diff --git a/lib/src/evaluator.dart b/lib/src/evaluator.dart
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+++ b/lib/src/evaluator.dart
@@ -0,0 +1,264 @@
+import 'dart:math' as math;
+
+import 'address.dart';
+import 'ast.dart';
+import 'names.dart';
+import 'sheet.dart';
+import 'values.dart';
+import 'workbook.dart';
+
+typedef CellFunction = CellValue Function(List<Expr> args, EvalContext ctx);
+
+class Area {
+  final List<CellValue> cells;
+  final int rows;
+  final int cols;
+  const Area(this.cells, this.rows, this.cols);
+
+  CellValue at(int row, int col) => cells[row * cols + col];
+}
+
+class EvalContext {
+  final Workbook workbook;
+  final Sheet currentSheet;
+  final Map<String, CellFunction> functions;
+
+  EvalContext({
+    required this.workbook,
+    required this.currentSheet,
+    required this.functions,
+  });
+
+  CellValue evaluateCell(Expr expr) {
+    try {
+      return eval(expr);
+    } on EvalException catch (error) {
+      return error.error;
+    }
+  }
+
+  CellValue eval(Expr expr) {
+    switch (expr) {
+      case NumberLit(:final value):
+        return NumberValue(value);
+      case TextLit(:final value):
+        return TextValue(value);
+      case BoolLit(:final value):
+        return BooleanValue(value);
+      case ErrorLit(:final kind):
+        throw EvalException(ErrorValue(kind));
+      case NameNode(:final name):
+        return _evalName(name);
+      case RefNode(:final address, :final sheet):
+        return cellValue(sheet, address);
+      case RangeNode():
+        throw const EvalException(ErrorValue(ErrorKind.value));
+      case UnaryOp(:final op, :final operand):
+        return _applyUnary(op, eval(operand));
+      case BinaryOp(:final op, :final left, :final right):
+        return _applyBinary(op, eval(left), eval(right));
+      case FuncCall(:final name, :final args):
+        final fn = functions[name.toUpperCase()];
+        if (fn == null) throw const EvalException(ErrorValue(ErrorKind.name));
+        return fn(args, this);
+    }
+  }
+
+  CellValue _evalName(String name) {
+    switch (workbook.resolveName(name)) {
+      case CellName(:final sheet, :final address):
+        return sheet.valueAt(address);
+      case RangeName():
+        throw const EvalException(ErrorValue(ErrorKind.value));
+      case null:
+        throw const EvalException(ErrorValue(ErrorKind.name));
+    }
+  }
+
+  CellValue cellValue(String? sheetName, CellAddress address) {
+    final sheet = workbook.sheetFor(sheetName, currentSheet);
+    if (sheet == null) throw const EvalException(ErrorValue(ErrorKind.ref));
+    return sheet.valueAt(address);
+  }
+
+  List<CellValue> rangeValues(String? sheetName, CellAddress start, CellAddress end) {
+    final sheet = workbook.sheetFor(sheetName, currentSheet);
+    if (sheet == null) throw const EvalException(ErrorValue(ErrorKind.ref));
+    return [for (final address in expandRange(start, end)) sheet.valueAt(address)];
+  }
+
+  Area resolveArea(Expr arg) {
+    switch (arg) {
+      case RangeNode(:final start, :final end, :final sheet):
+        return Area(
+          rangeValues(sheet, start, end),
+          rangeRowCount(start, end),
+          rangeColCount(start, end),
+        );
+      case RefNode(:final address, :final sheet):
+        return Area([cellValue(sheet, address)], 1, 1);
+      case NameNode(:final name):
+        switch (workbook.resolveName(name)) {
+          case RangeName(:final sheet, :final start, :final end):
+            return Area(
+              [for (final address in expandRange(start, end)) sheet.valueAt(address)],
+              rangeRowCount(start, end),
+              rangeColCount(start, end),
+            );
+          case CellName(:final sheet, :final address):
+            return Area([sheet.valueAt(address)], 1, 1);
+          case null:
+            throw const EvalException(ErrorValue(ErrorKind.name));
+        }
+      default:
+        return Area([eval(arg)], 1, 1);
+    }
+  }
+
+  List<CellValue> flatten(List<Expr> args) {
+    final values = <CellValue>[];
+    for (final arg in args) {
+      values.addAll(resolveArea(arg).cells);
+    }
+    return values;
+  }
+
+  List<double> numbersFrom(List<Expr> args) {
+    final numbers = <double>[];
+    for (final value in flatten(args)) {
+      switch (value) {
+        case NumberValue(:final value):
+          numbers.add(value);
+        case ErrorValue():
+          throw EvalException(value);
+        default:
+          break;
+      }
+    }
+    return numbers;
+  }
+
+  double asNumber(CellValue value) {
+    switch (value) {
+      case NumberValue(value: final number):
+        return number;
+      case BooleanValue(value: final flag):
+        return flag ? 1.0 : 0.0;
+      case BlankValue():
+        return 0.0;
+      case TextValue(value: final text):
+        final trimmed = text.trim();
+        if (trimmed.isEmpty) return 0.0;
+        final parsed = double.tryParse(trimmed);
+        if (parsed == null) throw const EvalException(ErrorValue(ErrorKind.value));
+        return parsed;
+      case ErrorValue():
+        throw EvalException(value);
+    }
+  }
+
+  bool asBoolean(CellValue value) {
+    switch (value) {
+      case BooleanValue(value: final flag):
+        return flag;
+      case NumberValue(value: final number):
+        return number != 0;
+      case BlankValue():
+        return false;
+      case TextValue(value: final text):
+        switch (text.trim().toUpperCase()) {
+          case 'TRUE':
+            return true;
+          case 'FALSE':
+            return false;
+          default:
+            throw const EvalException(ErrorValue(ErrorKind.value));
+        }
+      case ErrorValue():
+        throw EvalException(value);
+    }
+  }
+
+  String asText(CellValue value) {
+    switch (value) {
+      case TextValue(value: final text):
+        return text;
+      case NumberValue(value: final number):
+        return formatNumber(number);
+      case BooleanValue(value: final flag):
+        return flag ? 'TRUE' : 'FALSE';
+      case BlankValue():
+        return '';
+      case ErrorValue():
+        throw EvalException(value);
+    }
+  }
+
+  CellValue _applyUnary(String op, CellValue operand) {
+    if (op == '-') return NumberValue(-asNumber(operand));
+    return NumberValue(asNumber(operand));
+  }
+
+  CellValue _applyBinary(String op, CellValue left, CellValue right) {
+    switch (op) {
+      case '+':
+        return NumberValue(asNumber(left) + asNumber(right));
+      case '-':
+        return NumberValue(asNumber(left) - asNumber(right));
+      case '*':
+        return NumberValue(asNumber(left) * asNumber(right));
+      case '/':
+        final divisor = asNumber(right);
+        if (divisor == 0) throw const EvalException(ErrorValue(ErrorKind.divZero));
+        return NumberValue(asNumber(left) / divisor);
+      case '%':
+        final divisor = asNumber(right);
+        if (divisor == 0) throw const EvalException(ErrorValue(ErrorKind.divZero));
+        return NumberValue(asNumber(left) % divisor);
+      case '^':
+        return NumberValue(math.pow(asNumber(left), asNumber(right)).toDouble());
+      case '&':
+        return TextValue(asText(left) + asText(right));
+      case '=':
+        return BooleanValue(compare(left, right) == 0);
+      case '<>':
+        return BooleanValue(compare(left, right) != 0);
+      case '<':
+        return BooleanValue(compare(left, right) < 0);
+      case '<=':
+        return BooleanValue(compare(left, right) <= 0);
+      case '>':
+        return BooleanValue(compare(left, right) > 0);
+      case '>=':
+        return BooleanValue(compare(left, right) >= 0);
+      default:
+        throw const EvalException(ErrorValue(ErrorKind.value));
+    }
+  }
+
+  int compare(CellValue left, CellValue right) {
+    if (left is ErrorValue) throw EvalException(left);
+    if (right is ErrorValue) throw EvalException(right);
+    final leftNumber = _numericOrNull(left);
+    final rightNumber = _numericOrNull(right);
+    if (leftNumber != null && rightNumber != null) {
+      return leftNumber.compareTo(rightNumber);
+    }
+    return asText(left).toUpperCase().compareTo(asText(right).toUpperCase());
+  }
+
+  double? _numericOrNull(CellValue value) {
+    switch (value) {
+      case NumberValue(value: final number):
+        return number;
+      case BooleanValue(value: final flag):
+        return flag ? 1.0 : 0.0;
+      case BlankValue():
+        return 0.0;
+      case TextValue(value: final text):
+        return double.tryParse(text.trim());
+      case ErrorValue():
+        return null;
+    }
+  }
+}