streaming-with

with/bracket-style idioms for use with streaming

This library provides resource management for the streaming ecosystem of libraries using bracketed continuations.

Currently, these only contain file-handling utilities; if you can think of any more functions that fit in here please let me know!

There are two ways of using this library:

1. Explicitly pass around the continuations using Streaming.With.

2. If you have a lot of nested continuations, you may prefer using Streaming.With.Lifted with either ContT or managed; these will allow you to pass around the parameters to the continuations.

Motivation

The streaming library has some usages of MonadResource from the resourcet package to try and perform resource allocation; in comparison to conduit however, streaming doesn't support prompt finalisation. Furthermore, because in may ways Conduits can be considered to be a datatype representing monadic functions between two types whereas a single Stream is more akin to a producing-only Conduit or Pipe, attempts at having something like readFile that returns a Stream does not always lead to the resource being closed at the correct point. The consensus in the issue for promptness for streaming was that perhaps relying upon MonadResource was not the correct approach.

The Bracket pattern (also known as the with... idiom, or a subset of continuation passing style) allows for a convenient way to obtain and then guarantee the release of (possibly scarce) resources. This can be further enhanced when dealing with nested usages of these with the use of either the ContT managed transformer or the managed library (where Managed -- or at least its safe variant -- is isomorphic to ContT () IO).

The biggest downside of using bracket from the standard base library is that the types are not very convenient in the world of monad transformers: it is limited to IO computations only, which means for our use of streaming that we could only use Stream f IO r without any state, logging, etc. However, the exceptions library contains a more general purpose variant that provides us with extra flexibility; it doesn't even need to be in IO if you have a completely pure Stream! (A variant is also available in lifted-base, but it is less used and streaming already depends upon exceptions).

Disadvantages

Whilst the bracket pattern is powerful, it does have some downsides of which you should be aware (specifically compared to resourcet which is the main alternative).

First of all, independent of its usage with streaming, is that whilst bracket predates resourcet, the latter is more powerful. Whether this extra power is of use to you is up to you, but it does mean that you in effect have lots of nested resource management rather than just one overall resource control.

The obvious disadvantage of using the bracket pattern is that it does not fit in as nicely in the function composition style that usage of streaming enables compared to other stream processing libraries. This can be mitigated somewhat with using the lifted variants in this package which allows you to operate monadically (which still isn't as nice but may be preferable to lots of explicitly nested continuations).

Furthermore, without prompt finalisation the same "long running computation" issue is relevant. For example, consider something that looks like this:

withBinaryFile "myFile" ReadMode $
  doSomethingWithEachLine
  . B.lines
  . B.hGetContents

Ideally, after the last chunk from the file is read, the file handle would be closed. However, that is not the case: it's not until the entire computation is complete that the handle is closed. Note, however, that the same limitation is present when using MonadResource: this is a limitation of the Stream type, not on how we choose to allocate and manage resources.