On 5/22/12 3:27 PM, "Rick Waldron" <[email protected]> wrote:
> > >On Tue, May 22, 2012 at 5:57 PM, Herhut, Stephan A ><[email protected]> wrote: > >While exploring use cases for data-parallel concurrency in JavaScript >using the ParallelArray API (see > >http://wiki.ecmascript.org/doku.php?id=strawman:data_parallelism ><http://wiki.ecmascript.org/doku.php?id=strawman:data_parallelism>) I >found myself wanting better support for multiple results from a single >parallel operation. More precisely, I wanted a neat way to do >array-of-objects to object-of-arrays transformations. > >The same problem exists with the Array API, probably to a lesser extent >as one can always fall back to explicit iterators. So, for sake of >simplicity, I will use ordinary Array objects here. Consider the >following simple example: > >function foo (x) { > return {odd: (x%2), twice: 2*x}; >} > >[1,2,3,4].map(foo); > >This will produce an array of objects that have two properties: 'odd' >which encodes whether the corresponding source element was odd and >'twice' that gives twice the value of the source element. What I >typically want, though, are two arrays containing the odd > and twice values, respectively. For instance, I might want to use 'odd' >in a consecutive filter operation on 'twice'. One way to achieve this is >to write a little unzip helper: > >Array.unzip = function unzip(a) { > var r = {}; > for (var e in a) { > for (var k in a[e]) { > (r[k] || (r[k]=[]))[e] = a[e][k]; > } > } > return r; >} > > > > > > >I'm curious about this example implementation, as it won't do what you've >illustrated below - I assume you meant to write Array.prototype.unzip? Indeed, well spotted. >which then allows me to conveniently specify the array-of-objects to >object-of-arrays conversion like so: > >result = [1,2,3,4].map(foo).unzip(); > >Throw in destructuring as per >http://wiki.ecmascript.org/doku.php?id=harmony:destructuring ><http://wiki.ecmascript.org/doku.php?id=harmony:destructuring>, and you >get a pretty workable > >{odd, twice} = [1,2,3,4].map(foo).unzip(); > >Ideally, when implementing this, map should already materialize its >result as two separate arrays if its result is only used by an >application of unzip. A built in unzip with fixed, well known semantics >would make this feasible without the need to analyze what > a potential user defined unzip does. > >Improving on this some more, one could also extend destructuring to >include the required array-of-objects to object-of-arrays transformation >and thus directly allow the programmer to write > >{odd, twice} = [1,2,3,4].map(foo); > >The new semantics of destructuring would have an additional case that, >whenever the rhs is an array like value but the lhs is an object pattern, >the rhs is first transformed into an object of arrays before the pattern >is applied. An implementation is of course > free to perform the two steps at once, allowing for optimizations in >cases where only parts of the elements are selected, as in > >{odd} = [1,2,3,4].map(foo); > >Extending destructuring this way does not impact other cases of >destructuring. Consider the case where the programmer expected an object >but got an array of values. The expression > >{odd, twice} = [1,2,3,4]; > >would still yield odd and twice as undefined. If the lhs is an array >pattern, the extended semantics do not apply at all. > >To summarize, I propose two things. Either, rather conservatively, that >an unzip primitive > > > > >primitives in JavaScript are limited to Undefined, Null, Boolean, Number, >or String per 4.3.2 I was thinking of a built-in method of Array, rather than a primitive in the ES spec sense. Stephan _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
