Changed #atEnd primitive - #atEnd vs #next returning nil

Hi Sven,

Oh dear.  I feel as though I'm not getting my concerns across at all
well, and I'm pushing hard enough that all I'm going to do is make
people annoyed.  So let me try to restate the issue one last time
before answering your questions directly.

Pharo & Squeak have unwritten rules about stream usage that I suspect
have just emerged over time without being designed.

If you want to be able to iterate over any stream, and in particular
stdin from a terminal (which, as far as I know, is the outlier that
causes all the problems) you have to follow these rules:

1.  If the stream is character / byte oriented you have to check for
EOF using "stream next == nil".  #atEnd can be used, but you'll still
have to do the nil check.

2.  All other streams have to check for EOF (end of stream) using
#atEnd.  "stream next == nil" can be used, but you'll still need to
test #atEnd to determine whether nil is a value returned by the
stream.

If you write code that you want to be able to consume characters,
bytes or any other object, you'll have to test both "stream next ==
nil" and #atEnd.

The rules are the result of the original blue book design being that
#atEnd should be used, and then character input from a terminal being
added later, but always returning an EOF character (nil) before #atEnd
answers correctly.

At the moment, ZnCharacterEncoder uses #atEnd on character / byte
streams, so fails for stdin on a terminal.

Back to your questions:

On 11 April 2018 at 11:12, Sven Van Caekenberghe <[hidden email]> wrote:
>
>
>> On 11 Apr 2018, at 10:29, Alistair Grant <[hidden email]> wrote:
>>
>> Hi Denis,
>>
>> On 11 April 2018 at 10:02, Denis Kudriashov <[hidden email]> wrote:
>>>
>>> 2018-04-11 8:32 GMT+02:00 Alistair Grant <[hidden email]>:
>>>>
>>>>>>> Where is it being said that #next and/or #atEnd should be blocking or
>>>>>>> non-blocking ?
>>>>>>
>>>>>> There is existing code that assumes that #atEnd is non-blocking and
>>>>>> that #next is allowed block.  I believe that we should keep those
>>>>>> conditions.
>>>>>
>>>>> I fail to see where that is written down, either way. Can you point me
>>>>> to comments stating that, I would really like to know ?
>>>>
>>>> I'm not aware of it being written down, just that ever existing
>>>> implementation I'm aware of behaves this way.
>>>>
>>>> On the other hand, making #atEnd blocking breaks Eliot's REPL sample
>>>> (in Squeak).
>>>
>>>
>>> Could you write here this example, please?
>>
>> The code is loaded in squeak using:
>>
>> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/buildspurtrunkreaderimage.sh
>>
>> for 32 bit images.  It loads:
>>
>> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/LoadReader.st
>>
>> which loads package CogTools-Listener in http://source.squeak.org/VMMaker
>>
>> An image that automatically runs the code and nothing else is created in:
>>
>> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/StartReader.st
>>
>>
>> If you want to run it interactively you can load CogTools-Listener and
>> do something like:
>>
>> StdioListener new
>>    quitOnEof: false;
>>    run
>
> What does #quitOnEof: do ? Can the StdioListener code be browsed/viewed online somewhere ?

I just referenced this as an example of making #atEnd (really
FilePlugin>>primitiveFileAtEnd) blocking causing problems.  I wasn't
expecting people to go and look at the code or use it as a test.

If you really want to look at it (from Pharo):

1. Add http://source.squeak.org/VMMaker as a repository.
2. Browse the CogTools-Listener package


>> If you modify #atEnd to block it will result in the "squeak>" input
>> prompt being printed in the terminal after the input has been entered.
>
> How does one modify #atEnd to block ? I suppose you are talking about StdioStream>>#atEnd ?

I meant the primitive, i.e. FilePlugin>>primitiveFileAtEnd /
FilePluginPrims>>atEnd:.


>  ^ self peek isNil
>
> ?
>
> PS: I liked your runnable example better, I will try it later on. Thx!

Right.  My code is meant to be minimal and trigger the problem I'm
actually focused on - that ZnCharacterEncoder doesn't work with stdin
from a terminal.

Sven has expressed a hesitation to change the internal operation of
the Zinc streams from using #atEnd to "stream peek == nil" and this
whole discussion is really about us trying to resolve our different
perspective of the best path forward.  I respect Sven and his work so
I'm trying to justify the change (but I'm not expressing it at all
well, obviously).

Cheers,
Alistair

>> The code can be loaded in to Pharo and basically works, but the output
>> tends to be hidden behind the next input prompt because it uses #cr
>> instead of #lf.  You can easily modify StdioListener>>initialize to
>> set the line end convention in stdout.
>>
>> NOTE: It is not intended to be a release quality implementation of a
>> evaluation loop.  The whole purpose as I understand it is for it to be
>> as simple as possible to assist in tracking down issues using the VM
>> simulator.  It runs minimal code to get to the point of waiting for
>> user input and then allows an expression that causes problems to be
>> entered and traced using the simulator.
>>
>> Cheers,
>> Alistair
>>
>>
>>
>>>>>>> How is this related to how EOF is signalled ?
>>>>>>
>>>>>> Because, combined with terminal EOF not being known until the user
>>>>>> explicitly flags it (with Ctrl-D) it means that #atEnd can't be used
>>>>>> for iterating over input from stdin connected to a terminal.
>>>>>
>>>>> This seems to me like an exception that only holds for one particular
>>>>> stream in one particular scenario (interactive stdin). I might be wrong.
>>>>>
>>>>>>> It seems to me that there are quite a few classes of streams that are
>>>>>>> 'special' in the sense that #next could be blocking and/or #atEnd could be
>>>>>>> unclear - socket/network streams, serial streams, maybe stdio (interactive
>>>>>>> or not). Without a message like #isDataAvailable you cannot handle those
>>>>>>> without blocking.
>>>>>>
>>>>>> Right.  I think this is a distraction (I was trying to explain some
>>>>>> details, but it's causing more confusion instead of helping).
>>>>>>
>>>>>> The important point is that #atEnd doesn't work for iterating over
>>>>>> streams with terminal input
>>>>>
>>>>> Maybe you should also point to the actual code that fails. I mean you
>>>>> showed a partial stack trace, but not how you got there, precisely. How does
>>>>> the application reading from an interactive stdin do to get into trouble ?
>>>>
>>>> Included below.
>>>>
>>>>
>>>>>>> Reading from stdin seems like a very rare case for a Smalltalk system
>>>>>>> (not that it should not be possible).
>>>>>>
>>>>>> There's been quite a bit of discussion and several projects recently
>>>>>> related to using pharo for scripting, so it may become more common.
>>>>>> E.g.
>>>>>>
>>>>>>
>>>>>> https://www.quora.com/Can-Smalltalk-be-a-batch-file-scripting-language/answer/Philippe-Back-1?share=c19bfc95
>>>>>> https://github.com/rajula96reddy/pharo-cli
>>>>>
>>>>> Still, it is not common at all.
>>>>>
>>>>>>> I have a feeling that too much functionality is being pushed into too
>>>>>>> small an API.
>>>>>>
>>>>>> This is just about how should Zinc streams be iterating over the
>>>>>> underlying streams.  You didn't like checking the result of #next for
>>>>>> nil since it isn't general, correctly pointing out that nil is a valid
>>>>>> value for non-byte oriented streams.  But #atEnd doesn't work for
>>>>>> stdin from a terminal.
>>>>>>
>>>>>>
>>>>>> At this point I think there are three options:
>>>>>>
>>>>>> 1. Modify Zinc to check the return value of #next instead of using
>>>>>> #atEnd.
>>>>>>
>>>>>> This is what all existing character / byte oriented streams in Squeak
>>>>>> and Pharo do.  At that point the Zinc streams can be used on all file
>>>>>> / stdio input and output.
>>>>>
>>>>> I agree that such code exists in many places, but there is lots of
>>>>> stream reading that does not check for nils.
>>>>
>>>> Right.  Streams can be categorised in many ways, but for this
>>>> discussion I think streams are broken in to two types:
>>>>
>>>> 1) Byte / Character oriented
>>>> 2) All others
>>>>
>>>> For historical reasons, byte / character oriented streams need to
>>>> check for EOF by using "stream next == nil" and all other streams
>>>> should use #atEnd.
>>>>
>>>> This avoids the "nil being part of the domain" issue that was
>>>> discussed earlier in the thread.
>>>>
>>>>
>>>>>> 2. Modify all streams to signal EOF in some other way, i.e. a sentinel
>>>>>> or notification / exception.
>>>>>>
>>>>>> This is what we were discussing below.  But it is a decent chunk of
>>>>>> work with significant impact on the existing code base.
>>>>>
>>>>> Agreed. This would be a future extension.
>>>>>
>>>>>> 3. Require anyone who wants to read from stdin to code around Zinc's
>>>>>> inability to handle terminal input.
>>>>>>
>>>>>> I'd prefer to avoid this option if possible.
>>>>>
>>>>> See higher for a more concrete usage example request.
>>>>
>>>>
>>>> testAtEnd.st
>>>> --
>>>> | ch stream string stdin |
>>>>
>>>> 'stdio.cs' asFileReference fileIn.
>>>> "stdin := FileStream stdin."
>>>> stdin := ZnCharacterReadStream on:
>>>>    (ZnBufferedReadStream on:
>>>>        Stdio stdin).
>>>> stream := (String new: 100) writeStream.
>>>> ch := stdin next.
>>>> [ ch == nil ] whileFalse: [
>>>>    stream nextPut: ch.
>>>>    ch := stdin next. ].
>>>> string := stream contents.
>>>> FileStream stdout
>>>>    nextPutAll: string; lf;
>>>>    nextPutAll: 'Characters read: ';
>>>>    nextPutAll: string size asString;
>>>>    lf.
>>>> Smalltalk snapshot: false andQuit: true.
>>>> --
>>>>
>>>> Execute with:
>>>>
>>>> ./pharo --headless Pharo7.0-64bit-e76f1a2.image testAtEnd.st
>>>>
>>>> and type Ctrl-D gives:
>>>>
>>>>
>>>> 'Errors in script loaded from testAtEnd.st'
>>>> MessageNotUnderstood: receiver of "<" is nil
>>>> UndefinedObject(Object)>>doesNotUnderstand: #<
>>>> ZnUTF8Encoder>>nextCodePointFromStream:
>>>> ZnUTF8Encoder(ZnCharacterEncoder)>>nextFromStream:
>>>> ZnCharacterReadStream>>nextElement
>>>> ZnCharacterReadStream(ZnEncodedReadStream)>>next
>>>> UndefinedObject>>DoIt
>>>> OpalCompiler>>evaluate
>>>>
>>>>
>>>> Using #atEnd to control the loop instead of "stdin next == nil"
>>>> produces the same result.
>>>>
>>>> Replacing stdin with FileStream stdin makes the script work.
>>>>
>>>> stdio.cs fixes a bug in StdioStream which really isn't part of this
>>>> discussion (PR to be submitted).
>>>>
>>>> Cheers,
>>>> Alistair
>>>>
>>>>
>>>>
>>>>
>>>>>> Does that clarify the situation?
>>>>>
>>>>> Yes, it helps. Thanks. But questions remain.
>>>>>
>>>>>> Thanks,
>>>>>> Alistair
>>>>>>
>>>>>>
>>>>>>
>>>>>>>> On 10 Apr 2018, at 18:30, Alistair Grant <[hidden email]>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>> First a quick update:
>>>>>>>>
>>>>>>>> After doing some work on primitiveFileAtEnd, #atEnd now answers
>>>>>>>> correctly for files that don't report their size correctly, e.g.
>>>>>>>> /dev/urandom and /proc/cpuinfo, whether the files are opened directly
>>>>>>>> or
>>>>>>>> redirected through stdin.
>>>>>>>>
>>>>>>>> However determining whether stdin from a terminal has reached the end
>>>>>>>> of
>>>>>>>> file can't be done without making #atEnd blocking since we have to
>>>>>>>> wait
>>>>>>>> for the user to flag the end of file, e.g. by typing Ctrl-D.  And
>>>>>>>> #atEnd
>>>>>>>> is assumed to be non-blocking.
>>>>>>>>
>>>>>>>> So currently using ZnCharacterReadStream with stdin from a terminal
>>>>>>>> will
>>>>>>>> result in a stack dump similar to:
>>>>>>>>
>>>>>>>> MessageNotUnderstood: receiver of "<" is nil
>>>>>>>> UndefinedObject(Object)>>doesNotUnderstand: #<
>>>>>>>> ZnUTF8Encoder>>nextCodePointFromStream:
>>>>>>>> ZnUTF8Encoder(ZnCharacterEncoder)>>nextFromStream:
>>>>>>>> ZnCharacterReadStream>>nextElement
>>>>>>>> ZnCharacterReadStream(ZnEncodedReadStream)>>next
>>>>>>>> UndefinedObject>>DoIt
>>>>>>>>
>>>>>>>>
>>>>>>>> Going back through the various suggestions that have been made
>>>>>>>> regarding
>>>>>>>> using a sentinel object vs. raising a notification / exception, my
>>>>>>>> (still to be polished) suggestion is to:
>>>>>>>>
>>>>>>>> 1. Add an endOfStream instance variable
>>>>>>>> 2. When the end of the stream is reached answer the value of the
>>>>>>>> instance variable (i.e. the result of sending #value to the
>>>>>>>> variable).
>>>>>>>> 3. The initial default value would be a block that raises a
>>>>>>>> Deprecation
>>>>>>>> warning and then returns nil.  This would allow existing code to
>>>>>>>> function for a changeover period.
>>>>>>>> 4. At the end of the deprecation period the default value would be
>>>>>>>> changed to a unique sentinel object which would answer itself as its
>>>>>>>> #value.
>>>>>>>>
>>>>>>>> At any time users of the stream can set their own sentinel, including
>>>>>>>> a
>>>>>>>> block that raises an exception.
>>>>>>>>
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>> Alistair
>>>>>>>>
>>>>>>>>
>>>>>>>> On 4 April 2018 at 19:24, Stephane Ducasse <[hidden email]>
>>>>>>>> wrote:
>>>>>>>>> Thanks for this discussion.
>>>>>>>>>
>>>>>>>>> On Wed, Apr 4, 2018 at 1:37 PM, Sven Van Caekenberghe <[hidden email]>
>>>>>>>>> wrote:
>>>>>>>>>> Alistair,
>>>>>>>>>>
>>>>>>>>>> First off, thanks for the discussions and your contributions, I
>>>>>>>>>> really appreciate them.
>>>>>>>>>>
>>>>>>>>>> But I want to have a discussion at the high level of the definition
>>>>>>>>>> and semantics of the stream API in Pharo.
>>>>>>>>>>
>>>>>>>>>>> On 4 Apr 2018, at 13:20, Alistair Grant <[hidden email]>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>> On 4 April 2018 at 12:56, Sven Van Caekenberghe <[hidden email]>
>>>>>>>>>>> wrote:
>>>>>>>>>>>> Playing a bit devil's advocate, the idea is that, in general,
>>>>>>>>>>>>
>>>>>>>>>>>> [ stream atEnd] whileFalse: [ stream next. "..." ].
>>>>>>>>>>>>
>>>>>>>>>>>> is no longer allowed ?
>>>>>>>>>>>
>>>>>>>>>>> It hasn't been allowed "forever" [1].  It's just been misused for
>>>>>>>>>>> almost as long.
>>>>>>>>>>>
>>>>>>>>>>> [1] Time began when stdio stream support was introduced. :-)
>>>>>>>>>>
>>>>>>>>>> I am still not convinced. Another way to put it would be that the
>>>>>>>>>> old #atEnd or #upToEnd do not make sense for these streams and some new loop
>>>>>>>>>> is needed, based on a new test (it exists for socket streams already).
>>>>>>>>>>
>>>>>>>>>> [ stream isDataAvailable ] whileTrue: [ stream next ]
>>>>>>>>>>
>>>>>>>>>>>> And you want to replace it with
>>>>>>>>>>>>
>>>>>>>>>>>> [ stream next ifNil: [ false ] ifNotNil: [ :x | "..." true ]
>>>>>>>>>>>> whileTrue.
>>>>>>>>>>>>
>>>>>>>>>>>> That is a pretty big change, no ?
>>>>>>>>>>>
>>>>>>>>>>> That's the way quite a bit of code already operates.
>>>>>>>>>>>
>>>>>>>>>>> As Denis pointed out, it's obviously problematic in the general
>>>>>>>>>>> sense,
>>>>>>>>>>> since nil can be embedded in non-byte oriented streams.  I suspect
>>>>>>>>>>> that in practice not many people write code that reads streams
>>>>>>>>>>> from
>>>>>>>>>>> both byte oriented and non-byte oriented streams.
>>>>>>>>>>
>>>>>>>>>> Maybe yes, maybe no. As Denis' example shows there is a clear
>>>>>>>>>> definition problem.
>>>>>>>>>>
>>>>>>>>>> And I do use streams of byte arrays or strings all the time, this
>>>>>>>>>> is really important. I want my parsers to work on all kinds of streams.
>>>>>>>>>>
>>>>>>>>>>>> I think/feel like a proper EOF exception would be better, more
>>>>>>>>>>>> correct.
>>>>>>>>>>>>
>>>>>>>>>>>> [ [ stream next. "..." true ] on: EOF do: [ false ] ] whileTrue.
>>>>>>>>>>>
>>>>>>>>>>> I agree, but the email thread Nicolas pointed to raises some
>>>>>>>>>>> performance questions about this approach.  It should be
>>>>>>>>>>> straightforward to do a basic performance comparison which I'll
>>>>>>>>>>> get
>>>>>>>>>>> around to if other objections aren't raised.
>>>>>>>>>>
>>>>>>>>>> Reading in bigger blocks, using #readInto:startingAt:count: (which
>>>>>>>>>> is basically Unix's (2) Read sys call), would solve performance problems, I
>>>>>>>>>> think.
>>>>>>>>>>
>>>>>>>>>>>> Will we throw away #atEnd then ? Do we need it if we cannot use
>>>>>>>>>>>> it ?
>>>>>>>>>>>
>>>>>>>>>>> Unix file i/o returns EOF if the end of file has been reach OR if
>>>>>>>>>>> an
>>>>>>>>>>> error occurs.  You should still check #atEnd after reading past
>>>>>>>>>>> the
>>>>>>>>>>> end of the file to make sure no error occurred.  Another part of
>>>>>>>>>>> the
>>>>>>>>>>> primitive change I'm proposing is to return additional information
>>>>>>>>>>> about what went wrong in the event of an error.
>>>>>>>>>>
>>>>>>>>>> I am sorry, but this kind of semantics (the OR) is way too complex
>>>>>>>>>> at the general image level, it is too specific and based on certain
>>>>>>>>>> underlying implementation details.
>>>>>>>>>>
>>>>>>>>>> Sven
>>>>>>>>>>
>>>>>>>>>>> We could modify the read primitive so that it fails if an error
>>>>>>>>>>> has
>>>>>>>>>>> occurred, and then #atEnd wouldn't be required.
>>>>>>>>>>>
>>>>>>>>>>> Cheers,
>>>>>>>>>>> Alistair
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>> On 4 Apr 2018, at 12:41, Alistair Grant <[hidden email]>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Hi Nicolas,
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 4 April 2018 at 12:36, Nicolas Cellier
>>>>>>>>>>>>> <[hidden email]> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> 2018-04-04 12:18 GMT+02:00 Alistair Grant
>>>>>>>>>>>>>> <[hidden email]>:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> 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
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks for this, I'll definitely take a look.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Do you have a sense of how Denis' suggestion of using an
>>>>>>>>>>>>> EndOfStream
>>>>>>>>>>>>> object would compare?
>>>>>>>>>>>>>
>>>>>>>>>>>>> It would keep the same coding style, but avoid the problems with
>>>>>>>>>>>>> nil.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>> Alistair
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> 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
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> <stdio.cs>
>
>

Hi Sven & Dennis,

On 11 April 2018 at 12:04, Denis Kudriashov <[hidden email]> wrote:
> Hi Alistair.
>
> I don't think anybody is annoyed by you. You are doing really good job. And
> nice thing that you are super patient to continue :)
>

On 11 April 2018 at 12:13, Sven Van Caekenberghe <[hidden email]> wrote:
>
> Yes, Alistair, you are a top notch open source contributor !
>
> For me, this discussion is about the difference between looking from low level details/issues/changes up, vs, from a higher level down.

Thanks for your kind words.



> What I try to understand is why blocking atEnd is bad?
> Here is code from VMMaker:
>
> [stdin atEnd] whileFalse:
> [| nextChunk |
> stdout nextPutAll: 'squeak> '; flush.
> nextChunk := stdin nextChunkNoTag.
> [nextChunk notEmpty and: [nextChunk first isSeparator]] whileTrue:
> [nextChunk := nextChunk allButFirst].
> Transcript cr; nextPutAll: nextChunk; cr; flush.
> [stdout print: (Compiler evaluate: nextChunk); cr; flush]
> on: Error
> do: [:ex| self logError: ex description inContext: ex signalerContext to:
> stderr]].
> quitOnEof ifTrue:
> [SourceFiles at: 2 put: nil.
> Smalltalk snapshot: false andQuit: true]
>
>
> I am not see why it breaks with blocking #atEnd. Can you explain?


First consider the case where #atEnd doesn't block and we just want to
evaluate 4+3:

1. #atEnd will return false
2. the loop will print the prompt
3. wait for input (stdin nextChunkNoTag)
4. print the result
5. goto 1.

So the screen will look like:

squeak> 4+3!
7
squeak> [cursor here]

Which is what we expect (prompt, input, result, prompt).

If #atEnd is blocking the VM will hang at step 1 until the user enters
something in the terminal.  In Ubuntu at least terminal input appears to
be line buffered, so for the example above the terminal will look like:

4+3!
squeak> 7
[cursor here]

We don't get the prompt when the program is started, the result is
printed after the prompt, and then there's just a cursor sitting at the
start of the next line.

Obviously the program could be re-written to have the correct output
with #atEnd blocking.  But I'm arguing that this program is
representative of many others, and we don't want to break backward
compatibility in this case.

Cheers,
Alistair

> 2018-04-11 11:41 GMT+02:00 Alistair Grant <[hidden email]>:
>>
>> Hi Sven,
>>
>> Oh dear.  I feel as though I'm not getting my concerns across at all
>> well, and I'm pushing hard enough that all I'm going to do is make
>> people annoyed.  So let me try to restate the issue one last time
>> before answering your questions directly.
>>
>> Pharo & Squeak have unwritten rules about stream usage that I suspect
>> have just emerged over time without being designed.
>>
>> If you want to be able to iterate over any stream, and in particular
>> stdin from a terminal (which, as far as I know, is the outlier that
>> causes all the problems) you have to follow these rules:
>>
>> 1.  If the stream is character / byte oriented you have to check for
>> EOF using "stream next == nil".  #atEnd can be used, but you'll still
>> have to do the nil check.
>>
>> 2.  All other streams have to check for EOF (end of stream) using
>> #atEnd.  "stream next == nil" can be used, but you'll still need to
>> test #atEnd to determine whether nil is a value returned by the
>> stream.
>>
>> If you write code that you want to be able to consume characters,
>> bytes or any other object, you'll have to test both "stream next ==
>> nil" and #atEnd.
>>
>> The rules are the result of the original blue book design being that
>> #atEnd should be used, and then character input from a terminal being
>> added later, but always returning an EOF character (nil) before #atEnd
>> answers correctly.
>>
>> At the moment, ZnCharacterEncoder uses #atEnd on character / byte
>> streams, so fails for stdin on a terminal.
>>
>> Back to your questions:
>>
>> On 11 April 2018 at 11:12, Sven Van Caekenberghe <[hidden email]> wrote:
>> >
>> >
>> >> On 11 Apr 2018, at 10:29, Alistair Grant <[hidden email]> wrote:
>> >>
>> >> Hi Denis,
>> >>
>> >> On 11 April 2018 at 10:02, Denis Kudriashov <[hidden email]>
>> >> wrote:
>> >>>
>> >>> 2018-04-11 8:32 GMT+02:00 Alistair Grant <[hidden email]>:
>> >>>>
>> >>>>>>> Where is it being said that #next and/or #atEnd should be blocking
>> >>>>>>> or
>> >>>>>>> non-blocking ?
>> >>>>>>
>> >>>>>> There is existing code that assumes that #atEnd is non-blocking and
>> >>>>>> that #next is allowed block.  I believe that we should keep those
>> >>>>>> conditions.
>> >>>>>
>> >>>>> I fail to see where that is written down, either way. Can you point
>> >>>>> me
>> >>>>> to comments stating that, I would really like to know ?
>> >>>>
>> >>>> I'm not aware of it being written down, just that ever existing
>> >>>> implementation I'm aware of behaves this way.
>> >>>>
>> >>>> On the other hand, making #atEnd blocking breaks Eliot's REPL sample
>> >>>> (in Squeak).
>> >>>
>> >>>
>> >>> Could you write here this example, please?
>> >>
>> >> The code is loaded in squeak using:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/buildspurtrunkreaderimage.sh
>> >>
>> >> for 32 bit images.  It loads:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/LoadReader.st
>> >>
>> >> which loads package CogTools-Listener in
>> >> http://source.squeak.org/VMMaker
>> >>
>> >> An image that automatically runs the code and nothing else is created
>> >> in:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/StartReader.st
>> >>
>> >>
>> >> If you want to run it interactively you can load CogTools-Listener and
>> >> do something like:
>> >>
>> >> StdioListener new
>> >>    quitOnEof: false;
>> >>    run
>> >
>> > What does #quitOnEof: do ? Can the StdioListener code be browsed/viewed
>> > online somewhere ?
>>
>> I just referenced this as an example of making #atEnd (really
>> FilePlugin>>primitiveFileAtEnd) blocking causing problems.  I wasn't
>> expecting people to go and look at the code or use it as a test.
>>
>> If you really want to look at it (from Pharo):
>>
>> 1. Add http://source.squeak.org/VMMaker as a repository.
>> 2. Browse the CogTools-Listener package
>>
>>
>> >> If you modify #atEnd to block it will result in the "squeak>" input
>> >> prompt being printed in the terminal after the input has been entered.
>> >
>> > How does one modify #atEnd to block ? I suppose you are talking about
>> > StdioStream>>#atEnd ?
>>
>> I meant the primitive, i.e. FilePlugin>>primitiveFileAtEnd /
>> FilePluginPrims>>atEnd:.
>>
>>
>> >  ^ self peek isNil
>> >
>> > ?
>> >
>> > PS: I liked your runnable example better, I will try it later on. Thx!
>>
>> Right.  My code is meant to be minimal and trigger the problem I'm
>> actually focused on - that ZnCharacterEncoder doesn't work with stdin
>> from a terminal.
>>
>> Sven has expressed a hesitation to change the internal operation of
>> the Zinc streams from using #atEnd to "stream peek == nil" and this
>> whole discussion is really about us trying to resolve our different
>> perspective of the best path forward.  I respect Sven and his work so
>> I'm trying to justify the change (but I'm not expressing it at all
>> well, obviously).
>>
>> Cheers,
>> Alistair
>>
>>
>>
>> >> The code can be loaded in to Pharo and basically works, but the output
>> >> tends to be hidden behind the next input prompt because it uses #cr
>> >> instead of #lf.  You can easily modify StdioListener>>initialize to
>> >> set the line end convention in stdout.
>> >>
>> >> NOTE: It is not intended to be a release quality implementation of a
>> >> evaluation loop.  The whole purpose as I understand it is for it to be
>> >> as simple as possible to assist in tracking down issues using the VM
>> >> simulator.  It runs minimal code to get to the point of waiting for
>> >> user input and then allows an expression that causes problems to be
>> >> entered and traced using the simulator.
>> >>
>> >> Cheers,
>> >> Alistair
>> >>
>> >>
>> >>
>> >>>>>>> How is this related to how EOF is signalled ?
>> >>>>>>
>> >>>>>> Because, combined with terminal EOF not being known until the user
>> >>>>>> explicitly flags it (with Ctrl-D) it means that #atEnd can't be
>> >>>>>> used
>> >>>>>> for iterating over input from stdin connected to a terminal.
>> >>>>>
>> >>>>> This seems to me like an exception that only holds for one
>> >>>>> particular
>> >>>>> stream in one particular scenario (interactive stdin). I might be
>> >>>>> wrong.
>> >>>>>
>> >>>>>>> It seems to me that there are quite a few classes of streams that
>> >>>>>>> are
>> >>>>>>> 'special' in the sense that #next could be blocking and/or #atEnd
>> >>>>>>> could be
>> >>>>>>> unclear - socket/network streams, serial streams, maybe stdio
>> >>>>>>> (interactive
>> >>>>>>> or not). Without a message like #isDataAvailable you cannot handle
>> >>>>>>> those
>> >>>>>>> without blocking.
>> >>>>>>
>> >>>>>> Right.  I think this is a distraction (I was trying to explain some
>> >>>>>> details, but it's causing more confusion instead of helping).
>> >>>>>>
>> >>>>>> The important point is that #atEnd doesn't work for iterating over
>> >>>>>> streams with terminal input
>> >>>>>
>> >>>>> Maybe you should also point to the actual code that fails. I mean
>> >>>>> you
>> >>>>> showed a partial stack trace, but not how you got there, precisely.
>> >>>>> How does
>> >>>>> the application reading from an interactive stdin do to get into
>> >>>>> trouble ?
>> >>>>
>> >>>> Included below.
>> >>>>
>> >>>>
>> >>>>>>> Reading from stdin seems like a very rare case for a Smalltalk
>> >>>>>>> system
>> >>>>>>> (not that it should not be possible).
>> >>>>>>
>> >>>>>> There's been quite a bit of discussion and several projects
>> >>>>>> recently
>> >>>>>> related to using pharo for scripting, so it may become more common.
>> >>>>>> E.g.
>> >>>>>>
>> >>>>>>
>> >>>>>>
>> >>>>>> https://www.quora.com/Can-Smalltalk-be-a-batch-file-scripting-language/answer/Philippe-Back-1?share=c19bfc95
>> >>>>>> https://github.com/rajula96reddy/pharo-cli
>> >>>>>
>> >>>>> Still, it is not common at all.
>> >>>>>
>> >>>>>>> I have a feeling that too much functionality is being pushed into
>> >>>>>>> too
>> >>>>>>> small an API.
>> >>>>>>
>> >>>>>> This is just about how should Zinc streams be iterating over the
>> >>>>>> underlying streams.  You didn't like checking the result of #next
>> >>>>>> for
>> >>>>>> nil since it isn't general, correctly pointing out that nil is a
>> >>>>>> valid
>> >>>>>> value for non-byte oriented streams.  But #atEnd doesn't work for
>> >>>>>> stdin from a terminal.
>> >>>>>>
>> >>>>>>
>> >>>>>> At this point I think there are three options:
>> >>>>>>
>> >>>>>> 1. Modify Zinc to check the return value of #next instead of using
>> >>>>>> #atEnd.
>> >>>>>>
>> >>>>>> This is what all existing character / byte oriented streams in
>> >>>>>> Squeak
>> >>>>>> and Pharo do.  At that point the Zinc streams can be used on all
>> >>>>>> file
>> >>>>>> / stdio input and output.
>> >>>>>
>> >>>>> I agree that such code exists in many places, but there is lots of
>> >>>>> stream reading that does not check for nils.
>> >>>>
>> >>>> Right.  Streams can be categorised in many ways, but for this
>> >>>> discussion I think streams are broken in to two types:
>> >>>>
>> >>>> 1) Byte / Character oriented
>> >>>> 2) All others
>> >>>>
>> >>>> For historical reasons, byte / character oriented streams need to
>> >>>> check for EOF by using "stream next == nil" and all other streams
>> >>>> should use #atEnd.
>> >>>>
>> >>>> This avoids the "nil being part of the domain" issue that was
>> >>>> discussed earlier in the thread.
>> >>>>
>> >>>>
>> >>>>>> 2. Modify all streams to signal EOF in some other way, i.e. a
>> >>>>>> sentinel
>> >>>>>> or notification / exception.
>> >>>>>>
>> >>>>>> This is what we were discussing below.  But it is a decent chunk of
>> >>>>>> work with significant impact on the existing code base.
>> >>>>>
>> >>>>> Agreed. This would be a future extension.
>> >>>>>
>> >>>>>> 3. Require anyone who wants to read from stdin to code around
>> >>>>>> Zinc's
>> >>>>>> inability to handle terminal input.
>> >>>>>>
>> >>>>>> I'd prefer to avoid this option if possible.
>> >>>>>
>> >>>>> See higher for a more concrete usage example request.
>> >>>>
>> >>>>
>> >>>> testAtEnd.st
>> >>>> --
>> >>>> | ch stream string stdin |
>> >>>>
>> >>>> 'stdio.cs' asFileReference fileIn.
>> >>>> "stdin := FileStream stdin."
>> >>>> stdin := ZnCharacterReadStream on:
>> >>>>    (ZnBufferedReadStream on:
>> >>>>        Stdio stdin).
>> >>>> stream := (String new: 100) writeStream.
>> >>>> ch := stdin next.
>> >>>> [ ch == nil ] whileFalse: [
>> >>>>    stream nextPut: ch.
>> >>>>    ch := stdin next. ].
>> >>>> string := stream contents.
>> >>>> FileStream stdout
>> >>>>    nextPutAll: string; lf;
>> >>>>    nextPutAll: 'Characters read: ';
>> >>>>    nextPutAll: string size asString;
>> >>>>    lf.
>> >>>> Smalltalk snapshot: false andQuit: true.
>> >>>> --
>> >>>>
>> >>>> Execute with:
>> >>>>
>> >>>> ./pharo --headless Pharo7.0-64bit-e76f1a2.image testAtEnd.st
>> >>>>
>> >>>> and type Ctrl-D gives:
>> >>>>
>> >>>>
>> >>>> 'Errors in script loaded from testAtEnd.st'
>> >>>> MessageNotUnderstood: receiver of "<" is nil
>> >>>> UndefinedObject(Object)>>doesNotUnderstand: #<
>> >>>> ZnUTF8Encoder>>nextCodePointFromStream:
>> >>>> ZnUTF8Encoder(ZnCharacterEncoder)>>nextFromStream:
>> >>>> ZnCharacterReadStream>>nextElement
>> >>>> ZnCharacterReadStream(ZnEncodedReadStream)>>next
>> >>>> UndefinedObject>>DoIt
>> >>>> OpalCompiler>>evaluate
>> >>>>
>> >>>>
>> >>>> Using #atEnd to control the loop instead of "stdin next == nil"
>> >>>> produces the same result.
>> >>>>
>> >>>> Replacing stdin with FileStream stdin makes the script work.
>> >>>>
>> >>>> stdio.cs fixes a bug in StdioStream which really isn't part of this
>> >>>> discussion (PR to be submitted).
>> >>>>
>> >>>> Cheers,
>> >>>> Alistair
>> >>>>
>> >>>>
>> >>>>
>> >>>>
>> >>>>>> Does that clarify the situation?
>> >>>>>
>> >>>>> Yes, it helps. Thanks. But questions remain.
>> >>>>>
>> >>>>>> Thanks,
>> >>>>>> Alistair

Hi Sven & Dennis,

On 11 April 2018 at 12:04, Denis Kudriashov <[hidden email]> wrote:
> Hi Alistair.
>
> I don't think anybody is annoyed by you. You are doing really good job. And
> nice thing that you are super patient to continue :)
>

On 11 April 2018 at 12:13, Sven Van Caekenberghe <[hidden email]> wrote:
>
> Yes, Alistair, you are a top notch open source contributor !
>
> For me, this discussion is about the difference between looking from low level details/issues/changes up, vs, from a higher level down.

Thanks for your kind words.



> What I try to understand is why blocking atEnd is bad?
> Here is code from VMMaker:
>
> [stdin atEnd] whileFalse:
> [| nextChunk |
> stdout nextPutAll: 'squeak> '; flush.
> nextChunk := stdin nextChunkNoTag.
> [nextChunk notEmpty and: [nextChunk first isSeparator]] whileTrue:
> [nextChunk := nextChunk allButFirst].
> Transcript cr; nextPutAll: nextChunk; cr; flush.
> [stdout print: (Compiler evaluate: nextChunk); cr; flush]
> on: Error
> do: [:ex| self logError: ex description inContext: ex signalerContext to:
> stderr]].
> quitOnEof ifTrue:
> [SourceFiles at: 2 put: nil.
> Smalltalk snapshot: false andQuit: true]
>
>
> I am not see why it breaks with blocking #atEnd. Can you explain?


First consider the case where #atEnd doesn't block and we just want to
evaluate 4+3:

1. #atEnd will return false
2. the loop will print the prompt
3. wait for input (stdin nextChunkNoTag)
4. print the result
5. goto 1.

So the screen will look like:

squeak> 4+3!
7
squeak> [cursor here]

Which is what we expect (prompt, input, result, prompt).

If #atEnd is blocking the VM will hang at step 1 until the user enters
something in the terminal.  In Ubuntu at least terminal input appears to
be line buffered, so for the example above the terminal will look like:

4+3!
squeak> 7
[cursor here]

We don't get the prompt when the program is started, the result is
printed after the prompt, and then there's just a cursor sitting at the
start of the next line.

Obviously the program could be re-written to have the correct output
with #atEnd blocking.  But I'm arguing that this program is
representative of many others, and we don't want to break backward
compatibility in this case.

Cheers,
Alistair

> 2018-04-11 11:41 GMT+02:00 Alistair Grant <[hidden email]>:
>>
>> Hi Sven,
>>
>> Oh dear.  I feel as though I'm not getting my concerns across at all
>> well, and I'm pushing hard enough that all I'm going to do is make
>> people annoyed.  So let me try to restate the issue one last time
>> before answering your questions directly.
>>
>> Pharo & Squeak have unwritten rules about stream usage that I suspect
>> have just emerged over time without being designed.
>>
>> If you want to be able to iterate over any stream, and in particular
>> stdin from a terminal (which, as far as I know, is the outlier that
>> causes all the problems) you have to follow these rules:
>>
>> 1.  If the stream is character / byte oriented you have to check for
>> EOF using "stream next == nil".  #atEnd can be used, but you'll still
>> have to do the nil check.
>>
>> 2.  All other streams have to check for EOF (end of stream) using
>> #atEnd.  "stream next == nil" can be used, but you'll still need to
>> test #atEnd to determine whether nil is a value returned by the
>> stream.
>>
>> If you write code that you want to be able to consume characters,
>> bytes or any other object, you'll have to test both "stream next ==
>> nil" and #atEnd.
>>
>> The rules are the result of the original blue book design being that
>> #atEnd should be used, and then character input from a terminal being
>> added later, but always returning an EOF character (nil) before #atEnd
>> answers correctly.
>>
>> At the moment, ZnCharacterEncoder uses #atEnd on character / byte
>> streams, so fails for stdin on a terminal.
>>
>> Back to your questions:
>>
>> On 11 April 2018 at 11:12, Sven Van Caekenberghe <[hidden email]> wrote:
>> >
>> >
>> >> On 11 Apr 2018, at 10:29, Alistair Grant <[hidden email]> wrote:
>> >>
>> >> Hi Denis,
>> >>
>> >> On 11 April 2018 at 10:02, Denis Kudriashov <[hidden email]>
>> >> wrote:
>> >>>
>> >>> 2018-04-11 8:32 GMT+02:00 Alistair Grant <[hidden email]>:
>> >>>>
>> >>>>>>> Where is it being said that #next and/or #atEnd should be blocking
>> >>>>>>> or
>> >>>>>>> non-blocking ?
>> >>>>>>
>> >>>>>> There is existing code that assumes that #atEnd is non-blocking and
>> >>>>>> that #next is allowed block.  I believe that we should keep those
>> >>>>>> conditions.
>> >>>>>
>> >>>>> I fail to see where that is written down, either way. Can you point
>> >>>>> me
>> >>>>> to comments stating that, I would really like to know ?
>> >>>>
>> >>>> I'm not aware of it being written down, just that ever existing
>> >>>> implementation I'm aware of behaves this way.
>> >>>>
>> >>>> On the other hand, making #atEnd blocking breaks Eliot's REPL sample
>> >>>> (in Squeak).
>> >>>
>> >>>
>> >>> Could you write here this example, please?
>> >>
>> >> The code is loaded in squeak using:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/buildspurtrunkreaderimage.sh
>> >>
>> >> for 32 bit images.  It loads:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/LoadReader.st
>> >>
>> >> which loads package CogTools-Listener in
>> >> http://source.squeak.org/VMMaker
>> >>
>> >> An image that automatically runs the code and nothing else is created
>> >> in:
>> >>
>> >>
>> >> https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/Cog/image/StartReader.st
>> >>
>> >>
>> >> If you want to run it interactively you can load CogTools-Listener and
>> >> do something like:
>> >>
>> >> StdioListener new
>> >>    quitOnEof: false;
>> >>    run
>> >
>> > What does #quitOnEof: do ? Can the StdioListener code be browsed/viewed
>> > online somewhere ?
>>
>> I just referenced this as an example of making #atEnd (really
>> FilePlugin>>primitiveFileAtEnd) blocking causing problems.  I wasn't
>> expecting people to go and look at the code or use it as a test.
>>
>> If you really want to look at it (from Pharo):
>>
>> 1. Add http://source.squeak.org/VMMaker as a repository.
>> 2. Browse the CogTools-Listener package
>>
>>
>> >> If you modify #atEnd to block it will result in the "squeak>" input
>> >> prompt being printed in the terminal after the input has been entered.
>> >
>> > How does one modify #atEnd to block ? I suppose you are talking about
>> > StdioStream>>#atEnd ?
>>
>> I meant the primitive, i.e. FilePlugin>>primitiveFileAtEnd /
>> FilePluginPrims>>atEnd:.
>>
>>
>> >  ^ self peek isNil
>> >
>> > ?
>> >
>> > PS: I liked your runnable example better, I will try it later on. Thx!
>>
>> Right.  My code is meant to be minimal and trigger the problem I'm
>> actually focused on - that ZnCharacterEncoder doesn't work with stdin
>> from a terminal.
>>
>> Sven has expressed a hesitation to change the internal operation of
>> the Zinc streams from using #atEnd to "stream peek == nil" and this
>> whole discussion is really about us trying to resolve our different
>> perspective of the best path forward.  I respect Sven and his work so
>> I'm trying to justify the change (but I'm not expressing it at all
>> well, obviously).
>>
>> Cheers,
>> Alistair
>>
>>
>>
>> >> The code can be loaded in to Pharo and basically works, but the output
>> >> tends to be hidden behind the next input prompt because it uses #cr
>> >> instead of #lf.  You can easily modify StdioListener>>initialize to
>> >> set the line end convention in stdout.
>> >>
>> >> NOTE: It is not intended to be a release quality implementation of a
>> >> evaluation loop.  The whole purpose as I understand it is for it to be
>> >> as simple as possible to assist in tracking down issues using the VM
>> >> simulator.  It runs minimal code to get to the point of waiting for
>> >> user input and then allows an expression that causes problems to be
>> >> entered and traced using the simulator.
>> >>
>> >> Cheers,
>> >> Alistair
>> >>
>> >>
>> >>
>> >>>>>>> How is this related to how EOF is signalled ?
>> >>>>>>
>> >>>>>> Because, combined with terminal EOF not being known until the user
>> >>>>>> explicitly flags it (with Ctrl-D) it means that #atEnd can't be
>> >>>>>> used
>> >>>>>> for iterating over input from stdin connected to a terminal.
>> >>>>>
>> >>>>> This seems to me like an exception that only holds for one
>> >>>>> particular
>> >>>>> stream in one particular scenario (interactive stdin). I might be
>> >>>>> wrong.
>> >>>>>
>> >>>>>>> It seems to me that there are quite a few classes of streams that
>> >>>>>>> are
>> >>>>>>> 'special' in the sense that #next could be blocking and/or #atEnd
>> >>>>>>> could be
>> >>>>>>> unclear - socket/network streams, serial streams, maybe stdio
>> >>>>>>> (interactive
>> >>>>>>> or not). Without a message like #isDataAvailable you cannot handle
>> >>>>>>> those
>> >>>>>>> without blocking.
>> >>>>>>
>> >>>>>> Right.  I think this is a distraction (I was trying to explain some
>> >>>>>> details, but it's causing more confusion instead of helping).
>> >>>>>>
>> >>>>>> The important point is that #atEnd doesn't work for iterating over
>> >>>>>> streams with terminal input
>> >>>>>
>> >>>>> Maybe you should also point to the actual code that fails. I mean
>> >>>>> you
>> >>>>> showed a partial stack trace, but not how you got there, precisely.
>> >>>>> How does
>> >>>>> the application reading from an interactive stdin do to get into
>> >>>>> trouble ?
>> >>>>
>> >>>> Included below.
>> >>>>
>> >>>>
>> >>>>>>> Reading from stdin seems like a very rare case for a Smalltalk
>> >>>>>>> system
>> >>>>>>> (not that it should not be possible).
>> >>>>>>
>> >>>>>> There's been quite a bit of discussion and several projects
>> >>>>>> recently
>> >>>>>> related to using pharo for scripting, so it may become more common.
>> >>>>>> E.g.
>> >>>>>>
>> >>>>>>
>> >>>>>>
>> >>>>>> https://www.quora.com/Can-Smalltalk-be-a-batch-file-scripting-language/answer/Philippe-Back-1?share=c19bfc95
>> >>>>>> https://github.com/rajula96reddy/pharo-cli
>> >>>>>
>> >>>>> Still, it is not common at all.
>> >>>>>
>> >>>>>>> I have a feeling that too much functionality is being pushed into
>> >>>>>>> too
>> >>>>>>> small an API.
>> >>>>>>
>> >>>>>> This is just about how should Zinc streams be iterating over the
>> >>>>>> underlying streams.  You didn't like checking the result of #next
>> >>>>>> for
>> >>>>>> nil since it isn't general, correctly pointing out that nil is a
>> >>>>>> valid
>> >>>>>> value for non-byte oriented streams.  But #atEnd doesn't work for
>> >>>>>> stdin from a terminal.
>> >>>>>>
>> >>>>>>
>> >>>>>> At this point I think there are three options:
>> >>>>>>
>> >>>>>> 1. Modify Zinc to check the return value of #next instead of using
>> >>>>>> #atEnd.
>> >>>>>>
>> >>>>>> This is what all existing character / byte oriented streams in
>> >>>>>> Squeak
>> >>>>>> and Pharo do.  At that point the Zinc streams can be used on all
>> >>>>>> file
>> >>>>>> / stdio input and output.
>> >>>>>
>> >>>>> I agree that such code exists in many places, but there is lots of
>> >>>>> stream reading that does not check for nils.
>> >>>>
>> >>>> Right.  Streams can be categorised in many ways, but for this
>> >>>> discussion I think streams are broken in to two types:
>> >>>>
>> >>>> 1) Byte / Character oriented
>> >>>> 2) All others
>> >>>>
>> >>>> For historical reasons, byte / character oriented streams need to
>> >>>> check for EOF by using "stream next == nil" and all other streams
>> >>>> should use #atEnd.
>> >>>>
>> >>>> This avoids the "nil being part of the domain" issue that was
>> >>>> discussed earlier in the thread.
>> >>>>
>> >>>>
>> >>>>>> 2. Modify all streams to signal EOF in some other way, i.e. a
>> >>>>>> sentinel
>> >>>>>> or notification / exception.
>> >>>>>>
>> >>>>>> This is what we were discussing below.  But it is a decent chunk of
>> >>>>>> work with significant impact on the existing code base.
>> >>>>>
>> >>>>> Agreed. This would be a future extension.
>> >>>>>
>> >>>>>> 3. Require anyone who wants to read from stdin to code around
>> >>>>>> Zinc's
>> >>>>>> inability to handle terminal input.
>> >>>>>>
>> >>>>>> I'd prefer to avoid this option if possible.
>> >>>>>
>> >>>>> See higher for a more concrete usage example request.
>> >>>>
>> >>>>
>> >>>> testAtEnd.st
>> >>>> --
>> >>>> | ch stream string stdin |
>> >>>>
>> >>>> 'stdio.cs' asFileReference fileIn.
>> >>>> "stdin := FileStream stdin."
>> >>>> stdin := ZnCharacterReadStream on:
>> >>>>    (ZnBufferedReadStream on:
>> >>>>        Stdio stdin).
>> >>>> stream := (String new: 100) writeStream.
>> >>>> ch := stdin next.
>> >>>> [ ch == nil ] whileFalse: [
>> >>>>    stream nextPut: ch.
>> >>>>    ch := stdin next. ].
>> >>>> string := stream contents.
>> >>>> FileStream stdout
>> >>>>    nextPutAll: string; lf;
>> >>>>    nextPutAll: 'Characters read: ';
>> >>>>    nextPutAll: string size asString;
>> >>>>    lf.
>> >>>> Smalltalk snapshot: false andQuit: true.
>> >>>> --
>> >>>>
>> >>>> Execute with:
>> >>>>
>> >>>> ./pharo --headless Pharo7.0-64bit-e76f1a2.image testAtEnd.st
>> >>>>
>> >>>> and type Ctrl-D gives:
>> >>>>
>> >>>>
>> >>>> 'Errors in script loaded from testAtEnd.st'
>> >>>> MessageNotUnderstood: receiver of "<" is nil
>> >>>> UndefinedObject(Object)>>doesNotUnderstand: #<
>> >>>> ZnUTF8Encoder>>nextCodePointFromStream:
>> >>>> ZnUTF8Encoder(ZnCharacterEncoder)>>nextFromStream:
>> >>>> ZnCharacterReadStream>>nextElement
>> >>>> ZnCharacterReadStream(ZnEncodedReadStream)>>next
>> >>>> UndefinedObject>>DoIt
>> >>>> OpalCompiler>>evaluate
>> >>>>
>> >>>>
>> >>>> Using #atEnd to control the loop instead of "stdin next == nil"
>> >>>> produces the same result.
>> >>>>
>> >>>> Replacing stdin with FileStream stdin makes the script work.
>> >>>>
>> >>>> stdio.cs fixes a bug in StdioStream which really isn't part of this
>> >>>> discussion (PR to be submitted).
>> >>>>
>> >>>> Cheers,
>> >>>> Alistair
>> >>>>
>> >>>>
>> >>>>
>> >>>>
>> >>>>>> Does that clarify the situation?
>> >>>>>
>> >>>>> Yes, it helps. Thanks. But questions remain.
>> >>>>>
>> >>>>>> Thanks,
>> >>>>>> Alistair