| PackageDescription | high-performance left-fold enumerators
Typical buffer–based incremental I/O is based around a single loop,
which reads data from some source (such as a socket or file),
transforms it, and generates one or more outputs (such as a line
count, HTTP responses, or modified file). Although efficient and
safe, these loops are all single–purpose; it is difficult or
impossible to compose buffer–based processing loops.
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Haskell's concept of "lazy I/O" allows pure code to operate on data
from an external source. However, lazy I/O has several shortcomings.
Most notably, resources such as memory and file handles can be
retained for arbitrarily long periods of time, causing unpredictable
performance and error conditions.
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Enumerators are an efficient, predictable, and safe alternative to
lazy I/O. Discovered by Oleg Kiselyov, they allow large datasets to
be processed in near–constant space by pure code. Although somewhat
more complex to write, using enumerators instead of lazy I/O
produces more correct programs.
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This library contains an enumerator implementation for Haskell,
designed to be both simple and efficient. Three core types are
defined, along with numerous helper functions:
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Iteratee: Data sinks, analogous to left folds. Iteratees consume
a sequence of input values, and generate a single output value. Many
iteratees are designed to perform side effects (such as printing to
stdout), so they can also be used as monad transformers.
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Enumerator: Data sources, which generate input sequences. Typical
enumerators read from a file handle, socket, random number generator,
or other external stream. To operate, enumerators are passed an
iteratee, and provide that iteratee with input until either the
iteratee has completed its computation, or EOF.
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Enumeratee: Data transformers, which operate as both enumerators
and iteratees. Enumeratees read from an outer enumerator, and
provide the transformed data to an inner iteratee.
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This package provides a library for the Haskell programming language.
See http://www.haskell.org/ for more information on Haskell. |