[ANSI-Smalltalk] #contents on <collectionStream>

[Richard O'Keefe]

On 19 Oct 2008, at 11:59 pm, Paolo Bonzini wrote:
>>    for WriteStream and ReadWriteStream answering
>>        self upTo: position
>>    but otherwise (i.e. for read-only streams) answering
>>        self upTo: readLimit
>
> Shouldn't this be "position for ReadStream and ReadWriteStream, and
> readLimit for WriteStreams"?  It seems to me that the two are
> substantially different cases and don't fit very well in the
> do-what-I-mean nature of #contents.

In Squeak, ReadWriteStream is a subclass of WriteStream,
but doesn't agree with it.
But there is no obvious reason why ReadWriteStream *should*
be a subclass of WriteStream.
The Blue Book says it is (page 204), but that's not the
standard, and I found it much easier to make
ReadWriteStream a subclass of ReadStream.

I can't for the life of me see why WriteStream should disagree
with ReadStream and WriteStream, and evidently the ANSI crew
didn't either.

Let me tell you why I don't think WriteStream should truncate
its contents.

I sometimes build a binary data structure something like this:

	write some NULs at the beginning.
	fill in the data, remembering certain
	positions as I go.
	go back to the beginning.
	fill in the empty bits with forward links.

Let's make a toy example.

	|s p|
	s := WriteStream on: (ByteArray new: 1000).
	s nextPut: 0.
	s nextPut: 0.
	pairs do: [:each |
	    s nextPutAll: each key asByteArray].
	p := s position.
	pairs do: [:each |
	    s nextPutAll: each value asByteArray].
	s position: 0.
	s nextPut: p // 256.
	s nextPut: p \\ 256.
	^s contents

Perfectly clear, straightforward code, using no stream
operations outside the standard, with a perfectly good
result required by the standard.

So why should I have to stick
	s setToEnd.
before the last line if I don't want to lose all my data?

> For reading past and future values in WriteStream you could do "x
> setToEnd; contents" but I never found a usage for it.

There's one above.

>  The current
> behavior is sometimes useful to do something like
>
>   x do: [ :each | s print: each; nextPut: $, ].
>   s skip: -1.
>   ^s contents
>
> ... though these days #do:separatedBy: is much clearer and works with
> all streams.

Exactly so.  I have learned to be extremely wary of "skip: -1".