foreach, an analogy

[Chris Nicholson-Sauls]

Benji Smith wrote:
> clayasaurus wrote:
> 
>> Here is my view on foreach_reverse
>>
>> 1) I really like the idea of foreach being the defacto-iterator for D
> 
> 
> Me too.
> 
>> 2) foreach_reverse allows me to implement an opApply function to my 
>> doubly linked list template for not only forward iteration, but 
>> backwards iteration as well! Otherwise, I'd have to pass it a 
>> &list.reverse delegate which is hackish.
> 
> 
> In C#, the foreach construct operates over arrays or objects that 
> implement the IEnumerable<T> interface (which includes the 
> ICollection<T> subinterface).
> 
> In Java, the "enhanced for loop" operates over arrays or objects that 
> implement the Iterable<T> interface (which includes the Collection<T> 
> sub-interface).
> 
> As far as I'm concerned, each class should be responsible for 
> determining which iterators it offers. A doubly-linked list could provide:
> 
> DoublyLinkedList.getForwardIterator();
> DoublyLinkedList.getReverseIterator();
> 
> Whereas a binary tree might provide even more:
> 
> BinaryTree.getInOrderIterator();
> BinaryTree.getReverseOrderIterator();
> BinaryTree.getDepthFirstIterator();
> BinaryTree.getBreadthFirstIterator();
> 
> Then, the foreach just iterates through any object that's iterable. Like 
> this:
> 
> IEnumerable<T> iterator = myBinaryTree.getDepthFirstIterator();
> foreach(T item in iterator) {
>   // do stuff with item
> }

Or, using the current situation:
# foreach (item; &myBinaryTree.depthFirstIterator) {
#   // do stuff with item
# }

I think that is about as expressive as could be needed?

> A single mechanism for all different kinds of iteration scenarios.
> 
> The ECMA C# specification (starting on page 236) gives a thorough 
> description of how compilers should implement foreach:
> 
> http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-334.pdf
> 
> I think it's a very elegant solution.
> 
> In D, even overlooking the presence of a fifteen-letter keyword 
> ("foreach_reverse"), the ugliest part is the presence of the 
> opApplyReverse() method, which suggests that the collection object 
> should be responsible for its own iteration, whereas I think iteration 
> should be delegated to a separate object.
> 
> --Benji Smith

-- Chris Nicholson-Sauls