Smalltalk collections have a rich protocol for enumerating their elements: simple iteration with do:, mapping with collect:, filtering with select: and reject:, reducing with fold: and inject:into:, and more.

Many systems contain objects that are logically collections, but rightfully do not inherit from classes in the collection hierarchy. Perhaps they contain a collection internally, but expose a more restricted API for interacting with the collection.

Some systems contain complex data structures that need to be enumerated. It is often possible to expose some kind of a do: method for enumerating the elements of the structure, but the other enumeration methods are rarely provided. Behaviour>>subclassesDo: and VisualComponent>>childrenDo: are examples.

If we want to provide the full enumeration API in these cases, our choices are limited:

  • Expose an internal collection via a method. This allows client code access to the full enumeration API, but also allows access to the entire collection API. This can be dangerous, as the client code can then modify the internal collection without our class knowing about it. For complex data structures, there isn’t even an internal collection to expose.

  • Use traits, assuming there is a working implementation for your Smalltalk.

  • Re-implement the full enumeration API on the class, delegating the methods to the internal collection. This is a lot of tedious work that must be repeated for each collection-like object in the system.

ExternalEnumeration provides a new option to solve these problems. It defines Enumerator, an object that takes a target object and a do: selector and provides the full, rich enumeration API that Smalltalk programmers expect.

Enumerator is an implementation of the External Iterator pattern.

To create an Enumerator, send the #on:selector: message to the Enumerator class. The resulting enumerator responds to all of the normal collection enumeration messages. The selector must be the name of a method that provides do: semantics for the target object.

Simple Enumerator
subclasses := Enumerator on: Collection selector: #allSubclassesDo:.
(subclasses groupedBy: #superclass) inspect.

Some enumeration messages return a collection (collect:, select:, reject:, and groupedBy:). By default, the resulting collection will be an OrderedCollection (or a Dictionary whose values are OrderedCollections in the case of groupedBy:). If you’d like to use a different collection type (like Set or Array), you can use the #on:selector:species: method instead:

Enumerator With Species
^Enumerator on: internalElements selector: #do: species: Set

ExternalEnumeration’s primary home is the Cincom Public Store Repository. Check there for the latest version. I’ve put a snapshot of the current version of ExternalEnumeration on GitHub. I’ve also submitted it for inclusion as a contributed package in a forthcoming Visualworks release.

ExternalEnumeration was developed in VW 7.9.1, but is intended to be compatible with any version of Visualworks Smalltalk. The code could also easily be ported to any other Smalltalk.

ExternalEnumeration is released under the MIT license.