On Sun, Jun 9, 2013 at 2:17 AM, Axel Rauschmayer <[email protected]> wrote:
> Another use case for iterator support: retrieve the first element from an
iterator, then iterate over the remaining elements via for-of.
Right, this happens whenever you're doing a fold and your combining
function has no handy "zero" value. For the sake of concreteness let's say
we're writing code for a robot kitchen, and we've got a function for taking
a collection of bowls and dumping all the contents into the first bowl. In
Python:
def combine(bowls):
iterator = iter(bowls)
first_bowl = next(iterator, None)
for bowl in iterator:
dump_into(bowl, first_bowl)
In JS the same function would be:
from "yet_unspecified_standard_module" import iteratorSymbol;
function combine(bowls) {
let iterator = bowls[iteratorSymbol]();
let {value: firstBowl, done} = iterator.next();
if (done)
return; // no bowls at all
let rest = {};
Object.defineProperty(rest, iteratorSymbol, {value: () =>
iterator});
for (let bowl of rest)
dumpInto(bowl, firstBowl);
}
If you don't mind copying the source collection into an array, you can use
destructuring instead:
let [?first, ...rest] = bowls;
At least, I think that's the current proposed destructuring
syntax/semantics. Of course this only works if the use case has a structure
that the destructuring mini-language can express.
Other use cases come up too: (1) you want to process a sequence in one loop
until you reach some marker, and then break out of the first loop and
process the rest in another loop; or (2) you want to pass the iterator to a
separate function that consumes a subsequence of the input. (For example,
when the input sequence is from a file, and you're doing some light
parsing.)
I think it's a mistake for iterators not to be iterable. We will have error
messages like "Iterator object is not iterable", and users will say
"*really*, JavaScript?". Requiring users to build pseudo-collection objects
like `rest` in these use cases seems awfully bureaucratic, and hard to
justify without resorting to types.
-j
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