[Haskell-cafe] Re: Detecting system endianness

[Maurí­cio]

I actually don't need a pure function, IO
is OK. I'll try something in these lines.
It doesn't build yet, with an error message
I'll probably take a few months to understand:

Couldn't match expected type
  `forall a. (Storable a) => a -> IO a'
   against inferred type `a -> IO a'

Thanks,
Maurício

-----
import Control.Monad ;
import Foreign ;
import Foreign.C ;

type CUInt16 = CUShort ; type CUInt8 = CChar ;

littleEndianToHost,hostToLittleEndian
  :: forall a. (Storable a ) => a -> IO a ;

(littleEndianToHost,hostToLittleEndian) =
  (f,f) where {
         f :: forall a. ( Storable a ) => a -> IO a ;
         f a = with ( 0x0102 :: CUInt16 ) $ \p -> do {
             firstByte <- peek ( castPtr p :: Ptr CUInt8 ) ;
             littleEndian <- return $ firstByte == 0x02 ;
             halfSize <- return $ div ( alignment a ) 2;
             reverse <- with a $ \val ->
               zipWithM (swapByte (castPtr val :: Ptr CUInt8))
                [0..halfSize-1] [halfSize..2*halfSize-1]
               >> peek val ;
             return $ if littleEndian then a else reverse ;
         } ;

         swapByte p n1 n2 = do {
               v1 <- peekElemOff p n1 ;
               v2 <- peekElemOff p n2 ;
               pokeElemOff p n1 v2 ;
               pokeElemOff p n2 v1
         } >> return () }
-----

> On Thursday 18 December 2008 13:40:47 Ryan Ingram wrote:
>> Actually, this is probably safer:
>>
>> import Foreign.Marshal.Alloc
>> import Foreign.Ptr
>> import Foreign.Storable
>> import Data.Word
>> import System.IO.Unsafe
>>
>> endianCheck = unsafePerformIO $ alloca $ \p -> poke p (0x01020304 ::
>> Word32) >> peek (castPtr p :: Ptr Word8)
>>
>> littleEndian = endianCheck == 4
>> bigEndian = endianCheck == 1
>>
>> (...)