I am using ghc-5.04 and a code like:

with c ( \c' -> hPutBuf h c' (sizeOf c))

fails with "Fail: Prelude.undefined" when "c" is a user defined type, such as a pair:

instance (Storable at,Storable bt) => Storable (at,bt) where sizeOf (a,b) = sizeOf a + sizeOf b alignment (a,b) = max (alignment a) (alignment b) peek p = do a <- peek ((castPtr p):: Ptr at) b <- peekByteOff ((castPtr p):: Ptr bt) (sizeOf a) return (a,b) poke p (a,b) = do poke ((castPtr p):: Ptr at) a pokeByteOff ((castPtr p):: Ptr bt) (sizeOf a) b

sizeOf and alignment are not supposed to evaluate their arguments. You might try making the definitions in your instance above a little lazier: sizeOf z = sizeOf a + sizeOf b where (a,b) = z alignment z = max (alignment a) (alignment b) where (a,b) = z (feel free to use '~' if you prefer). Cheers, Simon