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ZnCharacterWriteStream — is it a WriteStream?

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AndrewBlack

ZnCharacterWriteStream — is it a WriteStream?

I’m making a stream to write on a file using

outputFile := FileLocator home / 'Desktop' / classBeingTranslated name , '.grace'.
outputFile writeStreamDo: [ :s | ... ]

Is that OK? When I do the above, in the writeStreamDo: block, s is bound to an instance of ZnCharacterWriteStream. But that class does nt understand many methods from WriteStream. For example, #contents is not understood, and nor is #ensureASpace.

To put things another way: I wrote tests using a stream created with String streamContents: [ :s | ... ], and everything worked fine. Then, for production use, I switch to a stream created using outputFile writeStreamDo: [ :s | ... ] and everything breaks.

I know that ZnCharacterWriteStream is fairly new. Am I doing something wrong, or is ZnCharacterWriteStream still under development?

Andrew

Richard O'Keefe

Re: ZnCharacterWriteStream — is it a WriteStream?

Without getting into a general debate about the new stream classes,

a stream class meant for writing to a file SHOULDN'T have #contents

in its interface.

(1) For example, if you open a channel for *output* in Unix, you have

no reason to expect that you have been granted a right to read it.

For example, suppose user=fred group=whizzy owns a file foo.grace

with permission bits rw--w---- and you are running a program as

user=mary group=whizzy.  You can open foo.grace for writing, but

you cannot read it.

(2) The thing you open might not be a file.  It might be, for

example, a symbolic link to /dev/null, a symbolic link to /dev/tty,

a named pipe, or even (in principle, I don't know if anyone has

done it, but with something like FUSE it *could* be done) a socket

to a remote machine.  In all these cases, *nobody* is storing the

contents.

(3) If the thing you open *is* a disc file, and you *do* have

permission to read it back, what should happen if you have written

more to the file than your memory can hold?

If you know that the output written to the stream is going to be

small enough that holding it in memory won't be a problem, could

you use a WriteStream and then write it to the output file in one

burst when it's finished?

Now, what about #ensureASpace?  It can *almost* be done on a

buffered stream by looking at the last character in the buffer

and adding a space if it isn't one.  But what if you called

#flush and the buffer is empty?  Well, in that case, if you have

a "monolithic" design where the stream records the last character

at every flush, it's still doable.  If you don't have a monolithic

design, you have to be able to read the last block before the

present one, and we've just seen why that is not in general

possible.  The one thing you can safely do is to say that if the

buffer is empty you will add a space just to be sure, so that the

contract for #ensureASpace becomes

   Ensures that the past contents of the receiver ends with a

   space.  Tries to avoid adding a space when there is already

   one, but may or may not succeed.

On 1 August 2018 at 10:34, Andrew P. Black <[hidden email]> wrote:

I’m making a stream to write on a file using

outputFile := FileLocator home / 'Desktop' / classBeingTranslated name , '.grace'.
outputFile writeStreamDo: [ :s | ... ]

Is that OK? When I do the above, in the writeStreamDo: block, s is bound to an instance of ZnCharacterWriteStream. But that class does nt understand many methods from WriteStream. For example, #contents is not understood, and nor is #ensureASpace.

To put things another way: I wrote tests using a stream created with String streamContents: [ :s | ... ], and everything worked fine. Then, for production use, I switch to a stream created using outputFile writeStreamDo: [ :s | ... ] and everything breaks.

I know that ZnCharacterWriteStream is fairly new. Am I doing something wrong, or is ZnCharacterWriteStream still under development?

Andrew

Sven Van Caekenberghe-2

Re: ZnCharacterWriteStream — is it a WriteStream?

In reply to this post by AndrewBlack

> On 1 Aug 2018, at 00:34, Andrew P. Black <[hidden email]> wrote:
>
> I’m making a stream to write on a file using
>
> outputFile := FileLocator home / 'Desktop' / classBeingTranslated name , '.grace'.
> outputFile writeStreamDo: [ :s | ... ]
>
> Is that OK?

Yes

> When I do the above, in the writeStreamDo: block, s is bound to an instance of ZnCharacterWriteStream. But that class does nt understand many methods from WriteStream. For example, #contents is not understood, and nor is #ensureASpace.
>
> To put things another way: I wrote tests using a stream created with String streamContents: [ :s | ... ], and everything worked fine. Then, for production use, I switch to a stream created using outputFile writeStreamDo: [ :s | ... ] and everything breaks.
>
> I know that ZnCharacterWriteStream is fairly new. Am I doing something wrong, or is ZnCharacterWriteStream still under development?
>
> Andrew

The new streams are intentionally simpler, with a lighter API. If they were exactly the same, API wise, there would not be much point in doing them.