codon/docs/language/classes.md

Codon supports classes just like Python. However, you must declare class members and their types in the preamble of each class (like you would do with Python's dataclasses):

class Foo:
    x: int
    y: int

    def __init__(self, x: int, y: int):  # constructor
        self.x, self.y = x, y

    def method(self):
        print(self.x, self.y)

f = Foo(1, 2)
f.method()  # prints "1 2"

Unlike Python, Codon supports method overloading:

class Foo:
    x: int
    y: int

    def __init__(self):                    # constructor
        self.x, self.y = 0, 0

    def __init__(self, x: int, y: int):    # another constructor
        self.x, self.y = x, y

    def __init__(self, x: int, y: float):  # yet another constructor
        self.x, self.y = x, int(y)

    def method(self: Foo):
        print(self.x, self.y)

Foo().method()          # prints "0 0"
Foo(1, 2).method()      # prints "1 2"
Foo(1, 2.3).method()    # prints "1 2"
Foo(1.1, 2.3).method()  # error: there is no Foo.__init__(float, float)

Classes can also be generic:

class Container[T]:
    elements: List[T]

    def __init__(self, elements: List[T]):
        self.elements = elements

Classes create objects that are passed by reference:

class Point:
    x: int
    y: int

p = Point(1, 2)
q = p  # this is a reference!
p.x = 2
print((p.x, p.y), (q.x, q.y))  # (2, 2), (2, 2)

If you need to copy an object's contents, implement the __copy__ magic method and use q = copy(p) instead.

Classes can inherit from other classes:

class NamedPoint(Point):
    name: str

    def __init__(self, x: int, y: int, name: str):
        super().__init__(x, y)
        self.name = name

{% hint style="warning" %} Currently, inheritance in Codon is static and polymorphism is not supported. {% endhint %}

Named tuples

Codon also supports pass-by-value types via the @tuple annotation, which are effectively named tuples (equivalent to Python's collections.namedtuple):

@tuple
class Point:
    x: int
    y: int

p = Point(1, 2)
q = p  # this is a copy!
print((p.x, p.y), (q.x, q.y))  # (1, 2), (1, 2)

However, named tuples are immutable. The following code will not compile:

p = Point(1, 2)
p.x = 2  # error: immutable type

You can also add methods to named tuples:

@tuple
class Point:
    x: int
    y: int

    def __new__():          # named tuples are constructed via __new__, not __init__
        return Point(0, 1)

    def some_method(self):
        return self.x + self.y

p = Point()             # p is (0, 1)
print(p.some_method())  # 1

Type extensions

Suppose you have a class that lacks a method or an operator that might be really useful. Codon provides an @extend annotation that allows programmers to add and modify methods of various types at compile time, including built-in types like int or str. This actually allows much of the functionality of built-in types to be implemented in Codon as type extensions in the standard library.

class Foo:
    ...

f = Foo(...)

# We need foo.cool() but it does not exist... not a problem for Codon
@extend
class Foo:
    def cool(self: Foo):
        ...

f.cool()  # works!

# Let's add support for adding integers and strings:
@extend
class int:
    def __add__(self: int, other: str):
        return self + int(other)

print(5 + '4')  # 9

Note that all type extensions are performed strictly at compile time and incur no runtime overhead.

Magic methods

Here is a list of useful magic methods that you might want to add and overload:

Magic method	Description
__copy__	copy-constructor for copy method
__len__	for len method
__bool__	for bool method and condition checking
__getitem__	overload obj[key]
__setitem__	overload obj[key] = value
__delitem__	overload del obj[key]
__iter__	support iterating over the object
__repr__	support printing and str conversion