| 3. Why Seek FileOffset is error message? Are you talking about John Lato's implementation [1]? Well, `Seek' is not an error message. It is one of constructors for ErrMsg, and ErrMsg is [2]
-- -- a message to the stream producer (e.g., to rewind the stream) -- or an error indication.
You know the overall idea behind Seek, don't you? It is an instrument to implement random IO [3]. Compare Oleg's code [4]
data SeekException = SeekException FileOffset deriving Show
instance Typeable SeekException where typeOf _ = mkTyConApp (mkTyCon "SeekException") []
instance Exception SeekException
and [2]
type ErrMsg = SomeException data Stream el = EOF (Maybe ErrMsg) | Chunk [el] deriving Show
with John Lato's implementation [1]:
data StreamG c el = EOF (Maybe ErrMsg) | Chunk (c el)
data ErrMsg = Err String | Seek FileOffset deriving (Show, Eq)
John makes Err and Seek to be the distinct constructors of ErrMsg. Errors (Err) in `iteratee' package are always Strings. Oleg's ErrMsg is SomeException. One of its instances (SeekException) is a ``rewind the stream'' message to the stream producer. And the user is free to have as many different ErrMsg'es as he needs to do the job. [1] http://inmachina.net/~jwlato/haskell/iteratee/src/Data/Iteratee/Base.hs [2] http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs [3] http://okmij.org/ftp/Streams.html#random-bin-IO [4] http://okmij.org/ftp/Haskell/Iteratee/RandomIO.hs -- vvv