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Traits

A trait is a set of requirements that a type must implement. You can think of it as a contract: a type that conforms to a trait guarantees that it implements all of the features of the trait.

Traits are similar to Java interfaces, C++ concepts, Swift protocols, and Rust traits. If you're familiar with any of those features, Mojo traits solve the same basic problem.

Background​

In dynamically-typed languages like Python, you don't need to explicitly declare that two classes are similar. This is easiest to show by example:

%%python
class Duck:
    def quack(self):
        print("Quack.")

class StealthCow:
    def quack(self):
        print("Moo!")

def make_it_quack_python(maybe_a_duck):
    try:
        maybe_a_duck.quack()
    except:
        print("Not a duck.")

make_it_quack_python(Duck())
make_it_quack_python(StealthCow())
%%python
class Duck:
    def quack(self):
        print("Quack.")

class StealthCow:
    def quack(self):
        print("Moo!")

def make_it_quack_python(maybe_a_duck):
    try:
        maybe_a_duck.quack()
    except:
        print("Not a duck.")

make_it_quack_python(Duck())
make_it_quack_python(StealthCow())

The Duck and StealthCow classes aren't related in any way, but they both define a quack() method, so they work the same in the make_it_quack() function. This works because Python uses dynamic dispatch—it identifies the methods to call at runtime. So make_it_quack_python() doesn't care what types you're passing it, only the fact that they implement the quack() method.

In a statically-typed environment, this approach doesn't work: fn functions require you to specify the type of each argument. If you wanted to write this example in Mojo without traits, you'd need to write a function overload for each input type. All of the examples from here on are in Mojo, so we'll just call the function make_it_quack() going forward.

@value
struct Duck:
    fn quack(self):
        print("Quack")

@value
struct StealthCow:
    fn quack(self):
        print("Moo!")

fn make_it_quack(definitely_a_duck: Duck):
    definitely_a_duck.quack()

fn make_it_quack(not_a_duck: StealthCow):
    not_a_duck.quack()

make_it_quack(Duck())
make_it_quack(StealthCow())
@value
struct Duck:
    fn quack(self):
        print("Quack")

@value
struct StealthCow:
    fn quack(self):
        print("Moo!")

fn make_it_quack(definitely_a_duck: Duck):
    definitely_a_duck.quack()

fn make_it_quack(not_a_duck: StealthCow):
    not_a_duck.quack()

make_it_quack(Duck())
make_it_quack(StealthCow())
Quack Moo!

This isn't too bad with only two classes. But the more classes you want to support, the less practical this approach is.

You might notice that the Mojo versions of make_it_quack() don't include the try/except statement. We don't need it because Mojo's static type checking ensures that you can only pass instances of Duck or StealthCow into the make_it_quack()function.

Using traits​

Traits solve this problem by letting you define a shared set of behaviors that types can implement. Then you can write a function that depends on the trait, rather than individual types. As an example, let's update the make_it_quack() example using traits. The first step is defining a trait:

trait Quackable:
    fn quack(self):
        ...
trait Quackable:
    fn quack(self):
        ...

A trait looks a lot like a struct, except it's introduced by the trait keyword. Right now, a trait can only contain method signatures, and cannot include method implementations. Each method signature must be followed by three dots (...) to indicate that the method is unimplemented.

Next we create some structs that conform to the Quackable trait. To indicate that a struct conforms to a trait, include the trait name in parenthesis after the struct name. You can also include multiple traits, separated by commas. (If you're familiar with Python, this looks just like Python's inheritance syntax.)

@value
struct Duck(Quackable):
    fn quack(self):
        print("Quack")

@value
struct StealthCow(Quackable):
    fn quack(self):
        print("Moo!")
@value
struct Duck(Quackable):
    fn quack(self):
        print("Quack")

@value
struct StealthCow(Quackable):
    fn quack(self):
        print("Moo!")

The struct needs to implement any methods that are declared in the trait. The compiler enforces conformance: if a struct says it conforms to a trait, it must implement everything required by the trait or the code won't compile.

Finally, you can define a function that takes a Quackable like this:

fn make_it_quack[T: Quackable](maybe_a_duck: T):
    maybe_a_duck.quack()
fn make_it_quack[T: Quackable](maybe_a_duck: T):
    maybe_a_duck.quack()

This syntax may look a little unfamiliar if you haven't dealt with Mojo parameters before. What this signature means is that maybe_a_duck is an argument of type T, where T is a type that must conform to the Quackable trait. TODO: This syntax is a little verbose, and we hope to make it more ergonomic in a future release.

Using the method is simple enough:

make_it_quack(Duck())
make_it_quack(StealthCow())
make_it_quack(Duck())
make_it_quack(StealthCow())
Quack Moo!

Note that you don't need the square brackets when you call make_it_quack(): the compiler infers the type of the argument, and ensures the type has the required trait.

One limitation of traits is that you can't add traits to existing types. For example, if you define a new Numeric trait, you can't add it to the standard library Float64 and Int types. However, the standard library already includes a few traits, and we'll be adding more over time.

Traits can require static methods​

In addition to regular instance methods, traits can specify required static methods.

trait HasStaticMethod:
    @staticmethod
    fn do_stuff(): ...

fn fun_with_traits[T: HasStaticMethod]():
    T.do_stuff()
trait HasStaticMethod:
    @staticmethod
    fn do_stuff(): ...

fn fun_with_traits[T: HasStaticMethod]():
    T.do_stuff()

Implicit trait conformance​

Mojo also supports implicit trait conformance. That is, if a type implements all of the methods required for a trait, it's treated as conforming to the trait, even if it doesn't explicitly include the trait in its declaration:

struct RubberDucky:
    fn quack(self):
        print("Squeak!")

make_it_quack(RubberDucky())
struct RubberDucky:
    fn quack(self):
        print("Squeak!")

make_it_quack(RubberDucky())

Implicit conformance can be handy if you're defining a trait and you want it to work with types that you don't control—such as types from the standard library, or a third-party library.

However, we still strongly recommend explicit trait conformance wherever possible. This has two advantages:

  • Documentation. It makes it clear that the type conforms to the trait, without having to scan all of its methods.

  • Future feature support. When default method implementations are added to traits, they'll only work for types that explicitly conform to traits.

Trait inheritance​

Traits can inherit from other traits. A trait that inherits from another trait includes all of the requirements declared by the parent trait. For example:

trait Animal:
    fn make_sound(self):
        ...

# Bird inherits from Animal
trait Bird(Animal):
    fn fly(self):
        ...
trait Animal:
    fn make_sound(self):
        ...

# Bird inherits from Animal
trait Bird(Animal):
    fn fly(self):
        ...

Since Bird inherits from Animal, a struct that conforms to the Bird trait needs to implement both make_sound() and fly(). And since every Bird conforms to Animal, a struct that conforms to Bird can be passed to any function that requires an Animal.

To inherit from multiple traits, add a comma-separated list of traits inside the parenthesis. For example, you could define a NamedAnimal trait that combines the requirements of the Animal trait and a new Named trait:

trait Named:
    fn get_name(self) -> String:
        ...

trait NamedAnimal(Animal, Named):
    pass
trait Named:
    fn get_name(self) -> String:
        ...

trait NamedAnimal(Animal, Named):
    pass

Traits and lifecycle methods​

Traits can specify required lifecycle methods, including constructors, copy constructors and move constructors.

For example, the following code creates a MassProducible trait. A MassProducible type has a default (no-argument) constructor and can be moved. It uses the built-in Movable trait, which requires the type to have a move constructor.

The factory[]() function returns a newly-constructed instance of a MassProducible type.

trait DefaultConstructible:
    fn __init__(inout self): ...

trait MassProducible(DefaultConstructible, Movable):
    pass

fn factory[T: MassProducible]() -> T:
    return T()

struct Thing(MassProducible):
    var id: Int

    fn __init__(inout self):
        self.id = 0

    fn __moveinit__(inout self, owned existing: Self):
        self.id = existing.id

var thing = factory[Thing]()
trait DefaultConstructible:
    fn __init__(inout self): ...

trait MassProducible(DefaultConstructible, Movable):
    pass

fn factory[T: MassProducible]() -> T:
    return T()

struct Thing(MassProducible):
    var id: Int

    fn __init__(inout self):
        self.id = 0

    fn __moveinit__(inout self, owned existing: Self):
        self.id = existing.id

var thing = factory[Thing]()

Note that @register_passable("trivial") types have restrictions on their lifecycle methods: they can't define copy or move constructors, because they don't require any custom logic.

For the purpose of trait conformance, the compiler treats trivial types as copyable and movable.

Built-in traits​

The Mojo standard library currently includes a few traits. They're implemented by a number of standard library types, and you can also implement these on your own types:

The API reference docs linked above include usage examples for each trait. The following sections discuss a few of these traits.

The Sized trait​

The Sized trait identifies types that have a measurable length, like strings and arrays.

Specifically, Sized requires a type to implement the __len__() method. This trait is used by the built-in len() function. For example, if you're writing a custom list type, you could implement this trait so your type works with len():

struct MyList(Sized):
    var size: Int
    # ...

    fn __init__(inout self):
        self.size = 0

    fn __len__(self) -> Int:
        return self.size

print(len(MyList()))
struct MyList(Sized):
    var size: Int
    # ...

    fn __init__(inout self):
        self.size = 0

    fn __len__(self) -> Int:
        return self.size

print(len(MyList()))
0

The Intable and IntableRaising traits​

The Intable trait identifies a type that can be implicitly converted to Int. The IntableRaising trait describes a type can be converted to an Int, but the conversion might raise an error.

Both of these traits require the type to implement the __int__() method. For example:

@value
struct Foo(Intable):
    var i: Int

    fn __int__(self) -> Int:
        return self.i

var foo = Foo(42)
print(int(foo) == 42)
@value
struct Foo(Intable):
    var i: Int

    fn __int__(self) -> Int:
        return self.i

var foo = Foo(42)
print(int(foo) == 42)
True

The Stringable, Representable, and Formattable traits​

The Stringable trait identifies a type that can be implicitly converted to String. The StringableRaising trait describes a type that can be converted to a String, but the conversion might raise an error. Any type that conforms to Stringable or StringableRaising also works with the built-in str() function to explicitly return a String. These traits also mean that the type can support both the {!s} and {} format specifiers of the String class' format() method. These traits require the type to define the __str__() method.

In contrast, the Representable trait that defines a type that can be used with the built-in repr() function, as well as the {!r} format specifier of the format() method. This trait requires the type to define the __repr__() method, which should compute the "official" string representation of a type. If at all possible, this should look like a valid Mojo expression that could be used to recreate a struct instance with the same value.

The StringRepresentable trait denotes a trait composition of the Stringable and Representable traits. It requires a type to implement both a __str__() and a __repr__() method.

The Formattable trait describes a type that can be converted to a stream of UTF-8 encoded data by writing to a formatter object. The print() function requires that its arguments conform to the Formattable trait. This enables efficient stream-based writing by default, avoiding unnecessary intermediate String heap allocations.

The Formattable trait requires a type to implement a format_to() method, which is provided with an instance of Formatter as an argument. You then invoke the Formatter instance's write() method to write a sequence of Formattable arguments constituting the String representation of your type.

While this might sound complex at first, in practice you can minimize boilerplate and duplicated code by using the String.format_sequence() static function to implement the type's Stringable implementation in terms of its Formattable implementation. Here is a simple example of a type that implements all of the Stringable, Representable, and Formattable traits:

@value
struct Dog(Stringable, Representable, Formattable):
    var name: String
    var age: Int

    fn __repr__(self) -> String:
        return "Dog(name=" + repr(self.name) + ", age=" + repr(self.age) + ")"

    fn __str__(self) -> String:
        return String.format_sequence(self)

    fn format_to(self, inout writer: Formatter) -> None:
        writer.write("Dog(", self.name, ", ", self.age, ")")

var dog = Dog("Rex", 5)
print(repr(dog))
print(dog)

var dog_info = String("String: {!s}\nRepresentation: {!r}").format(dog, dog)
print(dog_info)
@value
struct Dog(Stringable, Representable, Formattable):
    var name: String
    var age: Int

    fn __repr__(self) -> String:
        return "Dog(name=" + repr(self.name) + ", age=" + repr(self.age) + ")"

    fn __str__(self) -> String:
        return String.format_sequence(self)

    fn format_to(self, inout writer: Formatter) -> None:
        writer.write("Dog(", self.name, ", ", self.age, ")")

var dog = Dog("Rex", 5)
print(repr(dog))
print(dog)

var dog_info = String("String: {!s}\nRepresentation: {!r}").format(dog, dog)
print(dog_info)
Dog(name='Rex', age=5) Dog(Rex, 5) String: Dog(Rex, 5) Representation: Dog(name='Rex', age=5)

The AnyType trait​

When building a generic container type, one challenge is knowing how to dispose of the contained items when the container is destroyed. Any type that dynamically allocates memory needs to supply a destructor (__del__() method) that must be called to free the allocated memory. But not all types have a destructor, and your Mojo code has no way to determine which is which.

The AnyType trait solves this issue: every trait implicitly inherits from AnyType, and all structs conform to AnyType, which guarantees that the type has a destructor. For types that don't have one, Mojo adds a no-op destructor. This means you can call the destructor on any type.

This makes it possible to build generic collections without leaking memory. When the collection's destructor is called, it can safely call the destructors on every item it contains.

Generic structs with traits​

You can also use traits when defining a generic container. A generic container is a container (for example, an array or hashmap) that can hold different data types. In a dynamic language like Python it's easy to add different types of items to a container. But in a statically-typed environment the compiler needs to be able to identify the types at compile time. For example, if the container needs to copy a value, the compiler needs to verify that the type can be copied.

The List type is an example of a generic container. A single List can only hold a single type of data. For example, you can create a list of integer values like this:

from collections import List

var list = List[Int](1, 2, 3)
for i in range(len(list)):
    print(list[i], sep=" ", end="")
from collections import List

var list = List[Int](1, 2, 3)
for i in range(len(list)):
    print(list[i], sep=" ", end="")
1 2 3

You can use traits to define requirements for elements that are stored in a container. For example, List requires elements that can be moved and copied. To store a struct in a List, the struct needs to conform to the CollectionElement trait, which requires a copy constructor and a move constructor.

Building generic containers is an advanced topic. For an introduction, see the section on parameterized structs.

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