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PositionableStream >> peekBack

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Jakob Reschke-2

PositionableStream >> peekBack

Hi all,

I just stumbled upon the behavior of #peekBack. I expected that it
would give me a "preview" of what I would get when I send #back, but
without changing the position of the Stream (like #peek does for
#next). Or in other words, I expected to get the last read (or
written) element.

As it turned out it answers the next-to-last element instead. There
even is a test case in RWBinaryOrTextStreamTest for this behavior.

But does it make sense? I have my doubts mainly because of the
inconsistency with the return value of #back.

There is also #peekLast, which does what I expected, but it is only
defined for File- and WriteStream and the comment says it is intended
to get the last put element, not a last read/extracted/next-ed
element.

Kind regards,
Jakob

Ben Coman

Re: PositionableStream >> peekBack

When I read that first line of your post, my impression/expectation was the same as yours.

Intuitively #back and #peekBack should return the same value.

But my comments are face-value only, without a deep familiarity of streams. Maybe someone else can provide historical & future context.

Presuming it was broadly accepted this "should" be the behaviour of #peekBack, the problem with changing its behaviour is the effect on existing users, and also cross-dialect compatability. "Ideally" Squeak/Cuis would be ammendable to such a change, and then one approach could be a little dance using an interim #peekBackFromCursor so the existing #peekBack can be deprecated for one version (to avoid silent corruption of application logic) then restoring #peekBack with the new behaviour. A slow process, but such small changes accumulate over time to improve things.

Alternatively, we just leave #peekBack as-is and permanently introduce #peekBackFromCursor or similar.

Maybe such changes would be superseded by XStreams anyway (not really a reason stop improving current system anyway.)

cheers -ben

On Wed, Jun 21, 2017 at 9:00 PM, Jakob Reschke <[hidden email]> wrote:

Hi all,

I just stumbled upon the behavior of #peekBack. I expected that it
would give me a "preview" of what I would get when I send #back, but
without changing the position of the Stream (like #peek does for
#next). Or in other words, I expected to get the last read (or
written) element.

As it turned out it answers the next-to-last element instead. There
even is a test case in RWBinaryOrTextStreamTest for this behavior.

But does it make sense? I have my doubts mainly because of the
inconsistency with the return value of #back.

There is also #peekLast, which does what I expected, but it is only
defined for File- and WriteStream and the comment says it is intended
to get the last put element, not a last read/extracted/next-ed
element.

Kind regards,
Jakob

Nicolas Cellier

Re: PositionableStream >> peekBack

Hi Jakob,

wellcome to the club of peekBack deceived
http://lists.squeakfoundation.org/pipermail/squeak-dev/2014-July/179285.html

we can't reasonnably drag too many of these inconsistencies...

2017-06-22 5:48 GMT+02:00 Ben Coman <[hidden email]>:

When I read that first line of your post, my impression/expectation was the same as yours.
Intuitively #back and #peekBack should return the same value.
But my comments are face-value only, without a deep familiarity of streams. Maybe someone else can provide historical & future context.

Presuming it was broadly accepted this "should" be the behaviour of #peekBack, the problem with changing its behaviour is the effect on existing users, and also cross-dialect compatability. "Ideally" Squeak/Cuis would be ammendable to such a change, and then one approach could be a little dance using an interim #peekBackFromCursor so the existing #peekBack can be deprecated for one version (to avoid silent corruption of application logic) then restoring #peekBack with the new behaviour. A slow process, but such small changes accumulate over time to improve things.

Alternatively, we just leave #peekBack as-is and permanently introduce #peekBackFromCursor or similar.
Maybe such changes would be superseded by XStreams anyway (not really a reason stop improving current system anyway.)

cheers -ben

On Wed, Jun 21, 2017 at 9:00 PM, Jakob Reschke <[hidden email]> wrote:
Hi all,

I just stumbled upon the behavior of #peekBack. I expected that it
would give me a "preview" of what I would get when I send #back, but
without changing the position of the Stream (like #peek does for
#next). Or in other words, I expected to get the last read (or
written) element.

As it turned out it answers the next-to-last element instead. There
even is a test case in RWBinaryOrTextStreamTest for this behavior.

But does it make sense? I have my doubts mainly because of the
inconsistency with the return value of #back.

There is also #peekLast, which does what I expected, but it is only
defined for File- and WriteStream and the comment says it is intended
to get the last put element, not a last read/extracted/next-ed
element.

Kind regards,
Jakob

cbc

Re: PositionableStream >> peekBack

In reply to this post by Jakob Reschke-2

What gives you a view of what would be returned if you sent #back is:

PositionableString (and subclasses): #last

StandardFileStream (and subclasses): #peekLast

* this last one is matches to #back as defined in MutliByteFileStream. #last could be used, but since it uses the internal collection instVar, it may give odd results. I don't know - haven't tried it.

-cbc

On Wed, Jun 21, 2017 at 6:00 AM, Jakob Reschke <[hidden email]> wrote:

Hi all,

I just stumbled upon the behavior of #peekBack. I expected that it
would give me a "preview" of what I would get when I send #back, but
without changing the position of the Stream (like #peek does for
#next). Or in other words, I expected to get the last read (or
written) element.

As it turned out it answers the next-to-last element instead. There
even is a test case in RWBinaryOrTextStreamTest for this behavior.

But does it make sense? I have my doubts mainly because of the
inconsistency with the return value of #back.

There is also #peekLast, which does what I expected, but it is only
defined for File- and WriteStream and the comment says it is intended
to get the last put element, not a last read/extracted/next-ed
element.

Kind regards,
Jakob

Ben Coman

Re: PositionableStream >> peekBack

In reply to this post by Nicolas Cellier

On Fri, Jun 30, 2017 at 5:56 AM, Nicolas Cellier <[hidden email]> wrote:

"peekBack deceived"

Strangely, I can't shake the idea this is a good name for a SciFi Thriller movie.

cheers -ben

http://lists.squeakfoundation.org/pipermail/squeak-dev/2014-July/179285.html

we can't reasonnably drag too many of these inconsistencies...

2017-06-22 5:48 GMT+02:00 Ben Coman <[hidden email]>:
When I read that first line of your post, my impression/expectation was the same as yours.
Intuitively #back and #peekBack should return the same value.
But my comments are face-value only, without a deep familiarity of streams. Maybe someone else can provide historical & future context.

Presuming it was broadly accepted this "should" be the behaviour of #peekBack, the problem with changing its behaviour is the effect on existing users, and also cross-dialect compatability. "Ideally" Squeak/Cuis would be ammendable to such a change, and then one approach could be a little dance using an interim #peekBackFromCursor so the existing #peekBack can be deprecated for one version (to avoid silent corruption of application logic) then restoring #peekBack with the new behaviour. A slow process, but such small changes accumulate over time to improve things.

Alternatively, we just leave #peekBack as-is and permanently introduce #peekBackFromCursor or similar.
Maybe such changes would be superseded by XStreams anyway (not really a reason stop improving current system anyway.)

cheers -ben

On Wed, Jun 21, 2017 at 9:00 PM, Jakob Reschke <[hidden email]> wrote:
Hi all,

I just stumbled upon the behavior of #peekBack. I expected that it
would give me a "preview" of what I would get when I send #back, but
without changing the position of the Stream (like #peek does for
#next). Or in other words, I expected to get the last read (or
written) element.

As it turned out it answers the next-to-last element instead. There
even is a test case in RWBinaryOrTextStreamTest for this behavior.

But does it make sense? I have my doubts mainly because of the
inconsistency with the return value of #back.

There is also #peekLast, which does what I expected, but it is only
defined for File- and WriteStream and the comment says it is intended
to get the last put element, not a last read/extracted/next-ed
element.

Kind regards,
Jakob