Provide access to last

[yongqli]

It should be possible to access last in O(1).

[spl]

I'm assuming you mean the analog to the list last function. I don't see how you can do last in O(1). Even head and tail are O(n). If you can implement it, I'd certainly consider a pull request.

[yongqli]

Hmm, I guess we can't pattern match on the last snoc, so it's not possible with this definition.

[spl]

Since a DList is represented as a function, primarily to get the “free” append via function composition, it is really a “one-way” type for collecting a bunch of appends and dealing them out in the end when you want to reconstruct the desired list. Thus, most of the DList functions that provide access to elements of the implied list are really convenience functions on top of toList.

You may be interested in Data.Sequence for its O(1) viewr.