Dear all
I have what I think should be a relatively simple problem:
I have a C program which reads in data from a binary file, it is a
specific format so it knows that there are 3 integers followed by three
doubles or whatever. So I'm trying to serialise the data from within a
Haskell program and write it out to a file such that the C program can
read this.
I've gotten the integers to work no problem but I'm having difficulty
with the doubles.
My method is to use the Data.Binary library to write the data out as a
bytestring and then simply write this out to disk.
The problem is that the instance of Binary for Double does not
serialise the data in the format which is expected by the C program
(standard IEEE format using fread and a cast).
Now there was a thread on (essentially) this very same topic on
haskell-cafe back in April, see:
http://www.haskell.org/pipermail/haskell-cafe/2007-April/024596.html
There was some discussion about the 'right' thing to do and that the
library intended to be split into two separate portions; one for users
that wish to serialise their own data and have it readable by the same
haskell program, and those that wish to communicate with programs
written in other languages.
From the above thread:
Perhaps we just don't care about ARM or other arches where GHC runs that
do not use IEEE formats, I don't know. If that were the case we'd say
something like:
instance Binary Double where
put d = assert (isIEEE (undefined :: Double)) $ do
write (poke d)
Now for my specific problem I'd be happy if I could write a function:
putDouble :: Double -> Put
and so obviously I tried:
putDouble :: Double -> Put
putDouble d = assert (isIEEE (undefined :: Double)) $ do
write (poke d)
However I seem to be stuck at this point, where does the 'write'
function come from?
Essentially then, is there anyone that can write my 'putDouble' function
or give some hint as to how I might do it.
For the moment assume that I'm not really concerned with portability
across platforms at least for the time being (I certainly don't think
that the C program I'm attempting to communicate with is particularly
portable anyway).
kind regards
allan
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