[Haskell-cafe] Endian conversion

[Marc Ziegert]

you are right, that pice of code is ugly. i would write sth simmilar (Int32->[Word8]) like you did, iff it should be able to cross-compile or do not need to be fast or should not need TH.
well, i think, in the case of joel's project the last sentence means "..., iff true or true or undefined".

is there any architecture with sth like 0xaabbccdd->bb aa dd cc, ghc (or any other haskell-compiler) runs on? (i did know, that such architectures exist.)

- marc


Udo Stenzel wrote:
> > >Why don't you pull out 4 bytes and assemble them manually?
> 
> To that I'd like to add a snippet from NewBinary itself:
> 
> | instance Binary Word32 where
> |   put_ h w = do
> |     putByte h (fromIntegral (w `shiftR` 24))
> |     putByte h (fromIntegral ((w `shiftR` 16) .&. 0xff))
> |     putByte h (fromIntegral ((w `shiftR` 8)  .&. 0xff))
> |     putByte h (fromIntegral (w .&. 0xff))
> |   get h = do
> |     w1 <- getWord8 h
> |     w2 <- getWord8 h
> |     w3 <- getWord8 h
> |     w4 <- getWord8 h
> |     return $! ((fromIntegral w1 `shiftL` 24) .|.
> |            (fromIntegral w2 `shiftL` 16) .|.
> |            (fromIntegral w3 `shiftL`  8) .|.
> |            (fromIntegral w4))
> 
> This obviously writes a Word32 in big endian format, also known as
> "network byte order", and doesn't care how the host platform stores
> integers.  No need for `hton' and `ntoh'.  To convert it to write little
> endian, just copy it and reorder some lines.  (But I think, writing LE
> integers with no good reason and without an enclosing protocol that
> explicitly declares them (like IIOP) is a bad idea.)
> 
> [Which reminds me, has anyone ever tried implementing a Corba ORB in
> Haskell?  There's a binding to MICO, but that just adds to the uglyness
> of MICO and does Haskell a bit of injustice...]
> 
> 
> > Well, I liked that bit of Template Haskell code that Marc sent. I'm  
> > now stuck trying to adapt it to read Storables :-).
> 
> I don't.  It's complex machinery, it's ugly, it solves a problem that
> doesn't even exist and it solves it incompletely.  It will determine the
> byte order of the host system, not of the target, which fails when
> cross-compiling, and it doesn't work on machines with little endian
> words and big endian long words (yes, this has been seen in the wild,
> though might be extinct these days).  Use it only if You Know What You
> Are Doing, have a performance problem and also know that writing
> integers en bloc would help with it.
> 
> 
> > I could read a FastString from a socket since it has IO methods but I  
> > don't know how to convert the FS into a pointer suitable for  
> > Storable. So much to learn :-).
> 
> useAsCString might be your friend.  But so might be (fold (:) []).
> 
> 
> Udo.
> -- 
> "The greatest dangers to liberty lurk in insidious encroachment by men
> of zeal, well-meaning but without understanding."
> 	-- Brandeis
> 
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