Collections language uses a runtime library as its back end, which is designed to be extensible. Prior to version 1.5, the collections runtime library was written in Java and used only standard Java APIs. The release 1.5 brings a change: now the runtime library is available as an MPS model and uses constructs from jetbrains.mps.baseLanguage.closures language to facilitate passing of function-type parameters around.
In order to make the transition from Java interfaces to abstract function types possible, several of the former Java interfaces in the collections runtime library have been changed into abstract classes. While no existing code that uses collections runtime will be broken, unfortunately this breaks the so called binary compatibility, which means that a complete recompilation of all the generated code is required to avoid incompatibility with the changed classes in the runtime.
The classes which constitute the collections runtime library can be found in the collections.runtime solution, which is available from the jetbrains.mps.baseLanguage.collections language.