[Haskell-cafe] Re: Binary [was MD5]

[Andrew Coppin]

Aaron Denney wrote:
> On 2007-11-17, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
>   
>>   pack8into16 :: [Word8] -> Word16
>>   pack8into32 :: [Word8] -> Word32
>>   unpack16into8 :: Word16 -> [Word8]
>>   unpack32into8 :: Word32 -> [Word8]
>>   pack8into16s :: [Word8] -> [Word16]
>>   pack8into32s :: [Word8] -> [Word32]
>>   etc.
>>
>> I had to write all these myself, by hand, and then check that I got 
>> everything the right way round and so forth. (And every now and then I 
>> find an edge case where these functions go wrong.)
>>     
>
> Well, you know, some of these are really the wrong signatures:
>
> pack8into16 :: (Word8, Word8) -> Word16
> pack8into32 :: (Word8, Word8, Word8, Word8) -> Word32
> unpack16into8 :: Word16 -> (Word8, Word8)
> unpack32into8 :: Word32 -> (Word8, Word8, Word8, Word8)
>
> curry the above to taste.
>   

Yeah, but

  unpack16into8s = concatMap unpack16into8

;-)

Now if you just define some function splitN :: Int -> [x] -> [[x]] (I'm 
sure we've debated why this isn't defined already...), we can even write

  pack8into16s = map pack8into16 . splitN 16

And so we continue...