Although I have not yet looked at any of Clojure's internals yet, I suspect
the change won't be too difficult (for the right person). So I hope/wish
someone with enough knowledge, skills and influence will agree with me and
advocate a review of the design ("last" may not be the only one with
issues) and fix some of those things before things get too messy down the
road.
Looking "peek" from your point of view (actually from another abstraction
point of view), it seems useful. Thank you.
On Friday, June 29, 2012 5:17:53 PM UTC-4, puzzler wrote:
>
> It is clear that some collections *could* support a more efficient last.
> Anything with random access. Anything that supports rseq (e.g., sorted
> collections).
>
> There are multiple ways to accomplish this; I presume a protocol would do
> the trick.
>
> Perhaps the original design decision is easily justified in terms of the
> original way the collections were factored into various interfaces, but now
> that it is so easy to make functions polymorphic over different types via
> protocols, I can't imagine this would be a difficult change if anyone cared
> to do it.
>
> I don't think anyone is arguing that the current semantics aren't well
> documented; the claim is that it violates the "Principle of Least Surprise"
> in the sense that most people expect a built-in core function to be
> implemented efficiently for the cases when it can be implemented
> efficiently. Everyone knows that a vector, for example, is designed to
> provide very fast access to the last element, so it is counterintuitive
> that a function called "last" ignores this capability and just treats
> vector as a generic sequence, searching through the items one-by-one to get
> to the last element.
>
> BTW, I disagree with Warren's comment about how a better last would
> eliminate the need for peek. Even if you have last, peek is a very useful
> way of guaranteeing consistent stack semantics for a wide variety of
> collection types.
>
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