Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/polymer_expressions/eval.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library polymer_expressions.eval;
+
+import 'dart:async';
+import 'dart:collection';
+
+import 'package:observe/observe.dart';
+import 'package:smoke/smoke.dart' as smoke;
+
+import 'async.dart';
+import 'expression.dart';
+import 'filter.dart';
+import 'visitor.dart';
+
+final _BINARY_OPERATORS = {
+  '+':   (a, b) => a + b,
+  '-':   (a, b) => a - b,
+  '*':   (a, b) => a * b,
+  '/':   (a, b) => a / b,
+  '%':   (a, b) => a % b,
+  '==':  (a, b) => a == b,
+  '!=':  (a, b) => a != b,
+  '===': (a, b) => identical(a, b),
+  '!==': (a, b) => !identical(a, b),
+  '>':   (a, b) => a > b,
+  '>=':  (a, b) => a >= b,
+  '<':   (a, b) => a < b,
+  '<=':  (a, b) => a <= b,
+  '||':  (a, b) => a || b,
+  '&&':  (a, b) => a && b,
+  '|':   (a, f) {
+    if (f is Transformer) return f.forward(a);
+    if (f is Filter) return f(a);
+    throw new EvalException("Filters must be a one-argument function.");
+  }
+};
+
+final _UNARY_OPERATORS = {
+  '+': (a) => a,
+  '-': (a) => -a,
+  '!': (a) => !a,
+};
+
+final _BOOLEAN_OPERATORS = ['!', '||', '&&'];
+
+/**
+ * Evaluation [expr] in the context of [scope].
+ */
+Object eval(Expression expr, Scope scope) => new EvalVisitor(scope).visit(expr);
+
+/**
+ * Returns an [ExpressionObserver] that evaluates [expr] in the context of
+ * scope] and listens for any changes on [Observable] values that are
+ * returned from sub-expressions. When a value changes the expression is
+ * reevaluated and the new result is sent to the [onUpdate] stream of the
+ * [ExpressionObsserver].
+ */
+ExpressionObserver observe(Expression expr, Scope scope) {
+  var observer = new ObserverBuilder().visit(expr);
+  return observer;
+}
+
+/**
+ * Causes [expr] to be reevaluated a returns it's value.
+ */
+Object update(ExpressionObserver expr, Scope scope, {skipChanges: false}) {
+  new Updater(scope, skipChanges).visit(expr);
+  return expr.currentValue;
+}
+
+/**
+ * Assign [value] to the variable or field referenced by [expr] in the context
+ * of [scope].
+ *
+ * [expr] must be an /assignable/ expression, it must not contain
+ * operators or function invocations, and any index operations must use a
+ * literal index.
+ */
+Object assign(Expression expr, Object value, Scope scope,
+    {bool checkAssignability: true}) {
+
+  Expression expression;
+  var property;
+  bool isIndex = false;
+  var filters = <Expression>[]; // reversed order for assignment
+
+  while (expr is BinaryOperator) {
+    BinaryOperator op = expr;
+    if (op.operator != '|') {
+      break;
+    }
+    filters.add(op.right);
+    expr = op.left;
+  }
+
+  if (expr is Identifier) {
+    expression = empty();
+    property = expr.value;
+  } else if (expr is Index) {
+    expression = expr.receiver;
+    property = expr.argument;
+    isIndex = true;
+  } else if (expr is Getter) {
+    expression = expr.receiver;
+    property = expr.name;
+  } else {
+    if (checkAssignability) {
+      throw new EvalException("Expression is not assignable: $expr");
+    }
+    return null;
+  }
+
+  // transform the values backwards through the filters
+  for (var filterExpr in filters) {
+    var filter = eval(filterExpr, scope);
+    if (filter is! Transformer) {
+      if (checkAssignability) {
+        throw new EvalException("filter must implement Transformer to be "
+            "assignable: $filterExpr");
+      } else {
+        return null;
+      }
+    }
+    value = filter.reverse(value);
+  }
+  // evaluate the receiver
+  var o = eval(expression, scope);
+
+  // can't assign to a property on a null LHS object. Silently fail.
+  if (o == null) return null;
+
+  if (isIndex) {
+    var index = eval(property, scope);
+    o[index] = value;
+  } else {
+    smoke.write(o, smoke.nameToSymbol(property), value);
+  }
+  return value;
+}
+
+
+/**
+ * A scope in polymer expressions that can map names to objects. Scopes contain
+ * a set of named variables and a unique model object. The scope structure
+ * is then used to lookup names using the `[]` operator. The lookup first
+ * searches for the name in local variables, then in global variables,
+ * and then finally looks up the name as a property in the model.
+ */
+abstract class Scope implements Indexable<String, Object> {
+  Scope._();
+
+  /** Create a scope containing a [model] and all of [variables]. */
+  factory Scope({Object model, Map<String, Object> variables}) {
+    var scope = new _ModelScope(model);
+    return variables == null ? scope
+        : new _GlobalsScope(new Map<String, Object>.from(variables), scope);
+  }
+
+  /** Return the unique model in this scope. */
+  Object get model;
+
+  /**
+   * Lookup the value of [name] in the current scope. If [name] is 'this', then
+   * we return the [model]. For any other name, this finds the first variable
+   * matching [name] or, if none exists, the property [name] in the [model].
+   */
+  Object operator [](String name);
+
+  operator []=(String name, Object value) {
+    throw new UnsupportedError('[]= is not supported in Scope.');
+  }
+
+  /**
+   * Returns whether [name] is defined in [model], that is, a lookup
+   * would not find a variable with that name, but there is a non-null model
+   * where we can look it up as a property.
+   */
+  bool _isModelProperty(String name);
+
+  /** Create a new scope extending this scope with an additional variable. */
+  Scope childScope(String name, Object value) =>
+      new _LocalVariableScope(name, value, this);
+}
+
+/**
+ * A scope that looks up names in a model object. This kind of scope has no
+ * parent scope because all our lookup operations stop when we reach the model
+ * object. Any variables added in scope or global variables are added as child
+ * scopes.
+ */
+class _ModelScope extends Scope {
+  final Object model;
+
+  _ModelScope(this.model) : super._();
+
+  Object operator[](String name) {
+    if (name == 'this') return model;
+    var symbol = smoke.nameToSymbol(name);
+    if (model == null || symbol == null) {
+      throw new EvalException("variable '$name' not found");
+    }
+    return _convert(smoke.read(model, symbol));
+  }
+
+  Object _isModelProperty(String name) => name != 'this';
+
+  String toString() => "[model: $model]";
+}
+
+/**
+ * A scope that holds a reference to a single variable. Polymer expressions
+ * introduce variables to the scope one at a time. Each time a variable is
+ * added, a new [_LocalVariableScope] is created.
+ */
+class _LocalVariableScope extends Scope {
+  final Scope parent;
+  final String varName;
+  // TODO(sigmund,justinfagnani): make this @observable?
+  final Object value;
+
+  _LocalVariableScope(this.varName, this.value, this.parent) : super._() {
+    if (varName == 'this') {
+      throw new EvalException("'this' cannot be used as a variable name.");
+    }
+  }
+
+  Object get model => parent != null ? parent.model : null;
+
+  Object operator[](String name) {
+    if (varName == name) return _convert(value);
+    if (parent != null) return parent[name];
+    throw new EvalException("variable '$name' not found");
+  }
+
+  bool _isModelProperty(String name) {
+    if (varName == name) return false;
+    return parent == null ? false : parent._isModelProperty(name);
+  }
+
+  String toString() => "$parent > [local: $varName]";
+}
+
+/** A scope that holds a reference to a global variables. */
+class _GlobalsScope extends Scope {
+  final _ModelScope parent;
+  final Map<String, Object> variables;
+
+  _GlobalsScope(this.variables, this.parent) : super._() {
+    if (variables.containsKey('this')) {
+      throw new EvalException("'this' cannot be used as a variable name.");
+    }
+  }
+
+  Object get model => parent != null ? parent.model : null;
+
+  Object operator[](String name) {
+    if (variables.containsKey(name)) return _convert(variables[name]);
+    if (parent != null) return parent[name];
+    throw new EvalException("variable '$name' not found");
+  }
+
+  bool _isModelProperty(String name) {
+    if (variables.containsKey(name)) return false;
+    return parent == null ? false : parent._isModelProperty(name);
+  }
+
+  String toString() => "$parent > [global: ${variables.keys}]";
+}
+
+Object _convert(v) => v is Stream ? new StreamBinding(v) : v;
+
+abstract class ExpressionObserver<E extends Expression> implements Expression {
+  final E _expr;
+  ExpressionObserver _parent;
+
+  StreamSubscription _subscription;
+  Object _value;
+
+  StreamController _controller = new StreamController.broadcast();
+  Stream get onUpdate => _controller.stream;
+
+  ExpressionObserver(this._expr);
+
+  Expression get expression => _expr;
+
+  Object get currentValue => _value;
+
+  update(Scope scope) => _updateSelf(scope);
+
+  _updateSelf(Scope scope) {}
+
+  _invalidate(Scope scope) {
+    _observe(scope, false);
+    if (_parent != null) {
+      _parent._invalidate(scope);
+    }
+  }
+
+  _unobserve() {
+    if (_subscription != null) {
+      _subscription.cancel();
+      _subscription = null;
+    }
+  }
+
+  _observe(Scope scope, skipChanges) {
+    _unobserve();
+
+    var _oldValue = _value;
+
+    // evaluate
+    _updateSelf(scope);
+
+    if (!skipChanges && !identical(_value, _oldValue)) {
+      _controller.add(_value);
+    }
+  }
+
+  String toString() => _expr.toString();
+}
+
+class Updater extends RecursiveVisitor {
+  final Scope scope;
+  final bool skipChanges;
+
+  Updater(this.scope, [this.skipChanges = false]);
+
+  visitExpression(ExpressionObserver e) {
+    e._observe(scope, skipChanges);
+  }
+}
+
+class Closer extends RecursiveVisitor {
+  static final _instance = new Closer._();
+  factory Closer() => _instance;
+  Closer._();
+
+  visitExpression(ExpressionObserver e) {
+    e._unobserve();
+  }
+}
+
+class EvalVisitor extends Visitor {
+  final Scope scope;
+
+  EvalVisitor(this.scope);
+
+  visitEmptyExpression(EmptyExpression e) => scope.model;
+
+  visitParenthesizedExpression(ParenthesizedExpression e) => visit(e.child);
+
+  visitGetter(Getter g) {
+    var receiver = visit(g.receiver);
+    if (receiver == null) return null;
+    var symbol = smoke.nameToSymbol(g.name);
+    return smoke.read(receiver, symbol);
+  }
+
+  visitIndex(Index i) {
+    var receiver = visit(i.receiver);
+    if (receiver == null) return null;
+    var key = visit(i.argument);
+    return receiver[key];
+  }
+
+  visitInvoke(Invoke i) {
+    var receiver = visit(i.receiver);
+    if (receiver == null) return null;
+    var args = (i.arguments == null)
+        ? null
+        : i.arguments.map(visit).toList(growable: false);
+
+    if (i.method == null) {
+      assert(receiver is Function);
+      return Function.apply(receiver, args);
+    }
+
+    var symbol = smoke.nameToSymbol(i.method);
+    return smoke.invoke(receiver, symbol, args);
+  }
+
+  visitLiteral(Literal l) => l.value;
+
+  visitListLiteral(ListLiteral l) => l.items.map(visit).toList();
+
+  visitMapLiteral(MapLiteral l) {
+    var map = {};
+    for (var entry in l.entries) {
+      var key = visit(entry.key);
+      var value = visit(entry.entryValue);
+      map[key] = value;
+    }
+    return map;
+  }
+
+  visitMapLiteralEntry(MapLiteralEntry e) =>
+      throw new UnsupportedError("should never be called");
+
+  visitIdentifier(Identifier i) => scope[i.value];
+
+  visitBinaryOperator(BinaryOperator o) {
+    var operator = o.operator;
+    var left = visit(o.left);
+    var right = visit(o.right);
+
+    var f = _BINARY_OPERATORS[operator];
+    if (operator == '&&' || operator == '||') {
+      // TODO: short-circuit
+      return f(_toBool(left), _toBool(right));
+    } else if (operator == '==' || operator == '!=') {
+      return f(left, right);
+    } else if (left == null || right == null) {
+      return null;
+    }
+    return f(left, right);
+  }
+
+  visitUnaryOperator(UnaryOperator o) {
+    var expr = visit(o.child);
+    var f = _UNARY_OPERATORS[o.operator];
+    if (o.operator == '!') {
+      return f(_toBool(expr));
+    }
+    return (expr == null) ? null : f(expr);
+  }
+
+  visitTernaryOperator(TernaryOperator o) =>
+      visit(o.condition) == true ? visit(o.trueExpr) : visit(o.falseExpr);
+
+  visitInExpression(InExpression i) =>
+      throw new UnsupportedError("can't eval an 'in' expression");
+
+  visitAsExpression(AsExpression i) =>
+      throw new UnsupportedError("can't eval an 'as' expression");
+}
+
+class ObserverBuilder extends Visitor {
+  final Queue parents = new Queue();
+
+  ObserverBuilder();
+
+  visitEmptyExpression(EmptyExpression e) => new EmptyObserver(e);
+
+  visitParenthesizedExpression(ParenthesizedExpression e) => visit(e.child);
+
+  visitGetter(Getter g) {
+    var receiver = visit(g.receiver);
+    var getter = new GetterObserver(g, receiver);
+    receiver._parent = getter;
+    return getter;
+  }
+
+  visitIndex(Index i) {
+    var receiver = visit(i.receiver);
+    var arg = visit(i.argument);
+    var index =  new IndexObserver(i, receiver, arg);
+    receiver._parent = index;
+    arg._parent = index;
+    return index;
+  }
+
+  visitInvoke(Invoke i) {
+    var receiver = visit(i.receiver);
+    var args = (i.arguments == null)
+        ? null
+        : i.arguments.map(visit).toList(growable: false);
+    var invoke =  new InvokeObserver(i, receiver, args);
+    receiver._parent = invoke;
+    if (args != null) args.forEach((a) => a._parent = invoke);
+    return invoke;
+  }
+
+  visitLiteral(Literal l) => new LiteralObserver(l);
+
+  visitListLiteral(ListLiteral l) {
+    var items = l.items.map(visit).toList(growable: false);
+    var list = new ListLiteralObserver(l, items);
+    items.forEach((e) => e._parent = list);
+    return list;
+  }
+
+  visitMapLiteral(MapLiteral l) {
+    var entries = l.entries.map(visit).toList(growable: false);
+    var map = new MapLiteralObserver(l, entries);
+    entries.forEach((e) => e._parent = map);
+    return map;
+  }
+
+  visitMapLiteralEntry(MapLiteralEntry e) {
+    var key = visit(e.key);
+    var value = visit(e.entryValue);
+    var entry = new MapLiteralEntryObserver(e, key, value);
+    key._parent = entry;
+    value._parent = entry;
+    return entry;
+  }
+
+  visitIdentifier(Identifier i) => new IdentifierObserver(i);
+
+  visitBinaryOperator(BinaryOperator o) {
+    var left = visit(o.left);
+    var right = visit(o.right);
+    var binary = new BinaryObserver(o, left, right);
+    left._parent = binary;
+    right._parent = binary;
+    return binary;
+  }
+
+  visitUnaryOperator(UnaryOperator o) {
+    var expr = visit(o.child);
+    var unary = new UnaryObserver(o, expr);
+    expr._parent = unary;
+    return unary;
+  }
+
+  visitTernaryOperator(TernaryOperator o) {
+    var condition = visit(o.condition);
+    var trueExpr = visit(o.trueExpr);
+    var falseExpr = visit(o.falseExpr);
+    var ternary = new TernaryObserver(o, condition, trueExpr, falseExpr);
+    condition._parent = ternary;
+    trueExpr._parent = ternary;
+    falseExpr._parent = ternary;
+    return ternary;
+  }
+
+  visitInExpression(InExpression i) {
+    throw new UnsupportedError("can't eval an 'in' expression");
+  }
+
+  visitAsExpression(AsExpression i) {
+    throw new UnsupportedError("can't eval an 'as' expression");
+  }
+}
+
+class EmptyObserver extends ExpressionObserver<EmptyExpression>
+    implements EmptyExpression {
+
+  EmptyObserver(EmptyExpression value) : super(value);
+
+  _updateSelf(Scope scope) {
+    _value = scope.model;
+    // TODO(justin): listen for scope.model changes?
+  }
+
+  accept(Visitor v) => v.visitEmptyExpression(this);
+}
+
+class LiteralObserver extends ExpressionObserver<Literal> implements Literal {
+
+  LiteralObserver(Literal value) : super(value);
+
+  dynamic get value => _expr.value;
+
+  _updateSelf(Scope scope) {
+    _value = _expr.value;
+  }
+
+  accept(Visitor v) => v.visitLiteral(this);
+}
+
+class ListLiteralObserver extends ExpressionObserver<ListLiteral>
+    implements ListLiteral {
+
+  final List<ExpressionObserver> items;
+
+  ListLiteralObserver(ListLiteral value, this.items) : super(value);
+
+  _updateSelf(Scope scope) {
+    _value = items.map((i) => i._value).toList();
+  }
+
+  accept(Visitor v) => v.visitListLiteral(this);
+}
+
+class MapLiteralObserver extends ExpressionObserver<MapLiteral>
+    implements MapLiteral {
+
+  final List<MapLiteralEntryObserver> entries;
+
+  MapLiteralObserver(MapLiteral value, this.entries) : super(value);
+
+  _updateSelf(Scope scope) {
+    _value = entries.fold(new Map(),
+        (m, e) => m..[e.key._value] = e.entryValue._value);
+  }
+
+  accept(Visitor v) => v.visitMapLiteral(this);
+}
+
+class MapLiteralEntryObserver extends ExpressionObserver<MapLiteralEntry>
+    implements MapLiteralEntry {
+
+  final LiteralObserver key;
+  final ExpressionObserver entryValue;
+
+  MapLiteralEntryObserver(MapLiteralEntry value, this.key, this.entryValue)
+      : super(value);
+
+  accept(Visitor v) => v.visitMapLiteralEntry(this);
+}
+
+class IdentifierObserver extends ExpressionObserver<Identifier>
+    implements Identifier {
+
+  IdentifierObserver(Identifier value) : super(value);
+
+  String get value => _expr.value;
+
+  _updateSelf(Scope scope) {
+    _value = scope[value];
+    if (!scope._isModelProperty(value)) return;
+    var model = scope.model;
+    if (model is! Observable) return;
+    var symbol = smoke.nameToSymbol(value);
+    _subscription = (model as Observable).changes.listen((changes) {
+      if (changes.any((c) => c is PropertyChangeRecord && c.name == symbol)) {
+        _invalidate(scope);
+      }
+    });
+  }
+
+  accept(Visitor v) => v.visitIdentifier(this);
+}
+
+class ParenthesizedObserver extends ExpressionObserver<ParenthesizedExpression>
+    implements ParenthesizedExpression {
+  final ExpressionObserver child;
+
+  ParenthesizedObserver(ParenthesizedExpression expr, this.child) : super(expr);
+
+
+  _updateSelf(Scope scope) {
+    _value = child._value;
+  }
+
+  accept(Visitor v) => v.visitParenthesizedExpression(this);
+}
+
+class UnaryObserver extends ExpressionObserver<UnaryOperator>
+    implements UnaryOperator {
+  final ExpressionObserver child;
+
+  UnaryObserver(UnaryOperator expr, this.child) : super(expr);
+
+  String get operator => _expr.operator;
+
+  _updateSelf(Scope scope) {
+    var f = _UNARY_OPERATORS[_expr.operator];
+    if (operator == '!') {
+      _value = f(_toBool(child._value));
+    } else {
+      _value = (child._value == null) ? null : f(child._value);
+    }
+  }
+
+  accept(Visitor v) => v.visitUnaryOperator(this);
+}
+
+class BinaryObserver extends ExpressionObserver<BinaryOperator>
+    implements BinaryOperator {
+
+  final ExpressionObserver left;
+  final ExpressionObserver right;
+
+  BinaryObserver(BinaryOperator expr, this.left, this.right)
+      : super(expr);
+
+  String get operator => _expr.operator;
+
+  _updateSelf(Scope scope) {
+    var f = _BINARY_OPERATORS[operator];
+    if (operator == '&&' || operator == '||') {
+      _value = f(_toBool(left._value), _toBool(right._value));
+    } else if (operator == '==' || operator == '!=') {
+      _value = f(left._value, right._value);
+    } else if (left._value == null || right._value == null) {
+      _value = null;
+    } else {
+      if (operator == '|' && left._value is ObservableList) {
+        _subscription = (left._value as ObservableList).listChanges
+            .listen((_) => _invalidate(scope));
+      }
+      _value = f(left._value, right._value);
+    }
+  }
+
+  accept(Visitor v) => v.visitBinaryOperator(this);
+
+}
+
+class TernaryObserver extends ExpressionObserver<TernaryOperator>
+    implements TernaryOperator {
+
+  final ExpressionObserver condition;
+  final ExpressionObserver trueExpr;
+  final ExpressionObserver falseExpr;
+
+  TernaryObserver(TernaryOperator expr, this.condition, this.trueExpr,
+      this.falseExpr) : super(expr);
+
+  _updateSelf(Scope scope) {
+    _value = _toBool(condition._value) ? trueExpr._value : falseExpr._value;
+  }
+
+  accept(Visitor v) => v.visitTernaryOperator(this);
+
+}
+
+class GetterObserver extends ExpressionObserver<Getter> implements Getter {
+  final ExpressionObserver receiver;
+
+  GetterObserver(Expression expr, this.receiver) : super(expr);
+
+  String get name => _expr.name;
+
+  _updateSelf(Scope scope) {
+    var receiverValue = receiver._value;
+    if (receiverValue == null) {
+      _value = null;
+      return;
+    }
+    var symbol = smoke.nameToSymbol(_expr.name);
+    _value = smoke.read(receiverValue, symbol);
+
+    if (receiverValue is Observable) {
+      _subscription = (receiverValue as Observable).changes.listen((changes) {
+        if (changes.any((c) => c is PropertyChangeRecord && c.name == symbol)) {
+          _invalidate(scope);
+        }
+      });
+    }
+  }
+
+  accept(Visitor v) => v.visitGetter(this);
+}
+
+class IndexObserver extends ExpressionObserver<Index> implements Index {
+  final ExpressionObserver receiver;
+  final ExpressionObserver argument;
+
+  IndexObserver(Expression expr, this.receiver, this.argument) : super(expr);
+
+  _updateSelf(Scope scope) {
+    var receiverValue = receiver._value;
+    if (receiverValue == null) {
+      _value = null;
+      return;
+    }
+    var key = argument._value;
+    _value = receiverValue[key];
+
+    if (receiverValue is ObservableList) {
+      _subscription = (receiverValue as ObservableList).listChanges
+          .listen((changes) {
+        if (changes.any((c) => c.indexChanged(key))) _invalidate(scope);
+      });
+    } else if (receiverValue is Observable) {
+      _subscription = (receiverValue as Observable).changes.listen((changes) {
+        if (changes.any((c) => c is MapChangeRecord && c.key == key)) {
+          _invalidate(scope);
+        }
+      });
+    }
+  }
+
+  accept(Visitor v) => v.visitIndex(this);
+}
+
+class InvokeObserver extends ExpressionObserver<Invoke> implements Invoke {
+  final ExpressionObserver receiver;
+  final List<ExpressionObserver> arguments;
+
+  InvokeObserver(Expression expr, this.receiver, this.arguments)
+      : super(expr) {
+    assert(arguments != null);
+  }
+
+  String get method => _expr.method;
+
+  _updateSelf(Scope scope) {
+    var args = arguments.map((a) => a._value).toList();
+    var receiverValue = receiver._value;
+    if (receiverValue == null) {
+      _value = null;
+      return;
+    }
+    if (_expr.method == null) {
+      // top-level function or model method
+      // TODO(justin): listen to model changes to see if the method has
+      // changed? listen to the scope to see if the top-level method has
+      // changed?
+      assert(receiverValue is Function);
+      _value = _convert(Function.apply(receiverValue, args));
+    } else {
+      var symbol = smoke.nameToSymbol(_expr.method);
+      _value = smoke.invoke(receiverValue, symbol, args);
+
+      if (receiverValue is Observable) {
+        _subscription = (receiverValue as Observable).changes.listen(
+            (List<ChangeRecord> changes) {
+              if (changes.any(
+                  (c) => c is PropertyChangeRecord && c.name == symbol)) {
+                _invalidate(scope);
+              }
+            });
+      }
+    }
+  }
+
+  accept(Visitor v) => v.visitInvoke(this);
+}
+
+_toBool(v) => (v == null) ? false : v;
+
+class EvalException implements Exception {
+  final String message;
+  EvalException(this.message);
+  String toString() => "EvalException: $message";
+}