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72 lines
2.7 KiB
Markdown
72 lines
2.7 KiB
Markdown
dejafu [![Build Status][build-status]][build-log]
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======
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> [Déjà Fu is] A martial art in which the user's limbs move in time as
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> well as space, […] It is best described as "the feeling that you
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> have been kicked in the head this way before"
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>
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> -- Terry Pratchett, Thief of Time
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Concurrency is nice, deadlocks and race conditions not so much. The
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`Par` monad family, as defined in [abstract-par][] provides
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deterministic parallelism, but sometimes we can tolerate a bit of
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nondeterminism.
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This package provides a class of monads for potentially
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nondeterministic concurrency, with an interface in the spirit of GHC's
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normal concurrency abstraction.
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The documentation of the latest developmental version is
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[available online][docs].
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`MonadConc` and `IO`
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--------------------
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The intention of the `MonadConc` class is to provide concurrency where
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any apparent nondeterminism arises purely from the scheduling
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behaviour. To put it another way, a given computation, parametrised
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with a fixed set of scheduling decisions, is deterministic. This
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assumption is used by the testing functionality provided by
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Control.Monad.Conc.SCT.
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Whilst this assumption may not hold in general when `IO` is involved,
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you should strive to produce test cases where it does.
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Memory Model
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------------
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The testing functionality supports a few different memory models, for
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computations which use non-synchronised `CRef` operations. The
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supported models are:
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- **Sequential Consistency:** A program behaves as a simple
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interleaving of the actions in different threads. When a CRef is
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written to, that write is immediately visible to all threads.
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- **Total Store Order (TSO):** Each thread has a write buffer. A
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thread sees its writes immediately, but other threads will only
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see writes when they are committed, which may happen later. Writes
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are committed in the same order that they are created.
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- **Partial Store Order (PSO):** Each CRef has a write buffer. A
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thread sees its writes immediately, but other threads will only
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see writes when they are committed, which may happen later. Writes
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to different CRefs are not necessarily committed in the same order
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that they are created.
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If a testing function does not take the memory model as a parameter,
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it uses sequential consistency.
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Contributing
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------------
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Bug reports, pull requests, and comments are very welcome!
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Feel free to contact me on GitHub, through IRC (#haskell on freenode),
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or email (mike@barrucadu.co.uk).
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[build-status]: http://ci.barrucadu.co.uk/job/(dejafu)/job/dejafu/badge/icon?style=plastic
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[build-log]: http://ci.barrucadu.co.uk/job/(dejafu)/job/dejafu/
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[docs]: https://barrucadu.github.io/dejafu
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[abstract-par]: https://hackage.haskell.org/package/abstract-par/docs/Control-Monad-Par-Class.html
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