// Always thread-safe
sclass FixedVarContext > VarContext {
  S[] names; // always sorted, may be null
  O[] values; // same length as names, may also be null
  
  *() {}
  *(S[] *names) { this(null, names); }
  *(VarContext parent, S[] *names) {
    super(parent);
    if (names != null) values = new O[names.length];
  }
  
  int indexOfVar(S name) {
    ret indexOfInSortedArray(names, name);
  }
  
  int indexOfVarMandatory(S name) {
    int idx = indexOfVar(name);
    if (idx < 0) fail("Variable not found: " + name + ", known: " + joinWithComma(names));
    ret idx;
  }
  
  O get(int idx) {
    ret values[idx];
  }
  
  O get(S name) {
    int idx = indexOfVar(name);
    if (idx >= 0) ret values[idx];
    if (parent != null)
      ret parent.get(name);
    null;
  }
  
  void set(int idx, O value) {
    values[idx] = value;
  }
  
  void set(S name, O value) {
    int idx = indexOfVar(name);
    if (idx >= 0)
      values[idx] = value;
    else
      fail("Variable " + name + " not defined in context");
  }
  
  AutoCloseable tempSet(S name, O value) {
    int idx = indexOfVar(name);
    if (idx >= 0) {
      O old = values[idx];
      values[idx] = value;
      ret -> values[idx] = old;
    } else
      fail("Variable " + name + " not defined in context");
  }
  
  void unset(S name) {
    set(name, null);
  }
  
  MapSO varMap() {
    new MapSO map;
    int n = l(names);
    for i to n:
      map.put(names[i], values[i]);
    ret map;
  }
}