Immutable collections

[nozavroni]

Collections should be immutable. Refactor every method in collection class to return a new collection. Never change state of a collection. Ever.

• [x] Alter any methods that change internal state of collection to instead return a new collection
• [x] Create custom PHPUnit assertion that allows you to easily assert that an object's internal state has not changed
• [ ] Write unit tests for every method in CollectionInterface
• [ ] Use immutability assertions in all new unit tests
• [ ] Update existing unit tests with immutability assertions to ensure that Collection is truely immutable
• [ ] Find out if you can still implement Iterator without affecting immutability.
  • [ ] If Iterator and immutability are mutually exclusive, give iterator the boot?
  • [ ] maybe this is why linked lists?
  • [ ] If collection cannot be iterated over directly, use IsTraversable trait and use ->traverse() method instead to traverse a collection

Tons of interesting stuff here: http://clojure-doc.org/articles/language/collections_and_sequences.html

My main take-away... CollectionInterface should only contain a very small, core set of methods that will be available on ALL collection types (I suppose I already knew I was going to need to do this, but this reinforces it). Then take all the methods you peeled from CollectionInterface and use them to create SequenceInterface (numerically indexed, always in order), MapInterface (essentially what collection is now), SetInterface (unique set of values of a particular type), NumericInterface (all values are numeric), ListInterface (linked list), ArrayInterface (ArrayAccess), FixedCollectionInterface (size cannot change), StructInterface (I'm not too clear on this one) etc.

[nozavroni]

Need to research how linked lists are used in functional languages to help with performance when creating a new collection for every operation. Also use spl data structures if possible to do it yourself.