packages/petitparser/lib/src/lisp/environment.dart

part of petitparser.lisp;

/// Environment of bindings.
class Environment {

  /// The owning environment.
  final Environment _owner;

  /// The internal environment bindings.
  final Map<Name, dynamic> _bindings;

  /// Constructor for the nested environment.
  Environment([this._owner]) : _bindings = new Map();

  /// Constructor for a nested environment.
  Environment create() => new Environment(this);

  /// Return the binding for [key].
  operator [](Name key) {
    if (_bindings.containsKey(key)) {
      return _bindings[key];
    } else if (_owner != null) {
      return _owner[key];
    } else {
      return _invalidBinding(key);
    }
  }

  /// Updates the binding for [key] with a [value].
  void operator []=(Name key, value) {
    if (_bindings.containsKey(key)) {
      _bindings[key] = value;
    } else if (_owner != null) {
      _owner[key] = value;
    } else {
      _invalidBinding(key);
    }
  }

  /// Defines a new binding from [key] to [value].
  define(Name key, value) {
    return _bindings[key] = value;
  }

  /// Returns the keys of the bindings.
  Iterable<Name> get keys => _bindings.keys;

  /// Returns the parent of the bindings.
  Environment get owner => _owner;

  /// Called when a missing binding is accessed.
  _invalidBinding(Name key) {
    throw new ArgumentError('Unknown binding for $key');
  }
}