streaming-with-0.2.2.1: with/bracket-style idioms for use with streaming

Streaming.With.Lifted

Description

Both the ContT and Managed (which is a specialised variant of ContT) monads can help with writing heavily nested bracketed code, by being able to pass around the argument to each continuation.

This module - through the use of the Withable class - provides lifted variants of Streaming.With to be able to automatically use these functions for resource management in your choice of monad.

Note that you still need to use the specific monad's running function, as it is not possible to encapsulate those in a generic fashion (unless we wanted to constrain the ContT instance to ContT ()).

To ensure resources don't leak out, it is preferred that if using ContT, you keep the final result type to () (which is what Managed recommends with its runManaged function).

As an example using Managed, this function will copy the contents of two files into a third.:

copyBoth :: FilePath -> FilePath -> FilePath -> IO ()
copyBoth inF1 inF2 outF = runManaged $do bs1 <- withBinaryFileContents inF1 bs2 <- withBinaryFileContents inF2 writeBinaryFile outF bs1 appendBinaryFile outF bs2 Synopsis # Documentation class (Monad w, MonadMask (WithMonad w), MonadIO (WithMonad w)) => Withable w where Source # How to automatically lift bracket-style expressions into a monad. The constraints are common ones found throughout this module, and as such incorporated into this class to avoid repetition in all the type signatures. It is highly recommended that you do not try and layer extra transformers on top of this; the intent of this class is just to make writing all the underlying continuations in a nicer fashion without explicit nesting, rather than as the basis of lower-level code. Minimal complete definition Associated Types type WithMonad w :: * -> * Source # Methods liftWith :: (forall r. (a -> WithMonad w r) -> WithMonad w r) -> w a Source # liftAction :: WithMonad w a -> w a Source # Instances  Source # Associated Typestype WithMonad (Managed :: * -> *) :: * -> * Source # MethodsliftWith :: (forall r. (a -> WithMonad Managed r) -> WithMonad Managed r) -> Managed a Source # (MonadMask m, MonadIO m) => Withable (ContT * r m) Source # Associated Typestype WithMonad (ContT * r m :: * -> *) :: * -> * Source # MethodsliftWith :: (forall a. (a -> WithMonad (ContT * r m) a) -> WithMonad (ContT * r m) a) -> ContT * r m a Source #liftAction :: WithMonad (ContT * r m) a -> ContT * r m a Source # class Withable w => RunWithable w where Source # Safely run the provided continuation. A result of type '()' is required to ensure no resources are leaked. Note that you cannot write something like: copyBoth :: FilePath -> FilePath -> FilePath -> IO () copyBoth inF1 inF2 outF = runWith$ do
bs1 <- withBinaryFileContents inF1
bs2 <- withBinaryFileContents inF2
writeBinaryFile outF bs1
appendBinaryFile outF bs2

as the RunWithable instance cannot be inferred. As such, you will need to specify a type somewhere.

Since: 0.2.1.0

Minimal complete definition

runWith

Methods

runWith :: w () -> WithMonad w () Source #

Instances

 Source # Methods (MonadMask m, MonadIO m) => RunWithable (ContT * () m) Source # MethodsrunWith :: ContT * () m () -> WithMonad (ContT * () m) () Source #

within :: Withable w => w a -> (a -> WithMonad w b) -> w b Source #

A helper function to run a computation within a lifted resource management expression.

within w f = w >>= liftAction . f

Since: 0.2.1.0

liftActionIO :: Withable w => IO a -> w a Source #

A helper function for the common case of lifting an IO computation into a Withable.

liftActionIO = liftAction . liftIO.

Since: 0.2.1.0

liftThrow :: (Withable w, Exception e) => e -> w a Source #

A helper function for the common case of throwing an exception in the underlying monad.

liftThrow = liftAction . throwM.

Since: 0.2.2.0

# File-handling

withFile :: Withable w => FilePath -> IOMode -> w Handle Source #

A lifted variant of withFile.

You almost definitely don't want to use this; instead, use withBinaryFile in conjunction with Data.ByteString.Streaming.

A lifted variant of withBinaryFile.

## Common file-handling cases

writeBinaryFile :: Withable w => FilePath -> ByteString (WithMonad w) r -> w r Source #

Write to the specified file.

appendBinaryFile :: Withable w => FilePath -> ByteString (WithMonad w) r -> w r Source #

Append to the specified file.

withBinaryFileContents :: (Withable w, MonadIO n) => FilePath -> w (ByteString n ()) Source #

Apply a function to the contents of the file.

Note that a different monadic stack is allowed for the ByteString input, as long as it later gets resolved to the required output type (e.g. remove transformer).

## Temporary files

Arguments

 :: Withable w => String File name template. See openTempFile. -> w (FilePath, Handle)

Create and use a temporary file in the system standard temporary directory.

Behaves exactly the same as withTempFile, except that the parent temporary directory will be that returned by getCanonicalTemporaryDirectory.

Since: 0.1.1.0

Arguments

 :: Withable w => FilePath Temp dir to create the file in -> String File name template. See openTempFile. -> w (FilePath, Handle)

Use a temporary filename that doesn't already exist.

Creates a new temporary file inside the given directory, making use of the template. The temp file is deleted after use. For example:

withTempFile "src" "sdist." >>= \(tmpFile, hFile) -> ...

The tmpFile will be file in the given directory, e.g. src/sdist.342.

Since: 0.1.1.0

Arguments

 :: Withable w => String Directory name template. See openTempFile. -> w FilePath

Create and use a temporary directory in the system standard temporary directory.

Behaves exactly the same as withTempDirectory, except that the parent temporary directory will be that returned by getCanonicalTemporaryDirectory.

Since: 0.1.1.0

Arguments

 :: Withable w => FilePath Temp directory to create the directory in -> String Directory name template. See openTempFile. -> w FilePath

Create and use a temporary directory.

Creates a new temporary directory inside the given directory, making use of the template. The temp directory is deleted after use. For example:

withTempDirectory "src" "sdist." >>= \tmpDir -> ...

The tmpDir will be a new subdirectory of the given directory, e.g. src/sdist.342.

Since: 0.1.1.0

# Re-exports

These may assist in writing your own bracket-style functions.

Note that not everything is re-exported: for example, Handle isn't re-exported for use with withFile as it's unlikely that you will write another wrapper around it, and furthermore it wouldn't be a common enough extension to warrant it.

A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads are invalid instances of this class.

Instances should ensure that, in the following code:

fg = f finally g

The action g is called regardless of what occurs within f, including async exceptions. Some monads allow f to abort the computation via other effects than throwing an exception. For simplicity, we will consider aborting and throwing an exception to be two forms of "throwing an error".

If f and g both throw an error, the error thrown by fg depends on which errors we're talking about. In a monad transformer stack, the deeper layers override the effects of the inner layers; for example, ExceptT e1 (Except e2) a represents a value of type Either e2 (Either e1 a), so throwing both an e1 and an e2 will result in Left e2. If f and g both throw an error from the same layer, instances should ensure that the error from g wins.

Effects other than throwing an error are also overriden by the deeper layers. For example, StateT s Maybe a represents a value of type s -> Maybe (a, s), so if an error thrown from f causes this function to return Nothing, any changes to the state which f also performed will be erased. As a result, g will see the state as it was before f. Once g completes, f's error will be rethrown, so g' state changes will be erased as well. This is the normal interaction between effects in a monad transformer stack.

By contrast, lifted-base's version of finally always discards all of g's non-IO effects, and g never sees any of f's non-IO effects, regardless of the layer ordering and regardless of whether f throws an error. This is not the result of interacting effects, but a consequence of MonadBaseControl's approach.

Minimal complete definition

Instances

Generalized abstracted pattern of safe resource acquisition and release in the face of errors. The first action "acquires" some value, which is "released" by the second action at the end. The third action "uses" the value and its result is the result of the bracket.
Note that this is essentially a type-specialized version of generalBracket. This function has a more common signature (matching the signature from Control.Exception), and is often more convenient to use. By contrast, generalBracket is more expressive, allowing us to implement other functions like bracketOnError.