| encode :: Buffer from -> Buffer to -> IO (Buffer from, Buffer to) | The encode function translates elements of the buffer from
to the buffer to. It should translate as many elements as possible
given the sizes of the buffers, including translating zero elements
if there is either not enough room in to, or from does not
contain a complete multibyte sequence.
encode should raise an exception if, and only if, from
begins with an illegal sequence, or the first element of from
is not representable in the encoding of to. That is, if any
elements can be successfully translated before an error is
encountered, then encode should translate as much as it can
and not throw an exception. This behaviour is used by the IO
library in order to report translation errors at the point they
actually occur, rather than when the buffer is translated.
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| getState :: IO state | Return the current state of the codec.
Many codecs are not stateful, and in these case the state can be
represented as '()'. Other codecs maintain a state. For
example, UTF-16 recognises a BOM (byte-order-mark) character at
the beginning of the input, and remembers thereafter whether to
use big-endian or little-endian mode. In this case, the state
of the codec would include two pieces of information: whether we
are at the beginning of the stream (the BOM only occurs at the
beginning), and if not, whether to use the big or little-endian
encoding.
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