Re: [Haskell-cafe] Re: ANN: A triple of new packages for talking tothe outside world

On Wed, Jan 09, 2008 at 11:43:52PM +0100, Achim Schneider wrote:

"David Roundy" wrote:

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On Jan 9, 2008 10:10 AM, Dominic Steinitz wrote:

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Duncan Coutts writes:

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The difficulty is in deciding what the api should be. Does it give you a real bitstream or only a byte aligned one? If I ask for 3 bits then 15 bytes what does it do? Does it assume I meant 3 bits, then pad to the next byte boundary and get 15 bytes, or does it mean get 15 bytes but at this 3 bit shift offset?

I'd suggest an aligned and unaligned api.

So the aligned api would get 3 bits and the 15 bytes would start from the next byte boundary.

The unaligned api would get 3 bits and the 15 bytes (=15 x 8 bits) would finish still with an offset of 3.

Do you mean we'd have an unalignedGetBytes as well as getBytes (which would remain aligned)? That would make sense, but it would seem a bit heavy to duplicate all of the binary API.

getBytes per default unaligned and additionally snapToNextByte?

But I can't imagine an implementation in which this change wouldn't slow down getBytes for the normal case. Perhaps the slowdown would be small, but it seems unwise to enforce that slowness at the API level, when we've already got a perfectly good API for fast binary IO. Maybe there's some type hackery you could do to avoid a speed penalty, but that's a lot to add for a somewhat dubious benefit. Note that you could get a similar effect with getBytes always aligned, and an additional function shiftToByte which takes the remainder of the input and bitshifts everything so that the current read pointer is on a byte boundary. Obviously this would be an O(N) operation (where N is the remainder of the input), which could certainly be a problem. Another option, I suppose, would be to introduce a type class for bytewise-reading monads. That'd be a type hack, but not such a bad one. Then you could have the efficient implementation, and one that allows bitwise reading, and there could also be a function that allows bitwise parsing of a chunk of a byte-aligned data. -- David Roundy Department of Physics Oregon State University