calculator: add an evaluation visitor

The `calculator.eval` module defines an `Evaluator` visitor for the AST
hierarchy.

calculator/calculator/__init__.py

@@ -1 +1,2 @@
 import calculator.ast
+import calculator.eval

calculator/calculator/eval/__init__.py

@@ -0,0 +1 @@
+from .evaluator import Evaluator

calculator/calculator/eval/evaluator.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import dataclasses
+from typing import TYPE_CHECKING
+
+from calculator.ast.visit import Visitor
+from pydantic.dataclasses import dataclass
+
+if TYPE_CHECKING:
+    from calculator.ast import BinOp, Constant, Node, UnaryOp
+
+
+@dataclass
+class Evaluator(Visitor):
+    """
+    Evaluate a Tree and retrieve the calculated value.
+    """
+
+    value: int = dataclasses.field(default=0, init=False)
+
+    def visit_and_get_value(self, n: Node) -> int:
+        n.accept(self)
+        return self.value
+
+    def visit_constant(self, c: Constant) -> None:
+        self.value = c.value
+
+    def visit_binop(self, b: BinOp) -> None:
+        lhs_val = self.visit_and_get_value(b.lhs)
+        rhs_val = self.visit_and_get_value(b.rhs)
+        self.value = b.op(lhs_val, rhs_val)
+
+    def visit_unaryop(self, b: UnaryOp) -> None:
+        rhs_val = self.visit_and_get_value(b.rhs)
+        self.value = b.op(rhs_val)

calculator/calculator/eval/test_evaluator.py

@@ -0,0 +1,34 @@
+import pytest
+from calculator.ast import BinOp, Constant, UnaryOp
+
+from .evaluator import Evaluator
+
+
+@pytest.fixture
+def vis():
+    return Evaluator()
+
+
+def test_evaluate_constant(vis):
+    c = Constant(42)
+    assert vis.visit_and_get_value(c) == 42
+
+
+def test_evaluate_unaryop_identity(vis):
+    op = UnaryOp(lambda x: x, Constant(42))
+    assert vis.visit_and_get_value(op) == 42
+
+
+def test_evaluate_unaryop_negate(vis):
+    op = UnaryOp(lambda x: -x, Constant(-42))
+    assert vis.visit_and_get_value(op) == 42
+
+
+def test_evaluate_binop_plus(vis):
+    op = BinOp(lambda x, y: x + y, Constant(12), Constant(27))
+    assert vis.visit_and_get_value(op) == 39
+
+
+def test_evaluate_binop_minus(vis):
+    op = BinOp(lambda x, y: x - y, Constant(42), Constant(51))
+    assert vis.visit_and_get_value(op) == -9