In this post I'll try to sketch a number of related ideas about working with sequential collections, all of them expressed in php. Enumerations, enumerables, interators, iteratees and their cousins have been tortured to death, but as Eric Meijer and his team working on LINQ and Rx frameworks for .net show, it pays off doing it systematically. I'll revisit the Iterator and Subject Observer design patterns and will derive them systematically from first principles.
In php iterators come handy for all kinds of work on sequences - usually using
foreach( $sequence in $key,$value) { ... }
The problem with foreach is that it is not composable, but there are a lot of functions and operations on iterators which are, at least in theory. I will try to show how to achieve composability by, first, composing the processing of each individual step and, later, by composing iterators. This can lead to easily reconfigurable applications and a more declarative style of programming.
