On 3/6/2014 8:37 PM, H. S. Teoh wrote:
Unfortunately, input ranges are somewhat tedious to write -- nice foreach loops have to be broken up into .empty, .front, .popFront, which is a lot of boilerplate code and not so nice in inner loops (though I suppose the idea is to improve compiler inlining to handle these cases). I wonder if a mid- to long-term goal for D would be to ease writing input ranges by moving some of the boilerplate into the language, or providing range builder facilities in Phobos. The most nagging part of writing input ranges in the current language is the inability to use foreach to generate the returned elements. (Well, you *can* do that to a buffer array and then return the array, but that kinda defeats the purpose of GC avoidance.) Some kind of built-in coroutine syntax with 'yield' would help things a LOT.
Yes, this has been #1 on my wishlist for ranges for some time. We really need a way to make input ranges (maybe even forward ranges if we're clever enough about it) via "coroutines" that, like C#'s coroutines, are automatically converted behind-the-scenes into stackless fibers (ie, into state machines).
A while back, I tried doing a library solution for this via regular fibers, but there turned out to be a lot of overhead due to the fiber's context-switching [1].
We need to take inspiration from two things: C's "protothreads" library[2], and (as I said) C#'s coroutines. Both of those are fantastic IMO. In both cases, the body of a coroutine is automagically transformed into a big switch statement. Yield statements then become roughly "return tuple([yielded value], [resume point]); case [resume point]:...". And then invoking the coroutine again automatically passes in the last [resume point] returned so the big switch() can jump straight to where it left off. The main difference in our case would be that we'd also auto-generate all the boilerplate rigging for this to be an input range.
[1] http://semitwist.com/articles/article/view/combine-coroutines-and-input-ranges-for-dead-simple-d-iteration
[2] C's "protothreads" library: http://dunkels.com/adam/pt/expansion.html
