calculator: add initial AST implementation

The AST's class hierarchy is defined inside the `calculator.ast` module.

An abstract visitor is defined inside the `calculator.ast.visit`
package.

calculator/calculator/__init__.py

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+import calculator.ast

calculator/calculator/ast/__init__.py

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+from .node import Node  # isort:skip
+from .binop import BinOp
+from .constant import Constant
+from .unaryop import UnaryOp

calculator/calculator/ast/binop.py

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+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Callable
+
+from pydantic.dataclasses import dataclass
+
+from .node import Node
+
+if TYPE_CHECKING:
+    from calculator.ast.visit import Visitor
+
+
+class Config:
+    arbitrary_types_allowed = True
+
+
+@dataclass(config=Config)
+class BinOp(Node):
+    """
+    Node to represent a binary operation
+    """
+
+    op: Callable[[int, int], int]
+    lhs: Node
+    rhs: Node
+
+    def accept(self, v: Visitor) -> None:
+        v.visit_binop(self)

calculator/calculator/ast/constant.py

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+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from pydantic.dataclasses import dataclass
+
+from .node import Node
+
+if TYPE_CHECKING:
+    from calculator.ast.visit import Visitor
+
+
+class Config:
+    arbitrary_types_allowed = True
+
+
+@dataclass(config=Config)
+class Constant(Node):
+    """
+    Node to represent a constant value.
+    """
+
+    value: int
+
+    def accept(self, v: Visitor) -> None:
+        v.visit_constant(self)

calculator/calculator/ast/node.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from calculator.ast.visit import Visitor
+
+
+class Node(ABC):
+    """
+    Abstract Node class for calculator AST.
+    """
+
+    @abstractmethod
+    def accept(self, v: Visitor) -> None:
+        pass

calculator/calculator/ast/unaryop.py

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+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Callable
+
+from pydantic.dataclasses import dataclass
+
+from .node import Node
+
+if TYPE_CHECKING:
+    from calculator.ast.visit import Visitor
+
+
+class Config:
+    arbitrary_types_allowed = True
+
+
+@dataclass(config=Config)
+class UnaryOp(Node):
+    """
+    Node to represent a unary operation
+    """
+
+    op: Callable[[int], int]
+    rhs: Node
+
+    def accept(self, v: Visitor) -> None:
+        v.visit_unaryop(self)

calculator/calculator/ast/visit/__init__.py

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+from .visitor import Visitor  # isort:skip

calculator/calculator/ast/visit/visitor.py

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+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from calculator.ast import BinOp, Constant, Node, UnaryOp
+
+
+class Visitor(ABC):
+    """
+    Abstract Visitor class for the AST class hierarchy.
+    """
+
+    def visit(self, n: Node) -> None:
+        n.accept(self)
+
+    @abstractmethod
+    def visit_constant(self, c: Constant) -> None:
+        pass
+
+    @abstractmethod
+    def visit_binop(self, b: BinOp) -> None:
+        pass
+
+    @abstractmethod
+    def visit_unaryop(self, b: UnaryOp) -> None:
+        pass