System.IO.Streams

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

Two primitive operations are defined on InputStream:

• read :: InputStream c -> IO (Maybe c) reads a value from the stream, where "end of stream" is signaled by read returning Nothing.

• unRead :: c -> InputStream c -> IO () "pushes back" a value to the stream.

It is intended that InputStreams obey the following law:

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

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

• write :: Maybe c -> OutputStream c -> IO ()

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.)

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#pushback).

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.)

Reads one value from an InputStream.

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

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.

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

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.

Flipped version of write.

Since: 1.3.0.0.

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

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.

The connectTo function is just flip connect.

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

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

supply with the arguments flipped.

appendInputStream concatenates two InputStreams, analogous to (++) for lists.

The second InputStream continues where the first InputStream ends.

Note: values pushed back to appendInputStream are not propagated to either wrapped InputStream.

concatInputStreams concatenates a list of InputStreams, analogous to (++) for lists.

Subsequent InputStreams continue where the previous one InputStream ends.

Note: values pushed back to the InputStream returned by concatInputStreams are not propagated to any of the source InputStreams.

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.

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.

An empty InputStream that yields Nothing immediately.

An empty OutputStream that discards any input fed to it.

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.System.IO.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.

Turns a Generator into an InputStream.

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.