where you need a stable "category" class in the hierarchy that runtime systems can rely on, while still allowing further specialization below it.

Persistable

The <persistable> specifier marks types that can be saved and restored across game sessions, enabling permanent storage of player progress, achievements, and game state. This specifier transforms ephemeral gameplay into lasting progression, creating the foundation for meaningful player investment.

Persistence works through module-scoped weak_map(player, t) variables, where t is any persistable type. These special maps automatically synchronize with backend storage — when players join, their data loads; when they leave or data changes, it saves. The system handles all serialization, network transfer, and storage management transparently.

player_inventory := class<final><persistable>:
      Gold:int = 0
      Items:[]string = array{}
      UnlockedAreas:[]string = array{}

# This variable automatically persists across sessions
SavedInventories : weak_map(player, player_inventory) = map{}

The <persistable> specifier enforces strict structural requirements to guarantee data integrity across versions. Classes must be <final> because inheritance would complicate serialization schemas. They cannot contain var fields, preserving immutability guarantees even in persistent storage. They cannot be <unique> since identity-based equality doesn't survive serialization. These constraints ensure that what you save today can be reliably loaded tomorrow, next month, or next year.

Interfaces

Interfaces define contracts that classes can implement, specifying both the data and behavior that implementing classes must provide. Unlike many traditional languages where interfaces only declare method signatures, Verse interfaces are rich contracts that can include fields, default method implementations, and even custom accessor logic.

An interface can declare method signatures, provide default implementations, and define data members:

damageable := interface:
    # Abstract method - implementing classes must provide
    TakeDamage(Amount:int)<transacts>:void

    # Method with default implementation
    GetHealth()<computes>:int = 100

    # Data member - implementing classes inherit or must provide
    MaxHealth:int = 100

    IsAlive()<computes>:logic = logic{GetHealth() > 0}

healable := interface:
    Heal(Amount:int):void
    GetMaxHealth():int

Interfaces establish contracts that can be purely abstract (method signatures only), partially concrete (some default implementations), or fully implemented (complete behavior that classes inherit). Any class implementing an interface must provide implementations for abstract methods, but inherits concrete implementations and default field values.

Implementing Interfaces

Classes implement interfaces by inheriting from them and providing concrete implementations where required:

<!--versetest healable:=interface: TakeDamage(Amount:int)<transacts>:void ={} GetHealth():int = 0 Heal(Amount:int)<transacts>:void ={}

damageable:=interface{} -->

character := class(damageable, healable):
    var Health : int = 100
    MaxHealth : int = 100

    TakeDamage<override>(Amount:int)<transacts>:void =
        set Health = Max(0, Health - Amount)

    GetHealth<override>()<reads>:int = Health

    Heal<override>(Amount:int)<transacts>:void =
        set Health = Min(MaxHealth, Health + Amount)

A class can implement multiple interfaces, effectively achieving multiple inheritance of both behavior contracts and data specifications. This provides more flexibility than single class inheritance while maintaining type safety.

Interface Fields

Interfaces can declare data members that implementing classes must provide or inherit. These fields can be either immutable or mutable, and may include default values:

# Interface with various field types
entity_properties := interface:
    # Immutable field with default - classes inherit this value
    EntityID:int = 0

    # Mutable field with default
    var Health:float = 100.0

    # Field without default - classes must provide a value
    Name:string

    # Field that can be overridden
    MaxHealth:float = 100.0

player_entity := class(entity_properties):
    # Must provide Name (no default in interface)
    Name<override>:string = "Player"

    # Can override to change default
    MaxHealth<override>:float = 150.0

    # Inherits EntityID and Health with their defaults

When an interface field has a default value, implementing classes automatically inherit that default unless they override it. Fields without defaults must be provided either by the implementing class or through construction parameters.

Default Implementations

Interfaces can provide complete method implementations that implementing classes inherit automatically:

animated := interface:
    var CurrentFrame:int = 0
    TotalFrames:int = 10

    # Concrete implementation provided by interface
    NextFrame()<transacts><decides>:void =
        set CurrentFrame = Mod[(CurrentFrame + 1),TotalFrames] or 0

    # Can access interface fields
    ProgressPercent()<reads><decides>:rational =
        CurrentFrame / TotalFrames

sprite := class(animated):
    TotalFrames<override>:int = 20
    # Automatically inherits NextFrame and ProgressPercent implementations

Classes inherit these implementations without modification, allowing interfaces to provide reusable behavior. Implementing classes can override these methods if they need specialized behavior, but the interface provides a working default.

Overriding Members

Classes can override both fields and methods from interfaces to provide specialized implementations:

base_stats := interface:
    BaseHealth:int = 100

    CalculateFinalHealth():int = BaseHealth

warrior := class(base_stats):
    # Override field with different default
    BaseHealth<override>:int = 150

    # Override method for specialized calculation
    CalculateFinalHealth<override>():int =
        BaseHealth * 2  # Warriors get double health

mage := class(base_stats):
    BaseHealth<override>:int = 75

    CalculateFinalHealth<override>():int =
        BaseHealth + MagicBonus

    MagicBonus:int = 25

Field overrides can provide different default values or specialize to subtypes. Method overrides replace the interface's implementation entirely. All overrides must maintain type compatibility—fields can only be overridden with subtypes, and method signatures must match exactly.

Verse interfaces are more permissive than in many other languages — they can declare data fields, provide concrete method implementations, and a class can implement multiple interfaces even when they share member names. This design avoids the friction of requiring globally unique names across all interfaces. In practice, independent interface authors may naturally use the same names (Enable, Disable, Power, Update), and requiring every interface to use distinct names would create artificial naming conflicts that scale poorly — especially when interfaces form deep hierarchies with subinterfaces for specialized variants.

When a class implements multiple interfaces that declare fields or methods with the same name, you use qualified names to disambiguate:

magical := interface:
    Power:int = 50
    GetPowerLevel()<computes>:int = Power

physical := interface:
    Power:int = 75
    GetPowerLevel()<computes>:int = Power * 2

hybrid := class(magical, physical):
    UseHybridPowers():void =
       MagicPower := (magical:)Power         # Access magical's Power
       PhysicalPower := (physical:)Power     # Access physical's Power
       MagicLevel := (magical:)GetPowerLevel()
       PhysicalLevel := (physical:)GetPowerLevel()

The qualified name syntax (InterfaceName:)MemberName specifies which interface's member you're accessing. Each interface maintains its own instance of the field, allowing the class to support both contracts simultaneously without conflict.

Interface Hierarchies

Interfaces can extend other interfaces, creating hierarchies of contracts that combine data and behavior requirements:

combatant := interface(damageable, healable):
    var AttackPower:int = 10

    Attack(Target:damageable):void =
        Target.TakeDamage(AttackPower)

    GetAttackPower():int = AttackPower

boss := interface(combatant):
    Phase:int = 1

    UseSpecialAbility():void
    GetPhase():int = Phase

A class implementing boss inherits all fields and methods from the entire hierarchy—boss, combatant, damageable, and healable. Diamond inheritance (where an interface is inherited through multiple paths) is fully supported, with fields properly merged so each field exists only once in the implementing class.

Important: A class cannot directly inherit the same interface multiple times (e.g., class(interface1, interface1) is an error), but can inherit it indirectly through diamond inheritance. This means class(interface2, interface3) is valid even if both interface2 and interface3 inherit from the same base interface.

Fields with Accessors

Interfaces can define fields with custom getter and setter logic, encapsulating complex behavior behind simple field access syntax:

<!--versetest subscribable_property := interface: # External field with accessor methods var Value<getter(GetValue)><setter(SetValue)>:int = external{}

# Internal storage
var Storage:int = 100

# Getter adds computation
GetValue(:accessor):int = Storage + 10

# Setter adds validation
SetValue(:accessor, NewValue:int):void =
    if (NewValue >= 0):
        set Storage = NewValue

tracked_value := class(subscribable_property):

UseTrackedValue():void = Object := tracked_value{}

# Uses getter - returns 110 (Storage + 10)
Current := Object.Value

# Uses setter - validates and updates Storage
set Object.Value = 150

<# -->

subscribable_property := interface:
    # External field with accessor methods
    var Value<getter(GetValue)><setter(SetValue)>:int = external{}

    # Internal storage
    var Storage:int = 100

    # Getter adds computation
    GetValue(:accessor):int = Storage + 10

    # Setter adds validation
    SetValue(:accessor, NewValue:int):void =
        if (NewValue >= 0):
            set Storage = NewValue

tracked_value := class(subscribable_property):

UseTrackedValue():void =
    Object := tracked_value{}

    # Uses getter - returns 110 (Storage + 10)
    Current := Object.Value

    # Uses setter - validates and updates Storage
    set Object.Value = 150

The external{} keyword indicates the field has no direct storage—all access goes through the accessor methods. This pattern is powerful for implementing property change notifications, validation, computed properties, and other scenarios requiring logic around field access.

Important: Fields with accessors defined in interfaces cannot be overridden in implementing classes. The accessor implementation is fixed by the interface.

Classes and Interfaces

Classes and Interfaces

Classes

Object Construction

Methods

Blocks for Initialization

Let Clauses in Archetypes

Self

Inheritance

Super

Method Overriding

Constructor Functions

Overloading Constructors

Delegating Constructors

Subclass constructor delegates to parent constructor

Primary constructor

Convenience constructor forwards to primary

Primary constructor

Convenience constructor forwards to primary

Order of Execution

Shadowing and Qualification

Methods with Same Name

(super:) Qualified

Interface Collisions

Nested Module Qualification

Restrictions

Parametric Classes

Simple container that holds a single value

Instantiation and Identity

These are the same type

All three are equal - they're the same type

Covariant

Base class hierarchy

Covariant class - type parameter only in return position

Can use producer(player) where producer(entity) expected

Contravariant

Invariant

Invariant class - type parameter in both positions

Invariant class - type parameter in both positions

Bivariant

Bivariant class - type parameter not used in public interface

Bivariant class - type parameter not used in public interface

Recursive Parametric Types

Linked list node

Helper to create lists

Sum a linked list

Linked list node

Helper to create lists

Sum a linked list

Valid: Indirect recursion through class

Parametric Interfaces

Generic collection interface

Or with type parameters matching the class

Implement with specific types

Or with type parameters matching the class

Covariant interface - returns t

Specialize to a concrete type

Multiple Type Parameters

Implement with specific types

Advanced Parametric Types

Effects

Parametric class with effects

All instantiations inherit the effect

Multiple effects

This requires <computes> in the context

Aliases

Advanced Type Constraints

Constrain to subtype of a class

Valid: player is subtype of entity

PlayerContainer := bounded_container(player){}

Invalid: int is not subtype of entity

IntContainer := bounded_container(int){} # Type error

Requires castable subtype

Constraints propagate through function calls

Access Specifiers

Concrete

Unique

Interfaces

Child interface inherits <unique> from parent

Classes implementing any interface in the hierarchy become comparable

`(super:)` Qualified