Add expression-evaluator: DAGs & state machines tutorial project

Educational calculator teaching FSMs (explicit transition table tokenizer)
and DAGs (recursive descent parser with AST evaluation). Includes CLI with
REPL, graphviz visualization, and 61 tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/expression-evaluator/tests/test_parser.py

@@ -0,0 +1,136 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+import pytest
+from tokenizer import tokenize, TokenType
+from parser import Parser, ParseError, NumberNode, BinOpNode, UnaryOpNode
+
+
+def parse(expr):
+    """Helper: tokenize and parse in one step."""
+    return Parser(tokenize(expr)).parse()
+
+
+# ---------- Basic parsing ----------
+
+def test_parse_number():
+    ast = parse("42")
+    assert isinstance(ast, NumberNode)
+    assert ast.value == 42.0
+
+def test_parse_decimal():
+    ast = parse("3.14")
+    assert isinstance(ast, NumberNode)
+    assert ast.value == 3.14
+
+def test_parse_addition():
+    ast = parse("3 + 4")
+    assert isinstance(ast, BinOpNode)
+    assert ast.op == TokenType.PLUS
+    assert isinstance(ast.left, NumberNode)
+    assert isinstance(ast.right, NumberNode)
+
+
+# ---------- Precedence ----------
+
+def test_multiply_before_add():
+    """3 + 4 * 2 should parse as 3 + (4 * 2)."""
+    ast = parse("3 + 4 * 2")
+    assert ast.op == TokenType.PLUS
+    assert isinstance(ast.right, BinOpNode)
+    assert ast.right.op == TokenType.MULTIPLY
+
+def test_power_before_multiply():
+    """2 * 3 ^ 4 should parse as 2 * (3 ^ 4)."""
+    ast = parse("2 * 3 ^ 4")
+    assert ast.op == TokenType.MULTIPLY
+    assert isinstance(ast.right, BinOpNode)
+    assert ast.right.op == TokenType.POWER
+
+def test_parentheses_override_precedence():
+    """(3 + 4) * 2 should parse as (3 + 4) * 2."""
+    ast = parse("(3 + 4) * 2")
+    assert ast.op == TokenType.MULTIPLY
+    assert isinstance(ast.left, BinOpNode)
+    assert ast.left.op == TokenType.PLUS
+
+
+# ---------- Associativity ----------
+
+def test_left_associative_subtraction():
+    """10 - 3 - 2 should parse as (10 - 3) - 2."""
+    ast = parse("10 - 3 - 2")
+    assert ast.op == TokenType.MINUS
+    assert isinstance(ast.left, BinOpNode)
+    assert ast.left.op == TokenType.MINUS
+    assert isinstance(ast.right, NumberNode)
+
+def test_power_right_associative():
+    """2 ^ 3 ^ 4 should parse as 2 ^ (3 ^ 4)."""
+    ast = parse("2 ^ 3 ^ 4")
+    assert ast.op == TokenType.POWER
+    assert isinstance(ast.left, NumberNode)
+    assert isinstance(ast.right, BinOpNode)
+    assert ast.right.op == TokenType.POWER
+
+
+# ---------- Unary minus ----------
+
+def test_unary_minus():
+    ast = parse("-3")
+    assert isinstance(ast, UnaryOpNode)
+    assert ast.operand.value == 3.0
+
+def test_double_negation():
+    ast = parse("--3")
+    assert isinstance(ast, UnaryOpNode)
+    assert isinstance(ast.operand, UnaryOpNode)
+    assert ast.operand.operand.value == 3.0
+
+def test_unary_minus_precedence():
+    """-3^2 should parse as -(3^2), not (-3)^2."""
+    ast = parse("-3 ^ 2")
+    assert isinstance(ast, UnaryOpNode)
+    assert isinstance(ast.operand, BinOpNode)
+    assert ast.operand.op == TokenType.POWER
+
+def test_unary_minus_in_expression():
+    """2 * -3 should parse as 2 * (-(3))."""
+    ast = parse("2 * -3")
+    assert ast.op == TokenType.MULTIPLY
+    assert isinstance(ast.right, UnaryOpNode)
+
+
+# ---------- Nested parentheses ----------
+
+def test_nested_parens():
+    ast = parse("((3))")
+    assert isinstance(ast, NumberNode)
+    assert ast.value == 3.0
+
+def test_complex_nesting():
+    """((2 + 3) * (7 - 2))"""
+    ast = parse("((2 + 3) * (7 - 2))")
+    assert isinstance(ast, BinOpNode)
+    assert ast.op == TokenType.MULTIPLY
+
+
+# ---------- Errors ----------
+
+def test_missing_rparen():
+    with pytest.raises(ParseError):
+        parse("(3 + 4")
+
+def test_empty_expression():
+    with pytest.raises(ParseError):
+        parse("")
+
+def test_trailing_operator():
+    with pytest.raises(ParseError):
+        parse("3 +")
+
+def test_empty_parens():
+    with pytest.raises(ParseError):
+        parse("()")