BrendanEich ‏@BrendanEich Apr 23

@wavded yield* is in tail position there, but the expression it exhausts is not -- it's a divergent recursive call. JS is strict ("eager").

Marc Harter ‏@wavded Apr 23

@BrendanEich I must not be understanding TCO here. Would you have a sec to comment on that gist with an proper TCO version? Recur fib gen.

BrendanEich ‏@BrendanEich Apr 23

@wavded yield* is not return -- yield* E evaluates E eagerly, and exhausts it, yielding each value. return E where E is F() can tail-call F.

getify ‏@getify Apr 23

@wavded first yield* will, when recursing to gen(), wait for both of its yields.. that's why it's not a single-op in tail pos @brendaneich

Marc Harter ‏@wavded Apr 23

@getify @BrendanEich ok, so is it possible to infinitely recurse in a generator? if you have an example that may help me get it better.

BrendanEich ‏@BrendanEich Apr 23

@wavded Think of yield* E as do{for (let V of E) yield V;} and follow @getify's point. Also note yield is not return, G.next() goes back.

getify ‏@getify Apr 23

@brendaneich @wavded it doesn't seem like generators could ever be TCO'd. even one with only a single yield* in final pos is not a return.

Marc Harter ‏@wavded Apr 23

@BrendanEich so yield* X is just a fancy way of exhausting all the yields from another generator (looping) and then continuing on...

BrendanEich ‏@BrendanEich Apr 23

@getify Correct -- control can come back (send a value, even; also the close method, reborn as g.return() to force-return at yield). @wavded

BrendanEich ‏@BrendanEich Apr 23

@getify In contast, once a function has return'ed that's it! No more resumption. @wavded

getify ‏@getify Apr 23

@wavded FWIW, i like to think of yield* as delegating its own iterator control to another generator. @brendaneich

Marc Harter ‏@wavded Apr 23

@BrendanEich @getify background for my questions; generators enable infinite, lazy, sequences - while() yield - wondered about recursion...

Marc Harter ‏@wavded Apr 23

@getify @BrendanEich think I have it, it's like handing off "yields" to another generator, then continuing on?

JΞFF WAɌƉ ‏@jeffscottward Apr 23

@BrendanEich @wavded @getify I wish I could understand this madness :(

BrendanEich ‏@BrendanEich Apr 23

@wavded Just as noted yield* is eager. @getify

Marc Harter ‏@wavded Apr 23

@BrendanEich @getify got it now, thanks for your help! I keep on thinking about lazy, infinitely recursive, Haskell-like functions in JS :)

getify ‏@getify Apr 23

@wavded "handing off yields to another generator, then waiting for all of them to finish before going on." :) @brendaneich

getify ‏@getify Apr 23

@jeffscottward you don't know JS yet. i'm writing some books to help. ;-)

Marc Harter ‏@wavded Apr 23

@getify @BrendanEich I know, I just said that twice already right :) :)