io-streams-1.2.1.2: Simple, composable, and easy-to-use stream I/O

Safe HaskellNone
LanguageHaskell2010

System.IO.Streams.Core

Contents

Description

Core types and functions for the io-streams library.

Synopsis

Stream types

data InputStream a Source

An InputStream generates values of type c in the IO monad.

Two primitive operations are defined on InputStream:

It is intended that InputStreams obey the following law:

unRead c stream >> read stream === return (Just c)

Instances

data OutputStream a Source

An OutputStream consumes values of type c in the IO monad. The only primitive operation defined on OutputStream is:

Values of type c are written in an OutputStream by wrapping them in Just, and the end of the stream is indicated by supplying Nothing.

If you supply a value after a Nothing, the behavior is defined by the implementer of the given OutputStream. (All OutputStream definitions in this library will simply discard the extra input.)

Instances

Creating streams

makeInputStream :: IO (Maybe a) -> IO (InputStream a) Source

Creates an InputStream from a value-producing action.

(makeInputStream m) calls the action m each time you request a value from the InputStream. The given action is extended with the default pushback mechanism (see System.IO.Streams.Internal).

makeOutputStream :: (Maybe a -> IO ()) -> IO (OutputStream a) Source

Creates an OutputStream from a value-consuming action.

(makeOutputStream f) runs the computation f on each value fed to it.

Since version 1.2.0.0, makeOutputStream also ensures that output streams no longer receive data once EOF is received (i.e. you can now assume that makeOutputStream will feed your function Nothing at most once.)

Primitive stream operations

read :: InputStream a -> IO (Maybe a) Source

Reads one value from an InputStream.

Returns either a value wrapped in a Just, or Nothing if the end of the stream is reached.

unRead :: a -> InputStream a -> IO () Source

Pushes a value back onto an input stream. read and unRead should satisfy the following law, with the possible exception of side effects:

Streams.unRead c stream >> Streams.read stream === return (Just c)

Note that this could be used to add values back to the stream that were not originally drawn from the stream.

peek :: InputStream a -> IO (Maybe a) Source

Observes the first value from an InputStream without consuming it.

Returns Nothing if the InputStream is empty. peek satisfies the following law:

Streams.peek stream >> Streams.read stream === Streams.read stream

write :: Maybe a -> OutputStream a -> IO () Source

Feeds a value to an OutputStream. Values of type c are written in an OutputStream by wrapping them in Just, and the end of the stream is indicated by supplying Nothing.

atEOF :: InputStream a -> IO Bool Source

Checks if an InputStream is at end-of-stream.

Connecting streams together

connect :: InputStream a -> OutputStream a -> IO () Source

Connects an InputStream and OutputStream, supplying values from the InputStream to the OutputStream, and propagating the end-of-stream message from the InputStream through to the OutputStream.

The connection ends when the InputStream yields a Nothing.

connectTo :: OutputStream a -> InputStream a -> IO () Source

The connectTo function is just flip connect.

Useful for writing expressions like fromList [1,2,3] >>= connectTo foo.

supply :: InputStream a -> OutputStream a -> IO () Source

Connects an InputStream to an OutputStream without passing the end-of-stream notification through to the OutputStream.

Use this to supply an OutputStream with multiple InputStreams and use connect for the final InputStream to finalize the OutputStream, like so:

do Streams.supply  input1 output
   Streams.supply  input2 output
   Streams.connect input3 output

supplyTo :: OutputStream a -> InputStream a -> IO () Source

supply with the arguments flipped.

Thread safety / concurrency

lockingInputStream :: InputStream a -> IO (InputStream a) Source

Converts an InputStream into a thread-safe InputStream, at a slight performance penalty.

For performance reasons, this library provides non-thread-safe streams by default. Use the locking functions to convert these streams into slightly slower, but thread-safe, equivalents.

lockingOutputStream :: OutputStream a -> IO (OutputStream a) Source

Converts an OutputStream into a thread-safe OutputStream, at a slight performance penalty.

For performance reasons, this library provides non-thread-safe streams by default. Use the locking functions to convert these streams into slightly slower, but thread-safe, equivalents.

Utility streams

nullInput :: IO (InputStream a) Source

An empty InputStream that yields Nothing immediately.

nullOutput :: IO (OutputStream a) Source

An empty OutputStream that discards any input fed to it.

Generator monad

data Generator r a Source

A Generator is a coroutine monad that can be used to define complex InputStreams. You can cause a value of type Just r to appear when the InputStream is read by calling yield:

g :: Generator Int ()
g = do
    Streams.yield 1
    Streams.yield 2
    Streams.yield 3

A Generator can be turned into an InputStream by calling fromGenerator:

m :: IO [Int]
m = Streams.fromGenerator g >>= Streams.toList     -- value returned is [1,2,3]

You can perform IO by calling liftIO, and turn a Generator into an InputStream with fromGenerator.

As a general rule, you should not acquire resources that need to be freed from a Generator, because there is no guarantee the coroutine continuation will ever be called, nor can you catch an exception from within a Generator.

Instances

fromGenerator :: Generator r a -> IO (InputStream r) Source

Turns a Generator into an InputStream.

yield :: r -> Generator r () Source

Calling yield x causes the value Just x to appear on the input when this generator is converted to an InputStream. The rest of the computation after the call to yield is resumed later when the InputStream is read again.