[Haskell-cafe] Binary and Serialize classes

[Daniel Fischer]

On Thursday 28 April 2011 17:00:49, Evan Laforge wrote:
> 
> Speaking of that Double instance... both data-binary and cereal use
> decodeFloat and encodeFloat which mean they suffer from the same
> problems as realToFrac, namely that -0 becomes 0 and NaN becomes
> -Infinity (I'm not sure why the latter happens since the decoded
> values differ... must be a problem with encodeFloat).

Yes, encodeFloat - at least in ghc - first converts the mantissa (while 
there are words left, multiply the accumulator with 2^32 or 2^64, add next 
word to it), then adjusts the exponent via ldexp. That process can't 
produce NaNs. It would be possible to check for special patterns, but it's 
impossible to know whether that pattern arose from a decodeFloat and hence 
should be converted to NaN or from a calculation that simply overflowed and 
should be converted to ±Infinity. 

> It's tempting
> to just get the ieee754 bitmap out and serialize that.

binary-ieee754 does that, iirc.

> I know I've
> seen this question around before, but is it possible to somehow cast a
> D# directly to bytes?

Not safely. unsafeCoerce :: Double -> Word64 will usually work, but if you 
wantsomething reliable, write your Double to an STUArray and read a 
Word(64) from the result of castSTUArray. That's slower than unsafeCoerce, 
but supposed to be reliable (unless I misremember).

> I know I can write a C function and FFI that
> in, but it would be tidier to do it all in haskell.  I guess I can
> probably use castPtr and memCpy, but I don't see the required
> addressOf.  I.e. how would I write 'memcpy(buf, &d, sizeof(double));'?