Re: Why does for(x <- c; y = ...) yield {...} become c.map(x => (x, ...)).map((x,y) => {...})

The decomposition of inside the yield is no fun either when you look at the
bytecode.  Consider

  for (x <- xs; l = x.length) yield { x(0)+l }
  for (x <- xs) yield { val l = x.length; x(0) + l }

Same thing, right?  Except the top one takes twice as long because of the
double-map.  Similarly,

  for (x <- xs; c = x(x.length-1); if c < '5') yield x
  for (x <- xs if x(x.length-1) < '5') yield x
  xs.collect{ case x if x(x.length-1) < '5' => x }
  val nb = Coll.newBuilder[A]; xs.foreach(x => if (x(x.length-1) < '5') nb
+= x); nb.result

all do the same thing.  But they vary by a factor of 5 as to which is the
fastest, and which is which depends on the collection.  The first one is
always slowest, though.

I think a serious revisiting of the desugaring is appropriate.  I am a
little hesitant to recommend anything when moving to SAMs may change what
is possible / efficient.

  --Rex


On Sat, Jan 24, 2015 at 11:07 AM, Haoyi Li <[email protected]> wrote:

> I've been looking at http://docs.scala-lang.org/tutorials/FAQ/yield.html,
> and found this
>
> for(x <- c; y = ...) yield {...}
>
> is translated into
>
> c.map(x => (x, ...)).map((x,y) => {...})
>
>
> This seems rather odd to me. Why not
>
> for(x <- c; y = ...) yield {...}
>
> is translated into
>
> c.flatMap{ x => val y = ...; {...} }
>
>
> Isn't this basically the same, but much more straightforward?
>
> The reason I bumped into this is that I was trying to implement a bunch of
> custom for-comprehension generators, and it kept asking for *.map*, and I
> couldn't figure out why. These were pretty odd generators, e.g. a TryCatch
> generator which you can put anywhere in your loop and which propagates a
> Seq() back up if it catches an exception, and so don't really have
> meaningful values to pass into *map* or *flatMap*s.
>
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