On Sat, May 11, 2013 at 1:32 PM, Mike Stay <[email protected]> wrote: > On Fri, May 10, 2013 at 2:34 PM, Brendan Eich <[email protected]> wrote: >> So, just to be clear, now the issue isn't that comprehensions make arrays, >> it's that iteration isn't X. What is X? Quiz rule: don't say "Monad" or >> "Monadic" in your answer. :-| > > Actually, if we could provide a different flatten operation, that > would suffice, since the iterator could simply provide a single value > of the appropriate class. For example, given an iterator that > produced integers, we could do matrix multiplication if flatten summed > all the elements of the inner lists instead of concatenating the inner > lists. > > If > [for (x of foo) for (y of bar(x)) for (z of baz(x,y)) expr(x, y, z)] > iterated as usual over foo, bar(x) and baz(x,y) but used the flatten() > method of bar(x) on the innermost list of lists and the flatten() > method of foo on the outermost list of lists, that would do everything > I was hoping for. > > In particular, the expression > > [ for (w of obj) > for (x of foo(w)) > for (y of bar(w,x)) > for (z of baz(w,x,y)) > if (cond(w,x,y,z)) > expr(w,x,y,z) > ] > > would translate as > > let resultObj = []; > let onceObj = obj; > for (let w of onceObj) { > let resultFoo = []; > let onceFoo = foo(w); > for (let x of onceFoo) { > let resultBar = []; > let onceBar = bar(w,x); > for (let y of onceBar) { > let resultBaz = []; > let onceBaz = baz(w,x,y); > for (let z of onceBaz) { > if (cond(w,x,y,z)) { > resultBaz.push(expr(w,x,y,z)); > } > } > resultBar.push(resultBaz); > } > resultBar = (onceBar.flatten || [].flatten).call(resultBar); > resultFoo.push(resultBar); > } > resultFoo = (onceFoo.flatten || [].flatten).call(resultFoo); > resultObj.push(resultFoo); > } > resultObj = (onceObj.flatten || [].flatten).call(resultObj); > > No change to the syntax, no change to the iterator semantics, and a > compatible change to the flattening semantics (assuming the existence > of Array.prototype.flatten).
In addition to what Brendan said about the cost of actually generating nested arrays in the common case, I don't think this desugaring does what you want - if you have only a single for-of, you'll just get an array back out, without a chance to call the custom flatten on it. You can't handle this generically, either, because the innermost for-of is handled as a map rather than a flatMap. You really do need to go monadic all the way, and as Brendan said, that's too much cost for the common case of Arrays. Let's just add do-expressions, or something similar, in the future. We can leave list comprehensions to their array specialization for now. ~TJ _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
