The API of version 0.3.0.0 proved to be too convoluted, with bloated and obscure function signatures. Hopefully the API for the new version is more intuitive.
The central abstraction if that of a Siphon. A Siphon represents a computation that either drains a Producer completely, or fails early aborting the consumption of the Producer. stdout and stderr can only be consumed through Siphons. Siphons can be created out of regular Consumers, pipes folds, or Parsers from pipes-parse.
Why do we need Siphons? When consuming both stdin and stderr of an external process, it is important that the two are drained to completion, because otherwise the process may block due to an output buffer that is not being read and fills up.
Siphons have an Applicative instance that "forks" a producer and feeds it to two computations. This Applicative instance made the support for branching pipelines of processes easy to implement.
The test suite contains several examples of usage.