Wrapping Code

Dominic Steinitz dominic.steinitz at blueyonder.co.uk
Sat Oct 25 13:28:06 EDT 2003

Does anyone have any views on how to "wrap" code that has been designed to
take input from handles but you now want to take input from strings? I want
to make minimal changes to a module that already works and to code that uses
that module. Here's my thinking below. I'd be interested in other approaches
and pros and cons.


I thought I would only ever have to parse ASN.1 from a socket or a file. So
I have ended up with a lot of functions like this:

getTagOctets :: Handle -> IO OctetStream
getTagOctets h =
   do x <- hGetChar h
      let y = fromIntegral (ord x) in
         if not (testBit y msb)
            then return [y]
            else do ys <- getTagOctets h
                    return ((clearBit y msb):ys)

It turns out I need to be able to parse ASN.1 from a string. So my first
thought was:

tagOctets :: String -> (String,OctetStream)
tagOctets s =
   let y = fromIntegral $ ord $ head s in
      if not $ testBit y $ msb
         then (tail s,[y])
         else let (s',ys) = tagOctets $ tail s in
                  (s',((clearBit y msb):ys))

Now this is very similar and the type signature suggests state monads. So my
second thought was this:

tagOctets' :: State String OctetStream
   do (x:xs) <- get
      put xs
      let y = fromIntegral (ord x) in
         if not (testBit y msb)
            then return [y]
            else do ys <- tagOctets'
                    return ((clearBit y msb):ys)

Even nearer but still not good enough. So I went for type class:

class Monad m => Foo m where
   get' :: Handle -> m Char

instance Foo IO where
   get' = hGetChar

instance Foo (State String) where
   get' = \_ -> do (x:xs) <- get; put xs; return x

And now I can write:

tagOctets'' h =
   do x <- get' h
      let y = fromIntegral (ord x) in
         if not (testBit y msb)
            then return [y]
            else do ys <- tagOctets'' h
                    return ((clearBit y msb):ys)

I can now use this either with a handle or with a string:

test'' :: (OctetStream,String)
test'' = runState (tagOctets'' stdin) (map (chr . fromIntegral) (encode $
toASN NoTag NULL))

test =
   do ofh <- openFile "tst.txt" WriteMode
      hPutStr ofh (map (chr . fromIntegral) (encode (toASN NoTag NULL)))
      hClose ofh
      ifh <- openFile "tst.txt" ReadMode
      (ts::OctetStream) <- tagOctets'' ifh
      putStrLn $ show ts

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