On Thu, Jan 19, 2012 at 12:44 AM, Andreas Rossberg <rossb...@google.com>wrote:
> On 19 January 2012 07:51, Mark S. Miller <erig...@google.com> wrote: > > > > Because an instance of InfiniteMap conforms to the full Map contract > (given > > that baseMap does and is not otherwise used), we have full Liskov > > substitutability -- you can validly pass an InfiniteMap to an abstraction > > expecting a Map. > > PS: Won't the "full Map contract" eventually contain iterators, too? > An infinite map cannot sensibly support those. > > Depends what you mean by "sensibly". I see nothing at http://wiki.ecmascript.org/doku.php?id=harmony:iterators that implies that iterators must or even should terminate. And infinite iterators (in general though not here) are clearly useful. However, I agree that doing an infinite iteration over an InfiniteMap is unlikely to be useful. For example, the contract can almost be trivially and uselessly satisfied by having the InfiniteMap return the following "items" (key-value pair) iterator: { next: function() { return [{}, lazyFactory()]; } } Since each of these keys are fresh, we know we don't need to consult the baseMap for an overriding mapping. Since there are an infinite number of such keys, we know they'll never run out, so we can indefinitely postpone ever needing to return a meaningful mapping. However the "almost" qualifier above is because, to be correct, the iterator would have to store the new useless associations in the baseMap before returning them -- making this technique even more useless. Infinities are indeed weird. -- Cheers, --MarkM
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