streaming: a free monad transformer optimized for streaming applications, with a simple prelude

Stream can be used wherever FreeT is used. The compiler's standard range of optimizations work better for operations written in terms of Stream. FreeT f m r / Stream f m r is of course extremely general, and many functor-general combinators are exported by the general module Streaming.

See the examples in Streaming.Prelude for a sense of how simple the library is to use and think about. That module is focused on employment with such base functors (readings of the f in Stream f m r) that express different forms of effectful sequences. Some of these appear elsewhere under titles like

pipes:      Producer a m r, Producer a m (Producer a m r), FreeT (Producer a m) m r
io-streams: InputStream a, Generator a r
conduit:    Source m a, ConduitM () o m r

and the like. Streaming.Prelude closely follows Pipes.Prelude, but cleverly omits the pipes:

>>> S.stdoutLn $ S.take 2 S.stdinLn
let's<Enter>
let's
stream<Enter>
stream

And here we do a little connect and resume, as the streaming-io experts call it:

>>> rest <- S.print $ S.splitAt 3 $ S.each [1..10]
1
2
3
>>> S.sum rest
49

Somehow, we didn't even need a four-character operator for that, nor advice about best practices; just ordinary Haskell common sense.

The simplest form of interoperation with pipes is accomplished with this isomorphism:

Pipes.unfoldr Streaming.next        :: Stream (Of a) m r   -> Producer a m r
Streaming.unfoldr Pipes.next        :: Producer a m r      -> Stream (Of a) m r

Interoperation with io-streams is thus:

Streaming.reread IOStreams.read     :: InputStream a       -> Stream (Of a) IO ()
IOStreams.unfoldM Streaming.uncons  :: Stream (Of a) IO () -> IO (InputStream a)

A simple exit to conduit would be, e.g.:

Conduit.unfoldM Streaming.uncons    :: Stream (Of a) m ()  -> Source m a

These conversions should never be more expensive than a single >-> or =$=.

At a much more general level, we also of course have interoperation with free:

Free.iterTM  Stream.wrap              :: FreeT f m a -> Stream f m a
Stream.iterTM Free.wrap               :: Stream f m a -> FreeT f m a

For some simple ghci examples, see the commentary throughout the Prelude module. For slightly more advanced usage see the commentary in the haddocks of streaming-bytestring and e.g. these replicas of shell-like programs from the io-streams tutorial. Here's a simple streaming GET request with intrinsically streaming byte streams.

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• OliverCharles, MichaelThompson, andrewthad, chessai

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