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Adds an external Enumerator that provides the full richness of the Smalltalk enumeration protocol to an internal collection or complex data structure without exposing the entire collection.

ExternalEnumeration is licensed under the MIT license. See the copyright tab in the RB, the notice property of this package, or the LICENSE file on GitHub.

ExternalEnumeration's primary home is the Cincom Public Store Repository. Check there for the latest version. It is also on GitHub.

ExternalEnumeration was developed in VW 7.9.1, but is compatible with any version of VisualworksSmalltalk. If you find any incompatibilities, let me know (see below for contact information) or file an issue on GitHub.


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 have 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. 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 full collection API. This can be dangerous, as the client code can then modify the collection as well. For complex data structures, there isn't even an internal collection to expose.

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

  • 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 provides 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.

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 on: internalElements selector: #do: species: Set

Enumerator currently implements the following methods:

  • allSatisfy:
  • anySatisfy:
  • collect:
  • detect:
  • detect:ifFound:
  • detect:ifFound:ifNone:
  • detect:ifNone:
  • do:
  • do:separatedBy:
  • fold:
  • groupedBy:
  • inject:into:
  • reject:
  • select:


I'm happy to receive bug fixes and improvements to this package. If you'd like to contribute, please publish your changes as a "branch" (non-integer) version in the Public Store Repository and contact me as outlined below to let me know. I will consider merging your changes back into the "trunk" as soon as I can review them.

Contact Information

If you have any questions about ExternalEnumeration and how to use it, feel free to contact me.


Provides an external Enumerator for Visualworks Smalltalk







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