Codec.Picture.Png.Streaming.Util

This is a set of assorted utility functions for streaming and streaming-bytestring which are used in various places in this library, but may also be of use to others so they are exposed here.

Documentation

expectNull :: (MonadThrow m, Exception e) => e -> ByteString m r -> m r

If the input ByteString is empty, return its result. Otherwise throw the provided error value.

chunksOfBS :: Monad m => Int64 -> ByteString m r -> Stream (ByteString m) m r

Split a streaming ByteString up into a stream of chunks of the given size.

rememberPrevious :: Monad m => Stream (Of a) m r -> Stream (Of (Maybe a, a)) m r

Remember the previous value at each point in a stream of values.

mapWithMemory :: forall m a b r. Monad m => (Maybe b -> a -> m b) -> Stream (Of a) m r -> Stream (Of b) m r

Map a function across a stream, but also include in its arguments the result of applying it to the previous item in the stream (which is Nothing if the current item is the first in the stream).

filterMapped :: (Monad m, Functor f, Functor g) => (forall x. f x -> m (Sum Identity g x)) -> Stream f m r -> Stream g m r

For each functor wrapper f in the stream, either strip it off or convert it to some new functor g. Return the stream of g's. This can be seen to be analogous to a list function of type Monad m => (a -> m (Maybe b)) -> [a] -> m [b] if we consider what it looks like when f and g are Of a and Of b respectively:

filterMapped
  :: (forall x. Of a x -> m (Sum Identity (Of b) x))
  -> Stream (Of a) m r -> Stream (Of b) m r

Here, Sum Identity (Of b) x is isomorphic to Of (Maybe b) x.

filtered :: (Monad m, Functor f) => (forall x. f x -> m (Maybe x)) -> Stream f m r -> Stream f m r

For each functor wrapper in the stream, optionally strip it off. Those stripped off will be removed from the resulting stream.

buildByteString :: Monad m => (a -> m (Either r (ByteString m (), a))) -> a -> ByteString m r

Build a ByteString monadically from a seed.

bytestringToVector :: ByteString -> Vector Word8

Directly convert a ByteString into a storable Vector, in constant time.

vectorToBytestring :: Vector Word8 -> ByteString

Directly convert a storable Vector of Word8s into a ByteString, in constant time.