Hi Denis,

On Apr 4, 2018, at 7:32 AM, Denis Kudriashov <dionisiydk at gmail.com> wrote:

Hi Eliot.

2018-04-04 16:12 GMT+02:00 Eliot Miranda <eliot.miranda at gmail.com>:

> Hi Denis, Hi Sven,
>
> On Apr 4, 2018, at 3:16 AM, Denis Kudriashov <dionisiydk at gmail.com> wrote:
>
> Hi.
>
> #next returning nil is definitely looks bad because we will always have
> case when nil value will not means the end:
>
> #(1 nil 2) readSteam next; next; atEnd
>
> But I understand that in many cases analysing #next for nil is much more
> suitable than #atEnd. I am sure that it can be always avoided but it can be
> not easy to do.
> Alternatively we can reify real EndOfStream object which will be used as
> result of reading operation when stream is at end.
> It can provide nice API like #ifEndOfStream:. which will reflect intention
> in much cleaner way than ifNil: checks.
>
>
> There is a much better scheme which is to allow streams to have an
> end-of-file element in an inst var, "endOfStream".  It is initialised with
> the stream itself (nil cannot be used as we want it as a value).  If the
> end of the stream is reached and endOfStream == self an EndIfStream
> exception is raised.  Otherwise the value of endOfStream is answered.
>

Does it mean that at the end of stream:


aStream next == aStream

 ?


Of course not.  Did you read what I write above?  If the end of the stream
is reached and endOfStream == self an EndIfStream exception is raised.
The idea is that endOfStream == self is a market that the default behaviour
should occur at end of stream, and in all other cases the endOfStream
should be returned.

e.g.

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."

<primitive: 65>
^position >= readLimit
ifTrue: [*self pastEnd*]
ifFalse: [collection at: (position := position + 1)]

ReadStream>>pastEnd
"The stream is at its end; respond appropriately. If the endOfStream
element has
been set, answer it, otherwise raise an exception."
^endOfStream == self
ifTrue: [self class endOfStreamSignal signal]
ifFalse: [endOfStream]

then for the current behavior of answering nil on end of stream endOfStream
is set to nil. By default endOfStream would be initialized to self, meaning
that an exception is raised on end of stream by default.


And I'm wrong. This scheme is not used in VisualWorks. They ripped it out
after I left. It's not in vw7.7.

We can't answer nil because in some applications (streams of objects) nil
> is a valid element to read from the steam and we need to manufacture a
> unique sentinel.
>
> We made this change in VisualWorks and it works well.  One can avoid using
> atEnd and lion for the sentinel instead, which is much cleaner and faster
> and less problematic than atEnd.
>
>
>
> 2018-04-04 12:00 GMT+02:00 Sven Van Caekenberghe <sven at stfx.eu>:
>
>>
>>
>> > On 4 Apr 2018, at 11:38, Nicolas Cellier <nicolas.cellier.aka.nice at gmai
>> l.com> wrote:
>> >
>> > Hi Sven,
>> > See also discussion at https://github.com/OpenSmallta
>> lk/opensmalltalk-vm/pull/232
>>
>> Thanks Nicolas, I had already seen parts of it.
>>
>> Now, I still want image level changes to be based on clear semantic
>> definitions of the abstract stream API, we're not just talking about file
>> or unix streams, the general Smalltalk concept.
>>
>> For reading, they are IMHO,
>>
>> #next
>> #readInto:startingAt:count:
>> #peek
>> #atEnd
>> #upToEnd (can be derived but still the semantics are important in
>> relation to #atEnd)
>>
>> For writing, we have
>>
>> #nextPut:
>> #next:putAll:startingAt:
>> #flush
>>
>> For both, we have
>>
>> #atEnd
>> #close
>> #closed (new)
>>
>> So, I know, #next returning nil exists, but is it universally/officially
>> defined as such ? Where is that documented ?
>>
>> Positioning, sizing are not universal, IMHO, but should be clearly
>> defined as well.
>>
>> > 2018-04-04 11:32 GMT+02:00 Sven Van Caekenberghe <sven at stfx.eu>:
>> > Somehow, somewhere there was a change to the implementation of the
>> primitive called by some streams' #atEnd.
>> >
>> > IIRC, someone said it is implemented as 'remaining size being zero' and
>> some virtual unix files like /dev/random are zero sized.
>> >
>> > Now, all kinds of changes are being done image size to work around this.
>> >
>> > 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 ?
>> >
>> > 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
>> >
>> >
>> >
>> >
>>
>>
>>
>
>
>
>
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