2018-04-04 12:18 GMT+02:00 Alistair Grant <akgrant0710@gmail.com>:
Hi Sven,
On Wed, Apr 04, 2018 at 11:32:02AM +0200, Sven Van Caekenberghe wrote:
Somehow, somewhere there was a change to the implementation of the primitive called by some streams' #atEnd.
That's a proposed change by me, but it hasn't been integrated yet. So the discussion below should apply to the current stable vm (from August last year).
IIRC, someone said it is implemented as 'remaining size being zero' and some virtual unix files like /dev/random are zero sized.
Currently, for files other than sdio (stdout, stderr, stdin) it is effectively defined as:
atEnd := stream position >= stream size
And, as you say, plenty of virtual unix files report size 0.
Now, all kinds of changes are being done image size to work around this.
I would phrase this slightly differently :-)
Some code does the right thing, while other code doesn't. E.g.:
MultiByteFileStream>>upToEnd is good, while FileStream>>contents is incorrect
I am a strong believer in simple, real (i.e. infinite) streams, but I am not sure we are doing the right thing here.
Point is, I am not sure #next returning nil is official and universal.
Consider the comments:
Stream>>#next "Answer the next object accessible by the receiver."
ReadStream>>#next "Primitive. Answer the next object in the Stream represented by the receiver. Fail if the collection of this stream is not an Array or a String. Fail if the stream is positioned at its end, or if the position is out of bounds in the collection. Optional. See Object documentation whatIsAPrimitive."
Note how there is no talk about returning nil !
I think we should discuss about this first.
Was the low level change really correct and the right thing to do ?
The primitive change proposed doesn't affect this discussion. It will mean that #atEnd returns false (correctly) sometimes, while currently it returns true (incorrectly). The end result is still incorrect, e.g. #contents returns an empty string for /proc/cpuinfo.
You're correct about no mention of nil, but we have:
FileStream>>next
(position >= readLimit and: [self atEnd]) ifTrue: [^nil] ifFalse: [^collection at: (position := position + 1)]
which has been around for a long time (I suspect, before Pharo existed).
Having said that, I think that raising an exception is a better solution, but it is a much, much bigger change than the one I proposed in https://github.com/pharo-project/pharo/pull/1180.
Cheers, Alistair
Hi, yes, if you are after universal behavior englobing Unix streams, the Exception might be the best way. Because on special stream you can't allways say in advance, you have to try. That's the solution adopted by authors of Xtreams. But there is a runtime penalty associated to it. The penalty once was so high that my proposal to generalize EndOfStream usage was rejected a few years ago by AndreaRaab. http://forum.world.st/EndOfStream-unused-td68806.html I have regularly benched Xtreams, but stopped a few years ago. Maybe i can excavate and pass on newer VM. In the mean time, i had experimented a programmable end of stream behavior (via a block, or any other valuable) http://www.squeaksource.com/XTream.htm so as to reconcile performance and universality, but it was a source of complexification at implementation side. Nicolas
Note also that a Guille introduced something new, #closed which is related to the difference between having no more elements (maybe right now, like an open network stream) and never ever being able to produce more data.
Sven