[ADT in Python] Algebraic data type (sum type) in Python #demo

adt.py

from dataclasses import dataclass


@dataclass(frozen=True)
class Var:
    name: str


@dataclass(frozen=True)
class Int:
    val: int


@dataclass(frozen=True)
class Add:
    left: "Exp"
    right: "Exp"


@dataclass(frozen=True)
class Mul:
    left: "Exp"
    right: "Exp"


@dataclass(frozen=True)
class Assgn:
    var: str
    val: "Exp"
    exp: "Exp"


Exp = Var | Int | Add | Mul | Assgn
Env = dict[str, int]


def eval(exp: Exp, env: Env) -> int:
    match exp:
        case Var(name):
            return env[name]
        case Int(val):
            return val
        case Add(left, right):
            return eval(left, env) + eval(right, env)
        case Mul(left, right):
            return eval(left, env) * eval(right, env)
        case Assgn(var, val, exp):
            new_env = env.copy()
            new_env[var] = eval(val, env)
            return eval(exp, new_env)


exp: Exp = Assgn(
    "x",
    Int(2),
    Mul(
        Assgn("x", Int(7), Add(Var("x"), Assgn("y", Add(Int(3), Int(8)), Var("y")))),
        Var("x"),
    ),
)
print(eval(exp, {}))