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- Providing a 'select' and non-blocking IO would be enough to allow people to implement something like it themselves in haskell and are provided by most systems as primitives.
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- Things like the 'poor man's concurrency monad' can achieve some of the benefits
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## Proposal
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---
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# Proposal
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- Standardise on Concurrent Haskell without STM. It is our view that even in the presence of STM, `MVar`s offer
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- Use the semantics from [ Extending the Haskell FFI with Concurrency](http://www.haskell.org/~simonmar/papers/conc-ffi.pdf)
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- Standardise a way to write thread-safe libraries that work with implementations that don't provide full concurrency support.
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Questions:
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## Questions
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- Decide how much pre-emption is acceptable, and figure out how to specify this.
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- Decide whether the Haskell' report includes Concurrency (with a separate NoConcurrency addendum to specify how implementations without concurrency behave), or whether Concurrency is specified in a separate addendum.
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- Should we specify what an implementation that doesn't provide concurrency should do? (e.g. provide an implementation
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of MVars in terms of IORefs, so that concurrency-safe libraries can be written portably).
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- Decide how much pre-emption is acceptable, and figure out how to specify this.
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- Require bound thread support, or make it optional? (YHC has concurrency with non-blocking foreign calls, but doesn't
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have bound threads as yet.)
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## Thread-safety
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In order to write library code that is thread-safe when run in a multi-threaded environment, two things are needed:
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1. a way to protect mutable state against race conditions
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1. a way to declare that foreign calls are blocking
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For (1), we have two choices:
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- Provide `MVar`s. A non-concurrent implementation might implement them in terms of `IORef`,
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for example.
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- Provide STM. Not entirely trivial to implement, even in a single-threaded implementation, because
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exceptions have to abort a transaction.
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For (2), one option is [ForeignBlocking](foreign-blocking).
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---
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| ... | ... | |
