[Pharo-project] Small improvement for WriteStream >> nextPutAll:

Henrik Sperre Johansen henrik.s.johansen at veloxit.no
Mon Jan 9 11:06:43 CET 2012


On 09.01.2012 00:18, Eliot Miranda wrote:
>
>
> On Sun, Jan 8, 2012 at 5:46 AM, Mariano Martinez Peck 
> <marianopeck at gmail.com <mailto:marianopeck at gmail.com>> wrote:
>
>     Hi guys. Some time ago Henry spot to us a small improvement for
>     WriteStream >> nextPutAll: that we are using in Fuel, but I think
>     it can be general. Henry comment was exactly:
>
>     /Also: For variableBytes classes, if you rewrite:
>
>     WriteStream
>     nextPutAll: aCollection
>
>          | newEnd |
>     *collection class instSpec == aCollection class instSpec ifFalse:*
>              [^ super nextPutAll: aCollection ].
>
>          newEnd := position + aCollection size.
>          newEnd > writeLimit ifTrue:
>              [self growTo: newEnd + 10].
>
>     collection replaceFrom: position+1 to: newEnd  with: aCollection
>     startingAt: 1.
>          position := newEnd.
>
>         ^ aCollection
>
>     You can now pass all variableByte classes (Like ByteString)
>     directly to a stream with a ByteArray collection, and the
>     replaceFrom:to:with:/
>     /startingAt: primitive will work correctly, just like the file
>     primitive does
>     This means you don't need special Serializers for these either, 
>     using f.ex. clunky nextStringPutAll: methods with manual
>     asByteArray conversions./
>
>
>     So...if you agree, I can commit the patch.
>
>
> Surely this will break special encoded strings where the encode/decode 
> is done in at:/at:put:.  Moving the bytes isn't safe in general. 
>  However, if the programmer knows they themselves can use 
> next:putAll:startingAt: or some such.  I don't think your suggestion 
> is safe in general for WriteStream.  Instead if 
>  next:putAll:startingAt: doesn't do the job invent some new protocol 
> that will.
In principle I agree, though the only such case I can think of in an 
image atm is the newly introduced:
wa := WordArray new: 2.
ws := wa writeStream.
ws nextPutAll: 1.0

The string example in itself is rather theoretical, the "convention" is 
to use ByteStrings for all encodings, and let the programmer fend for 
himself when it comes to remembering which ByteStrings he's converted to 
which encoding.
(A practice which, sadly, is only enabled by the collection class = 
aCollection class check (the one in next:putAll:startingAt:), as 
encoding/decoding a ByteString to a ByteString is much faster than the 
equivalent operation resulting in a ByteArray... )
So in practice, even the current class check does not protect against 
what would be the common error.

Though, for a general, collection-agnostic approach which can still use 
primitives between compatible classes, I guess giving the nextPutAll: 
argument a say in what method is actually used to do a 
next:putAll:startingAt: would be a better approach than simply removing 
the check and crossing our fingers. :)

Cheers,
Henry



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