invariant_collection 1.0.1

Invariant versions of standard collection types using extension types. As of version 1.0.0, the only kind which is implemented is `List`.

Invariant Collections

The repository invariant_collection provides the library invariant_list.dart. Future versions of this repository will provide similar libraries for other kinds of collections from 'dart:core'.

invariant_list.dart provides an extension type IList<E> which is intended to be used as a replacement for types of the form List<E> (where List is the built-in class of that name). An easy way to obtain an expression e of type IList<T> from an expression of type List<T> for some T is to use e.iList.

IList<E> differs from List<E> in that the former is invariant in its type parameter, whereas the latter uses dynamically checked covariance.

It is well-known that dynamically checked covariance relies on run-time type checks to ensure soundness of the heap in situations where the available type information does not suffice to establish that guarantee at compile-time. The standard example goes as follows:

void main() {
  List<num> xs = <int>[1];
  xs.add(1.5); // No compile-time error, yet throws at run time.
}

The corresponding example using the type IList differs by having a compile-time error (which means that there will not be any run-time errors because the unsafe kind of subtyping is prevented at compile tim):

void main() {
  IList<num> xs = <int>[1].iList; // Compile-time error.
  ... // It doesn't matter what we would have done.
}